The scope of Baker's law

Pollinator-mediated effects of landscape-scale land use on grassland plant community composition and ecosystem functioning - seven hypotheses

Environmental change is disrupting mutualisms between organisms worldwide. Reported declines in insect populations and changes in pollinator community compositions in response to land use and other environmental drivers have put the spotlight on the need to conserve pollinators. While this is often motivated by their role in supporting crop yields, the role of pollinators for reproduction and resulting taxonomic and functional assembly in wild plant communities has received less attention. Recent findings suggest that observed and experimental gradients in pollinator availability can affect plant community composition, but we know little about when such shifts are to be expected, or the impact they have on ecosystem functioning. Correlations between plant traits related to pollination and plant traits related to other important ecosystem functions, such as productivity, nitrogen uptake or palatability to herbivores, lead us to expect non-random shifts in ecosystem functioning in response to changes in pollinator communities. At the same time, ecological and evolutionary processes may counteract these effects of pollinator declines, limiting changes in plant community composition, and in ecosystem functioning. Despite calls to investigate community- and ecosystem-level impacts of reduced pollination, the study of pollinator effects on plants has largely been confined to impacts on plant individuals or single-species populations. With this review we aim to break new ground by bringing together aspects of landscape ecology, ecological and evolutionary plant-insect interactions, and biodiversity-ecosystem functioning research, to generate new ideas and hypotheses about the ecosystem-level consequences of pollinator declines in response to land-use change, using grasslands as a focal system. Based on an integrated set of seven hypotheses, we call for more research investigating the putative pollinator-mediated links between landscape-scale land use and ecosystem functioning. In particular, future research should use combinations of experimental and observational approaches to assess the effects of changes in pollinator communities over multiple years and across species on plant communities and on trait distributions both within and among species.

Support for Baker's law: Facultative self-fertilization ability decreases pollen limitation in experimental colonization

PREMISE

The ability to self-fertilize is predicted to provide an advantage in colonization because a single individual can reproduce and establish a next generation in a new location regardless of the density of mates. While there is theoretical and correlative support for this idea, the strength of mate limitation as a selective agent has not yet been delineated from other factors that can also select for self-fertilization in colonization of new habitats. We used known mating-system variation in the American bellflower (Campanula americana) to explore how plants' ability to self-fertilize can mitigate density-dependent reproduction and impact colonization success.

METHODS

We created experimental populations of single individuals or a small number of plants to emulate isolated colonization events. These populations were composed of plants that differed in their ability to self-fertilize. We compared pollen limitation of the single individuals to that of small populations.

RESULTS

Experimental populations of plants that readily self-fertilize produced consistent seed numbers regardless of population size, whereas plants with lower ability to self-fertilize had density-dependent reproduction with greater seed production in small populations than in populations composed of a single individual.

CONCLUSIONS

We experimentally isolated the effect of mate limitation in colonization and found that it can select for increased self-fertilization. We show the benefit of self-fertilization in colonization, which helps to explain geographic patterns of self-fertilization and shows support for Baker's law, a long-held hypothesis in the field of mating-system evolution.

Island plants with newly discovered reproductive traits have higher capacity for uniparental reproduction, supporting Baker's law

Uniparental reproduction is advantageous when lack of mates limits outcrossing opportunities in plants. Baker's law predicts an enrichment of uniparental reproduction in habitats colonized via long-distance dispersal, such as volcanic islands. To test it, we analyzed reproductive traits at multiple hierarchical levels and compared seed-set after selfing and crossing experiments in both island and mainland populations of Limonium lobatum, a widespread species that Baker assumed to be self-incompatible because it had been described as pollen-stigma dimorphic, i.e., characterized by floral morphs differing in pollen-surface morphology and stigma-papillae shape that are typically self-incompatible. We discovered new types and combinations of pollen and stigma traits hitherto unknown in the literature on pollen-stigma dimorphism and a lack of correspondence between such combinations and pollen compatibility. Contrary to previous reports, we conclude that Limonium lobatum comprises both self-compatible and self-incompatible plants characterized by both known and previously undescribed combinations of reproductive traits. Most importantly, plants with novel combinations are overrepresented on islands, selfed seed-set is higher in islands than the mainland, and insular plants with novel pollen-stigma trait-combinations disproportionally contribute to uniparental reproduction on islands. Our results thus support Baker's law, connecting research on reproductive and island biology.

Death caps (Amanita phalloides) frequently establish from sexual spores, but individuals can grow large and live for more than a decade in invaded forests

Global change is reshaping Earth's biodiversity, but the changing distributions of nonpathogenic fungi remain largely undocumented, as do mechanisms enabling invasions. The ectomycorrhizal Amanita phalloides is native to Europe and invasive in North America. Using population genetics and genomics, we sought to describe the life history traits of this successfully invading symbiotic fungus. To test whether death caps spread underground using hyphae, or aboveground using sexual spores, we mapped and genotyped mushrooms from European and US sites. Larger genetic individuals (genets) would suggest spread mediated by vegetative growth, while many small genets would suggest dispersal mediated by spores. To test whether genets are ephemeral or persistent, we also sampled from populations over time. At nearly every site and across all time points, mushrooms resolve into small genets. Individuals frequently establish from sexual spores. But at one Californian site, a single individual measuring nearly 10 m across dominated. At two Californian sites, the same genetic individuals were discovered in 2004, 2014, and 2015, suggesting single individuals (both large and small) can reproduce repeatedly over relatively long timescales. A flexible life history strategy combining both mycelial growth and spore dispersal appears to underpin the invasion of this deadly perennial ectomycorrhizal fungus.

Gametic selection favours polyandry and selfing

Competition among pollen or sperm (gametic selection) can cause evolution. Mating systems shape the intensity of gametic selection by determining the competitors involved, which can in turn cause the mating system itself to evolve. We model the bidirectional relationship between gametic selection and mating systems, focusing on variation in female mating frequency (monandry-polyandry) and self-fertilisation (selfing-outcrossing). First, we find that monandry and selfing both reduce the efficiency of gametic selection in removing deleterious alleles. This means that selfing can increase mutation load, in contrast to cases without gametic selection where selfing purges deleterious mutations and decreases mutation load. Second, we explore how mating systems evolve via their effect on gametic selection. By manipulating gametic selection, polyandry can evolve to increase the fitness of the offspring produced. However, this indirect advantage of post-copulatory sexual selection is weak and is likely to be overwhelmed by any direct fitness effects of mating systems. Nevertheless, gametic selection can be potentially decisive for selfing evolution because it significantly reduces inbreeding depression, which favours selfing. Thus, the presence of gametic selection could be a key factor driving selfing evolution.

Linking pollen limitation and seed dispersal effectiveness

Seed production and dispersal are crucial ecological processes impacting plant demography, species distributions and community assembly. Plant-animal interactions commonly mediate both seed production and seed dispersal, but current research often examines pollination and seed dispersal separately, which hinders our understanding of how pollination services affect downstream dispersal services. To fill this gap, we propose a conceptual framework exploring how pollen limitation can impact the effectiveness of seed dispersal for endozoochorous and myrmecochorous plant species. We summarize the quantitative and qualitative effects of pollen limitation on plant reproduction and use Optimal Foraging Theory to predict its impact on the foraging behaviour of seed dispersers. In doing so, we offer a new framework that poses numerous hypotheses and empirical tests to investigate links between pollen limitation and seed dispersal effectiveness and, consequently, post-dispersal ecological processes occurring at different levels of biological organization. Finally, considering the importance of pollination and seed dispersal outcomes to plant eco-evolutionary dynamics, we discussed the implications of our framework for future studies exploring the demographic and evolutionary impacts of pollen limitation for animal-dispersed plants.

The evolutionary responses of life-history strategies to climatic variability in flowering plants

The evolution of annual or perennial strategies in flowering plants likely depends on a broad array of temperature and precipitation variables. Previous documented climate life-history correlations in explicit phylogenetic frameworks have been limited to certain clades and geographic regions. To gain insights which generalize to multiple lineages we employ a multi-clade approach analyzing 32 groups of angiosperms across eight climatic variables. We utilize a recently developed method that accounts for the joint evolution of continuous and discrete traits to evaluate two hypotheses: annuals tend to evolve in highly seasonal regions prone to extreme heat and drought; and annuals tend to have faster rates of climatic niche evolution than perennials. We find that temperature, particularly highest temperature of the warmest month, is the most consistent climatic factor influencing the evolution of annual strategy in flowering plants. Unexpectedly, we do not find significant differences in rates of climatic niche evolution between perennial and annual lineages. We propose that annuals are consistently favored in areas prone to extreme heat due to their ability to escape heat stress as seeds, but they tend to be outcompeted by perennials in regions where extreme heat is uncommon or nonexistent.

Invasive Californian death caps develop mushrooms unisexually and bisexually

Canonical sexual reproduction among basidiomycete fungi involves the fusion of two haploid individuals of different mating types, resulting in a heterokaryotic mycelial body made up of genetically different nuclei. Using population genomics data and experiments, we discover mushrooms of the invasive and deadly Amanita phalloides can also be homokaryotic; evidence of sexual reproduction by single, unmated individuals. In California, genotypes of homokaryotic mushrooms are also found in heterokaryotic mushrooms, implying nuclei of homokaryotic mycelia are also involved in outcrossing. We find death cap mating is controlled by a single mating type locus, but the development of homokaryotic mushrooms appears to bypass mating type gene control. Ultimately, sporulation is enabled by nuclei able to reproduce alone as well as with others, and nuclei competent for both unisexuality and bisexuality have persisted in invaded habitats for at least 17 but potentially as long as 30 years. The diverse reproductive strategies of invasive death caps are likely facilitating its rapid spread, suggesting a profound similarity between plant, animal and fungal invasions.

Phylogeographical evidence for historical long-distance dispersal in the flightless stick insect Ramulus mikado

Exploring how organisms overcome geographical barriers to dispersal is a fundamental question in biology. Passive long-distance dispersal events, although infrequent and unpredictable, have a considerable impact on species range expansions. Despite limited active dispersal capabilities, many stick insect species have vast geographical ranges, indicating that passive long-distance dispersal is vital for their distribution. A potential mode of passive dispersal in stick insects is via the egg stage within avian digestive tracts, as suggested by experimental evidence. However, detecting such events under natural conditions is challenging due to their rarity. Therefore, to indirectly assess the potential of historical avian-mediated dispersal, we examined the population genetic structure of the flightless stick insect Ramulus mikado across Japan, based on a multifaceted molecular approach [cytochrome oxidase subunit I (COI) haplotypes, nuclear simple sequence repeat markers and genome-wide single nucleotide polymorphisms]. Subsequently, we identified unique phylogeographic patterns, including the discovery of identical COI genotypes spanning considerable distances, which substantiates the notion of passive long-distance genotypic dispersal. Overall, all the molecular data revealed the low and mostly non-significant genetic differentiation among populations, with identical or very similar genotypes across distant populations. We propose that long-distance dispersal facilitated by birds is the plausible explanation for the unique phylogeographic pattern observed in this flightless stick insect.

An epigenetic clock in plants

DNA methylation can identify evolutionary relationships among close plant lineages.

Population Rescue through an Increase in the Selfing Rate under Pollen Limitation: Plasticity versus Evolution

AbstractIncreased rates of self-fertilization offer reproductive assurance when plant populations experience pollen limitation, but self-fertilization may reduce fitness by exposing deleterious mutations. If an environmental change responsible for pollen limitation also induces plastic mating system shifts toward self-pollination, the reproductive assurance benefit and inbreeding depression cost of increased self-fertilization occur immediately, while the benefit and cost happen more gradually when increased self-fertilization occur through evolution. I built eco-evolutionary models to explore the demographic and genetic conditions in which higher self-fertilization by plasticity and/or evolution rescues populations, following deficits due to a sudden onset of pollen limitation. Rescue is most likely under an intermediate level of selfing rate increase, either through plasticity or evolution, and this critical level of selfing rate increase is higher under stronger pollen limitation. Generally, rescue is more likely through plasticity than through evolution. Under weak pollen limitation, rescue by enhanced self-fertilization may mainly occur through purging of deleterious mutations rather than reproductive assurance. The selfing rate increase conferring the highest rescue probability is lower when the initial population size is smaller. This article shows the importance of plasticity during plant population rescue and offers insights for future studies of the evolution of mating system plasticity.

Tetrasporophytic bias coupled with heterozygote deficiency in Antarctic Plocamium sp. (Florideophyceae, Rhodophyta)

Meiosis and syngamy generate an alternation between two ploidy stages, but the timing of these two processes varies widely across taxa, thereby generating life cycle diversity. One hypothesis suggests that life cycles with long-lived haploid stages are correlated with selfing, asexual reproduction, or both. Though mostly studied in angiosperms, selfing and asexual reproduction are often associated with marginal habitats. Yet, in haploid-diploid macroalgae, these two reproductive modes have subtle but unique consequences whereby predictions from angiosperms may not apply. Along the western Antarctic Peninsula, there is a thriving macroalgal community, providing an opportunity to explore reproductive system variation in haploid-diploid macroalgae at high latitudes where endemism is common. Plocamium sp. is a widespread and abundant red macroalga observed within this ecosystem. We sampled 12 sites during the 2017 and 2018 field seasons and used 10 microsatellite loci to describe the reproductive system. Overall genotypic richness and evenness were high, suggesting sexual reproduction. Eight sites were dominated by tetrasporophytes, but there was strong heterozygote deficiency, suggesting intergametophytic selfing. We observed slight differences in the prevailing reproductive mode among sites, possibly due to local conditions (e.g., disturbance) that may contribute to site-specific variation. It remains to be determined whether high levels of selfing are characteristic of macroalgae more generally at high latitudes, due to the haploid-diploid life cycle, or both. Further investigations of algal life cycles will likely reveal the processes underlying the maintenance of sexual reproduction more broadly across eukaryotes, but more studies of natural populations are required.

Clones on the run: The genomics of a recently expanded partially clonal species

Why species that in their core areas mainly reproduce sexually become enriched with clones in marginal populations ("geographic parthenogenesis") remains unclear. Earlier hypotheses have emphasized that selection might promote clonality because it protects locally adapted genotypes. On the other hand, it also hampers recombination and adaptation to changing conditions. The aim of the present study was to investigate the early stages of range expansion in a partially clonal species and what drives an increase in cloning during such expansion. We used genome-wide sequencing to investigate the origin and evolution of large clones formed in a macroalgal species (Fucus vesiculosus) during a recent expansion into the postglacial Baltic Sea. We found low but persistent clonality in core populations, while at range margins, large dominant clonal lineages had evolved repeatedly from different sexual populations. A range expansion model showed that even when asexual recruitment is less favourable than sexual recruitment in core populations, repeated bottlenecks at the expansion front can establish a genetically eroded clonal wave that spreads ahead of a sexual wave into the new area. Genetic variation decreases by drift following repeated bottlenecks at the expansion front. This results in the emerging clones having low expected heterozygosity, which corroborated our empirical observations. We conclude that Baker's Law (clones being favoured by uniparental reproductive assurance in new areas) can play an important role during range expansion in partially clonal species, resulting in a complex spatiotemporal mosaic of clonal and sexual lineages that might persist during thousands of generations.

Drivers of strong isolation and small effective population size at a leading range edge of a widespread plant

Climate change has influenced species distributions worldwide with upward elevational shifts observed in many systems. Leading range edge populations, like those at upper elevation limits, are crucial for climate change responses but can exhibit low genetic diversity due to founder effects, isolation, or limited outbreeding. These factors can hamper local adaptation at range limits. Using the widespread herb, Argentina anserina, we measured ecological attributes (population density on the landscape, area of population occupancy, and plant and flower density) spanning a 1000 m elevation gradient, with high elevation populations at the range limit. We measured vegetative clonal potential in the greenhouse for populations spanning the gradient. We combined these data with a ddRAD-seq dataset to test the hypotheses that high elevation populations would exhibit ecological and genomic signatures of leading range edge populations. We found that population density on the landscape declined towards the high elevation limit, as is expected towards range edges. However, plant density was elevated within edge populations. In the greenhouse, high elevation plants exhibited stronger clonal potential than low elevation plants, likely explaining increased plant density in the field. Phylogeographic analysis supported more recent colonization of high elevation populations which were also more genetically isolated, had more extreme heterozygote excess and had smaller effective population size than low. Results support that colonization of high elevations was likely accompanied by increased asexuality, contributing to a decline in effective population size. Despite high plant density in leading edge populations, their small effective size, isolation and clonality could constrain adaptive potential.

Fruit Size in Indo-Malayan Island Plants Is More Strongly Influenced by Filtering than by In Situ Evolution

AbstractCommunity trait assembly, the formation of distributions of phenotypic characteristics across coexisting species, can occur via two main processes: filtering of trait distributions from the regional pool and in situ phenotypic evolution in local communities. But the relative importance of these processes remains unclear, largely because of the difficulty in determining the timing of evolutionary trait changes and biogeographic dispersal events in phylogenies. We assessed evolutionary and biogeographic transitions in woody plant species across the Indo-Malay archipelago, a series of island groups where the same plant lineages interact with different seed disperser and seed predator assemblages. Fruit size in 2,650 taxa spanning the angiosperm tree of life tended to be smaller in the Sulawesi and Maluku island groups, where frugivores are less diverse and smaller bodied, than in the regional source pool. While numerous plant lineages (not just small-fruited ones) reached the isolated islands, colonists tended to be the smaller-fruited members of each clade. Nearly all of the evolutionary transitions to smaller fruit size predated, often substantially, organismal dispersal to the islands. Our results suggest that filtering rather than within-island evolution largely determined the distribution of fruit sizes in these regions.

Invasive Californian death caps develop mushrooms unisexually and bisexually

Canonical sexual reproduction among basidiomycete fungi involves the fusion of two haploid individuals of different sexes, resulting in a heterokaryotic mycelial body made up of genetically different nuclei 1 . Using population genomics data, we discovered mushrooms of the deadly invasive Amanita phalloides are also homokaryotic, evidence of sexual reproduction by single individuals. In California, genotypes of homokaryotic mushrooms are also found in heterokaryotic mushrooms, implying nuclei of homokaryotic mycelia also promote outcrossing. We discovered death cap mating is controlled by a single mating-type locus ( A. phalloides is bipolar), but the development of homokaryotic mushrooms appears to bypass mating-type gene control. Ultimately, sporulation is enabled by nuclei able to reproduce alone as well as with others, and nuclei competent for both unisexuality and bisexuality have persisted in invaded habitats for at least 17 but potentially as long as 30 years. The diverse reproductive strategies of invasive death caps are likely facilitating its rapid spread, revealing a profound similarity between plant, animal and fungal invasions 2,3 .

A trait-based approach to determining principles of plant biogeography

Lineage-specific traits determine how plants interact with their surrounding environment. Unrelated species may evolve similar phenotypic characteristics to tolerate, persist in, and invade environments with certain characteristics, resulting in some traits becoming relatively more common in certain types of habitats. Analyses of these general patterns of geographical trait distribution have led to the proposal of general principles to explain how plants diversify in space over time. Trait-environment correlation analyses quantify to what extent unrelated lineages have similar evolutionary responses to a given type of habitat. In this synthesis, I give a short historical overview on trait-environment correlation analyses, from some key observations from classic naturalists to modern approaches using trait evolution models, large phylogenies, and massive data sets of traits and distributions. I discuss some limitations of modern approaches, including the need for more realistic models, the lack of data from tropical areas, and the necessary focus on trait scoring that goes beyond macromorphology. Overcoming these limitations will allow the field to explore new questions related to trait lability and niche evolution and to better identify generalities and exceptions in how plants diversify in space over time.

Multiple introductions, polyploidy and mixed reproductive strategies are linked to genetic diversity and structure in the most widespread invasive plant across Southern Ocean archipelagos

Biological invasions in remote areas that experience low human activity provide unique opportunities to elucidate processes responsible for invasion success. Here we study the most widespread invasive plant species across the isolated islands of the Southern Ocean, the annual bluegrass, Poa annua. To analyse geographical variation in genome size, genetic diversity and reproductive strategies, we sampled all major sub-Antarctic archipelagos in this region and generated microsatellite data for 470 individual plants representing 31 populations. We also estimated genome sizes for a subset of individuals using flow cytometry. Occasional events of island colonization are expected to result in high genetic structure among islands, overall low genetic diversity and increased self-fertilization, but we show that this is not the case for P. annua. Microsatellite data indicated low population genetic structure and lack of isolation by distance among the sub-Antarctic archipelagos we sampled, but high population structure within each archipelago. We identified high levels of genetic diversity, low clonality and low selfing rates in sub-Antarctic P. annua populations (contrary to rates typical of continental populations). In turn, estimates of selfing declined in populations as genetic diversity increased. Additionally, we found that most P. annua individuals are probably tetraploid and that only slight variation exists in genome size across the Southern Ocean. Our findings suggest multiple independent introductions of P. annua into the sub-Antarctic, which promoted the establishment of genetically diverse populations. Despite multiple introductions, the adoption of convergent reproductive strategies (outcrossing) happened independently in each major archipelago. The combination of polyploidy and a mixed reproductive strategy probably benefited P. annua in the Southern Ocean by increasing genetic diversity and its ability to cope with the novel environmental conditions.

Genomic insights into inter- and intraspecific mating system shifts in Primulina

The mating system shift from outcrossing to selfing is one of the most frequent evolutionary trends in flowering plants. However, the genomic consequences of this shift remain poorly understood. Specifically, the relative importance of the demographic and genetic processes causing changes in genetic variation and selection efficacy associated with the evolution of selfing is unclear. Here we sequenced the genomes of two Primulina species with contrasting mating systems: P. eburnea (outcrossing) versus P. tabacum (outcrossing, mixed-mating and selfing populations). Whole-genome resequencing data were used to investigate the genomic consequences of mating system shifts within and between species. We found that highly selfing populations of P. tabacum display loss of genetic diversity, increased deleterious mutations, higher genomic burden and fewer adaptive substitutions. However, compared with outcrossing populations, mixed-mating populations did not display loss of genetic diversity and accumulation of genetic load. We find no evidence of population bottlenecks associated with the shift to selfing, which suggests that the genetic effects of selfing on N_e and possibly linked selection, rather than demographic history, are the primary drivers of diversity reduction in highly selfing populations. Our results highlight the importance of distinguishing the relative contribution of mating system and demography on the genomic consequences associated with mating system evolution in plants.

Large-scale and small-scale population genetic structure of the medically important gastropod species Bulinus truncatus (Gastropoda, Heterobranchia)

Background

Gastropod snails remain strongly understudied, despite their important role in transmitting parasitic diseases. Knowledge of their distribution and population dynamics increases our understanding of the processes driving disease transmission. We report the first study to use high-throughput sequencing (HTS) to elucidate the population genetic structure of the hermaphroditic snail Bulinus truncatus (Gastropoda, Heterobranchia) on a regional (17–150 km) and inter-regional (1000–5400 km) scale. This snail species acts as an intermediate host of Schistosoma haematobium and Schistosoma bovis, which cause human and animal schistosomiasis respectively.

Methods

Bulinus truncatus snails were collected in Senegal, Cameroon, Egypt and France and identified through DNA barcoding. A single-end genotyping-by-sequencing (GBS) library, comprising 87 snail specimens from the respective countries, was built and sequenced on an Illumina HiSeq 2000 platform. Reads were mapped against S. bovis and S. haematobium reference genomes to identify schistosome infections, and single nucleotide polymorphisms (SNPs) were scored using the Stacks pipeline. These SNPs were used to estimate genetic diversity, assess population structure and construct phylogenetic trees of B. truncatus.

Results

A total of 10,750 SNPs were scored and used in downstream analyses. The phylogenetic analysis identified five clades, each consisting of snails from a single country but with two distinct clades within Senegal. Genetic diversity was low in all populations, reflecting high selfing rates, but varied between locations due to habitat variability. Significant genetic differentiation and isolation by distance patterns were observed at both spatial scales, indicating that gene flow is not strong enough to counteract the effects of population bottlenecks, high selfing rates and genetic drift. Remarkably, the population genetic differentiation on a regional scale (i.e. within Senegal) was as large as that between populations on an inter-regional scale. The blind GBS technique was able to pick up parasite DNA in snail tissue, demonstrating the potential of HTS techniques to further elucidate the role of snail species in parasite transmission.

Conclusions

HTS techniques offer a valuable toolbox to further investigate the population genetic patterns of intermediate schistosome host snails and the role of snail species in parasite transmission.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05445-x.

Contrasting patterns of genetic diversity and differentiation across the continental disjunct range of a sexually polymorphic aquatic plant

BACKGROUND AND AIMS

Reproductive systems enabling opportunities for self-fertilization influence population genetic structure and play a key role in colonization and genetic differentiation during range expansion. Because of their well-developed powers of dispersal, aquatic plants often have widespread disjunct geographical distributions, providing opportunities to investigate the role of reproductive systems in structuring genetic variation between parts of the range that differ in migration history and ecology.

METHODS

We compared reproductive systems and spatial genetic structure of the freshwater aquatic macrophyte Sagittaria latifolia between disjunct western and eastern ranges of North America (NA). Populations of this species are most commonly either monoecious or dioecious. We examined chloroplast DNA haplotype variation to test the hypothesis that the western range of this species represents a secondary colonization from the east, and evaluated the roles of reproductive system differences and geography in structuring contemporary patterns of genetic variation at 11 polymorphic SSR (simple sequence repeat) loci.

KEY RESULTS

Chloroplast haplotyping revealed a single haplotype in western NA compared to numerous haplotypes in eastern NA, consistent with a genetic bottleneck during westward migration. Estimates of genetic diversity in eastern NA populations differed significantly between reproductive systems, but this pattern was not evident in the western range. Eastern populations could be reliably assigned to genetic clusters based on their reproductive systems, whereas western populations clustered primarily by geographical location.

CONCLUSIONS

The sparser distribution of aquatic habitats in the drier western range of S. latifolia, combined with secondary colonization of this region, probably cause the lower genetic diversity and increased differentiation among populations, possibly overriding the effects of reproductive system evident in the eastern portion of the range. Our findings demonstrate that the complex interplay between migratory history, reproductive systems and habitat availability plays an important role in structuring spatial patterns of genetic variation in disjunct plant populations.

Dioecy and chromosomal sex determination are maintained through allopolyploid speciation in the plant genus Mercurialis

Polyploidization may precipitate dramatic changes to the genome, including chromosome rearrangements, gene loss, and changes in gene expression. In dioecious plants, the sex-determining mechanism may also be disrupted by polyploidization, with the potential evolution of hermaphroditism. However, while dioecy appears to have persisted through a ploidy transition in some species, it is unknown whether the newly formed polyploid maintained its sex-determining system uninterrupted, or whether dioecy re-evolved after a period of hermaphroditism. Here, we develop a bioinformatic pipeline using RNA-sequencing data from natural populations to demonstrate that the allopolyploid plant Mercurialis canariensis directly inherited its sex-determining region from one of its diploid progenitor species, M. annua, and likely remained dioecious through the transition. The sex-determining region of M. canariensis is smaller than that of its diploid progenitor, suggesting that the non-recombining region of M. annua expanded subsequent to the polyploid origin of M. canariensis. Homeologous pairs show partial sexual subfunctionalization. We discuss the possibility that gene duplicates created by polyploidization might contribute to resolving sexual antagonism.

The relative role of plasticity and demographic history in Capsella bursa-pastoris: a common garden experiment in Asia and Europe

The colonization success of a species depends on the interplay between its phenotypic plasticity, adaptive potential and demographic history. Assessing their relative contributions during the different phases of a species range expansion is challenging, and requires large-scale experiments. Here, we investigated the relative contributions of plasticity, performance and demographic history to the worldwide expansion of the shepherd's purse, Capsella bursa-pastoris. We installed two large common gardens of the shepherd's purse, a young, self-fertilizing, allopolyploid weed with a worldwide distribution. One common garden was located in Europe, the other in Asia. We used accessions from three distinct genetic clusters (Middle East, Europe and Asia) that reflect the demographic history of the species. Several life-history traits were measured. To explain the phenotypic variation between and within genetic clusters, we analysed the effects of (i) the genetic clusters, (ii) the phenotypic plasticity and its association to fitness and (iii) the distance in terms of bioclimatic variables between the sampling site of an accession and the common garden, i.e. the environmental distance. Our experiment showed that (i) the performance of C. bursa-pastoris is closely related to its high phenotypic plasticity; (ii) within a common garden, genetic cluster was a main determinant of phenotypic differences; and (iii) at the scale of the experiment, the effect of environmental distance to the common garden could not be distinguished from that of genetic clusters. Phenotypic plasticity and demographic history both play important role at different stages of range expansion. The success of the worldwide expansion of C. bursa-pastoris was undoubtedly influenced by its strong phenotypic plasticity.

S-RNase Alleles Associated With Self-Compatibility in the Tomato Clade: Structure, Origins, and Expression Plasticity

The self-incompatibility (SI) system in the Solanaceae is comprised of cytotoxic pistil S-RNases which are countered by S-locus F-box (SLF) resistance factors found in pollen. Under this barrier-resistance architecture, mating system transitions from SI to self-compatibility (SC) typically result from loss-of-function mutations in genes encoding pistil SI factors such as S-RNase. However, the nature of these mutations is often not well characterized. Here we use a combination of S-RNase sequence analysis, transcript profiling, protein expression and reproductive phenotyping to better understand different mechanisms that result in loss of S-RNase function. Our analysis focuses on 12 S-RNase alleles identified in SC species and populations across the tomato clade. In six cases, the reason for gene dysfunction due to mutations is evident. The six other alleles potentially encode functional S-RNase proteins but are typically transcriptionally silenced. We identified three S-RNase alleles which are transcriptionally silenced under some conditions but actively expressed in others. In one case, expression of the S-RNase is associated with SI. In another case, S-RNase expression does not lead to SI, but instead confers a reproductive barrier against pollen tubes from other tomato species. In the third case, expression of S-RNase does not affect self, interspecific or inter-population reproductive barriers. Our results indicate that S-RNase expression is more dynamic than previously thought, and that changes in expression can impact different reproductive barriers within or between natural populations.

Evolution of floral characters and biogeography of Heloniadeae (Melanthiaceae): an example of breeding system shifts with inflorescence change

Heloniadeae (Melanthiaceae) presents an East Asia–North America disjunct distribution. Different molecular and morphological data nevertheless support the tribe as a monophyletic group. However, their phylogenetic relationships and biogeographic history, together with the character evolution, are not clear. Therefore, we constructed a Bayesian phylogenetic tree for Heloniadeae using cpDNA and inferred the historical biogeography and floral character evolution. The results revealed that Heloniadeae was distributed in high-latitudes of East Asia and North America, originating since 22.2 mya. The East Asia clade migrated into southwest China, and subsequently colonized the Korean Peninsula, Taiwan, the Ryukyus, and spread northward to Japan and southern Sakhalin. The evolution of the inflorescence and number of flowers were phylogenetically conserved, associated with the historical biogeography of Heloniadeae. The inflorescences transferred from raceme to sub-umbel, and the number of flowers decreased during the dispersal process, which may be accompanied by changes in the breeding system. Besides, the anthesis period was more affected by the habitat environment than phylogenetic constraints. The flowering temperature of Heloniadeae was below 20 °C in most species, except H. kawanoi. Such a low temperature might not be conductive to pollinator activities, but it could be compensated by sustaining seed production with long-lasting flowers.

Greater reproductive assurance of asexual plant compared with sexual relative in a low-density sympatric population: Experimental evidence for pollen limitation

High reproductive assurance is regarded as a key advantage of uniparentally reproducing organisms for establishing a new population. This demographic benefit should especially be relevant for plants with autonomous apomixis, that is those which produce seeds completely independently from mates and pollinators. Indeed, many autonomous apomicts occupy larger distributional ranges when compared to their sexual relatives, showing geographical parthenogenesis patterns. However, uniparental reproduction advantage has only rarely been quantified in natural populations and results provided a mixed support, partly because allopatric sexual and asexual populations were exposed to different environmental and pollination conditions causing considerable between-population variation in the level of reproductive assurance. Here, we compared the level and stability of reproductive assurance between sexual self-incompatible and asexual autonomously apomictic plants of Hieracium alpinum (Asteraceae) cultivated in a sympatric low-density population with two levels of spatial clumping of sexual plants. Overall, we found that the realized seed set (i.e. proportion of well-developed seeds per capitulum) of asexuals was ca. 3 times greater than that of sexuals (83% vs. 27%), whereas the variance of this trait expressed as coefficient of variation was ca. 4 times smaller in asexuals compared with sexuals (19% vs. 83%). Solitary sexual plants had more than 2 times lower realized seed set when compared to clumps composed of two spatially close (20-30 cm) sexual plants (13% vs. 34%). Our study provides experimental evidence for benefit of uniparental reproduction of asexuals in a sympatric situation when the availability of mates is limited. This, together with unpredictability of pollinator environment could provide autonomous apomicts with an ultimate demographic superiority during colonization reflected in geographical parthenogenesis observed in this species.

Breeding systems of naturalized versus indigenous species provide support for Baker's law on Pohnpei island

The factors that facilitate successful colonization of islands should be especially evident where the establishment filter is strongest. Colonizers of small, remote oceanic islands should be initially rare, extremely mate-limited and often without pollinators. Hence, plant communities on such islands should reflect an establishment history in which young 'naturalized' species are most likely to display self-compatibility and autonomous selfing, whereas 'indigenous' species may exhibit more diverse reproductive strategies. To test this prediction, we characterized breeding systems of 28 species on Pohnpei, in the Federated States of Micronesia, a group of remote Pacific islands that are considered a global biodiversity hotspot. Three families with both naturalized and indigenous species were selected-Fabaceae, Malvaceae and Melastomataceae. Measurements included field observations of dichogamy/herkogamy and floral attraction traits, pollen:ovule (P:O) ratios and experimental hand-pollinations for self-compatibility and pollen limitation. Phylogenetic generalized least squares analyses tested for trait correlations between naturalized and indigenous species. Flowers of all 28 species were bisexual, and pollinator attraction features were common. Pollen:ovule ratios ranged from 9 to 557 (median = 87), and all 11 hand-pollinated species were self-compatible. All species had >5 ovules and <3500 pollen grains per flower. Indigenous species did not differ significantly from naturalized species for any trait. There is a dearth of data from remote islands bearing on the question of establishment history. In this study, we inferred all species to have some degree of autogamy and indigenous species were no more likely than naturalized species to display outcrossing mechanisms. On Pohnpei, high ovule numbers, and the inaccessibility of wind pollination and obligate outcrossing strategies, reflect the importance of retaining reproductive assurance mechanisms in the face of pollinator uncertainty.

In situ radiation explains the frequency of dioecious palms on islands

BACKGROUND AND AIMS

Dioecy has evolved up to 5000 times in angiosperms, despite the potentially high intrinsic costs to unisexuality. Dioecy prevents inbreeding, which is especially relevant on isolated islands when gene pools are small. Dioecy is also associated with certain dispersal traits, such as fruit size and type. However, the influence of dioecy on other life history traits and island distribution remains poorly understood. Here, we test the effect of dioecy on palm (Arecaceae) speciation rates, fruit size and frequency on islands.

METHODS

We used phylogenetic comparative methods to estimate the ancestral state of the sexual system and its impact on speciation rates and fruit size. Frequency of sexual systems, effect of insularity on the probability of being dioecious, and phylogenetic clustering of island dioecious vs. mainland species were inferred. Lastly, we determined the interplay of insularity and sexual system on speciation rates.

KEY RESULTS

Palms repeatedly evolved different sexual systems (dioecy, monoecy and polygamy) from a hermaphrodite origin. Differences in speciation rates and fruit size among the different sexual systems were not identified. An effect of islands on the probability of the palms being dioecious was also not found. However, we found a high frequency and phylogenetic clustering of dioecious palms on islands, which were not correlated with higher speciation rates.

CONCLUSIONS

The high frequency and phylogenetic clustering may be the result of in situ radiation and suggest an 'island effect' for dioecious palms, which was not explained by differential speciation rates. This island effect also cannot be attributed to long-distance dispersal due to the lack of fruit size difference among sexual systems, and particularly because palm dispersal to islands is highly constrained by the interaction between the sizes of fruit and frugivores. Taken together, we suggest that trait flexibility in sexual system evolution and the in situ radiation of dioecious lineages are the underlying causes of the outstanding distribution of palms on islands.

Spread of self-compatibility constrained by an intrapopulation crossing barrier

Mating system transitions from self-incompatibility (SI) to self-compatibility (SC) are common in plants. In the absence of high levels of inbreeding depression, SC alleles are predicted to spread due to transmission advantage and reproductive assurance. We characterized mating system and pistil-expressed SI factors in 20 populations of the wild tomato species Solanum habrochaites from the southern half of the species range. We found that a single SI to SC transition is fixed in populations south of the Rio Chillon valley in central Peru. In these populations, SC correlated with the presence of the hab-6 S-haplotype that encodes a low activity S-RNase protein. We identified a single population segregating for SI/SC and hab-6. Intrapopulation crosses showed that hab-6 typically acts in the expected codominant fashion to confer SC. However, we found one specific S-haplotype (hab-10) that consistently rejects pollen of the hab-6 haplotype, and results in SI hab-6/hab-10 heterozygotes. We suggest that the hab-10 haplotype could act as a genetic mechanism to stabilize mixed mating in this population by presenting a disadvantage for the hab-6 haplotype. This barrier may represent a mechanism allowing for the persistence of SI when an SC haplotype appears in or invades a population.

Migration through a Major Andean Ecogeographic Disruption as a Driver of Genetic and Phenotypic Diversity in a Wild Tomato Species

Evolutionary dynamics at the population level play a central role in creating the diversity of life on our planet. In this study, we sought to understand the origins of such population-level variation in mating systems and defensive acylsugar chemistry in Solanum habrochaites—a wild tomato species found in diverse Andean habitats in Ecuador and Peru. Using Restriction-site-Associated-DNA-Sequencing (RAD-seq) of 50 S. habrochaites accessions, we identified eight population clusters generated via isolation and hybridization dynamics of 4–6 ancestral populations. Detailed characterization of mating systems of these clusters revealed emergence of multiple self-compatible (SC) groups from progenitor self-incompatible populations in the northern part of the species range. Emergence of these SC groups was also associated with fixation of deleterious alleles inactivating acylsugar acetylation. The Amotape-Huancabamba Zone—a geographical landmark in the Andes with high endemism and isolated microhabitats—was identified as a major driver of differentiation in the northern species range, whereas large geographical distances contributed to population structure and evolution of a novel SC group in the central and southern parts of the range, where the species was also inferred to have originated. Findings presented here highlight the role of the diverse ecogeography of Peru and Ecuador in generating population differentiation, and enhance our understanding of the microevolutionary processes that create biological diversity.

The rapid dissolution of dioecy by experimental evolution

Evolutionary transitions from hermaphroditism to dioecy have been common in flowering plants,^1,2 but recent analysis also points to frequent reversions from dioecy to hermaphroditism.^2-4 Here, we use experimental evolution to expose a mechanism for such reversions, validating an explanation for the scattered phylogenetic distribution of dioecy. We removed males from dioecious populations of the wind-pollinated plant Mercurialis annua and allowed natural selection to act on the remaining females that occasionally produced male flowers; such "leaky" sex expression is common in both males and females of dioecious plants.⁵ Over the course of four generations, females evolved a 23-fold increase in average male flower production. This phenotypic masculinization of females coincided with the evolution of partial self-fertilization, high average seed set in the continued absence of males, and a capacity to sire progeny when males were re-introduced into their populations. Our study thus validates a mechanism for the rapid dissolution of dioecy and the evolution of functional hermaphroditism under conditions that may frequently occur during periods of low population density, repeated colonization, or range expansion.^6,7 Our results illustrate the power of natural selection, acting in replicated experimental populations, to bring about transitions in the mating behavior of plants.

Biosystematic studies on the status of Solanum chilense

PREMISE

Common taxonomic practices, which condition species' descriptions on diagnostic morphological traits, may systematically lump outcrossing species and unduly split selfing species. Specifically, higher effective population sizes and genetic diversity of obligate outcrossers are expected to result less reliable phenotypic diagnoses. Wild tomatoes, members of Solanum sect. Lycopersicum, are commonly used as a source of exotic germplasm for improvement of the cultivated tomato, and are increasingly employed in basic research. Although the section experienced significant early work, which continues presently, the taxonomic status of many wild species has undergone a number of significant revisions and remains uncertain. Species in this section vary in their breeding systems, notably the expression of self-incompatibility, which determines individual propensity for outcrossing METHODS: Here, we examine the taxonomic status of obligately outcrossing Chilean wild tomato (Solanum chilense) using reduced-representation sequencing (RAD-seq), a range of phylogenetic and population genetic analyses, as well as analyses of crossing and morphological data.

RESULTS

Overall, each of our analyses provides a considerable weight of evidence that the Pacific coastal populations and Andean inland populations of the currently described Solanum chilense represent separately evolving populations, and conceal at least one undescribed cryptic species.

CONCLUSIONS

Despite its vast economic importance, Solanum sect. Lycopersicon still exhibits considerable taxonomic instability. A pattern of under-recognition of outcrossing species may be common, not only in tomatoes, but across flowering plants. We discuss the possible causes and implications of this observation, with a focus on macroevolutionary inference.

Population Genomics of the "Arcanum" Species Group in Wild Tomatoes: Evidence for Separate Origins of Two Self-Compatible Lineages

Given their diverse mating systems and recent divergence, wild tomatoes (Solanum section Lycopersicon) have become an attractive model system to study ecological divergence, the build-up of reproductive barriers, and the causes and consequences of the breakdown of self-incompatibility. Here we report on a lesser-studied group of species known as the "Arcanum" group, comprising the nominal species Solanum arcanum, Solanum chmielewskii, and Solanum neorickii. The latter two taxa are self-compatible but are thought to self-fertilize at different rates, given their distinct manifestations of the morphological "selfing syndrome." Based on experimental crossings and transcriptome sequencing of a total of 39 different genotypes from as many accessions representing each species' geographic range, we provide compelling evidence for deep genealogical divisions within S. arcanum; only the self-incompatible lineage known as "var. marañón" has close genealogical ties to the two self-compatible species. Moreover, there is evidence under multiple inference schemes for different geographic subsets of S. arcanum var. marañón being closest to S. chmielewskii and S. neorickii, respectively. To broadly characterize the population-genomic consequences of these recent mating-system transitions and their associated speciation events, we fit demographic models indicating strong reductions in effective population size, congruent with reduced nucleotide and S-locus diversity in the two independently derived self-compatible species.

Propagule pressure and the establishment of emergent polyploid populations

BACKGROUND

Whereas the incidence or rate of polyploid speciation in flowering plants is modest, the production of polyploid individuals within local populations is widespread. Explanations for this disparity primarily have focused on properties or interactions of polyploids that limit their persistence.

HYPOTHESIS

The emergence of local polyploid populations within diploid populations is similar to the arrival of invasive species at new, suitable sites, with the exception that polyploids suffer interference from their progenitor(s). The most consistent predictor of successful colonization by invasive plants is propagule pressure, i.e. the number of seeds introduced. Therefore, insufficient propagule pressure, i.e. the formation of polyploid seeds within diploid populations, ostensibly is a prime factor limiting the establishment of newly emergent polyploids within local populations. Increasing propagule number reduces the effects of genetic, environmental and demographic stochasticity, which thwart population survival. As with invasive species, insufficient seed production within polyploid populations limits seed export, and thus reduces the chance of polyploid expansion.

CONCLUSION

The extent to which propagule pressure limits the establishment of local polyploid populations remains to be determined, because we know so little. The numbers of auto- or allopolyploid seed in diploid populations rarely have been ascertained, as have the numbers of newly emergent polyploid plants within diploid populations. Moreover, seed production by these polyploids has yet to be assessed.

Enhanced leaky sex expression in response to pollen limitation in the dioecious plant Mercurialis annua

In dioecious plants, males and females frequently show ‘leaky’ sex expression, with individuals occasionally producing flowers of the opposite sex. This leaky sex expression may have enabled the colonization of oceanic islands by dioecious plant species, and it is likely to represent the sort of variation upon which selection acts to bring about evolutionary transitions from dioecy to hermaphroditism. Although leakiness is commonly reported for dioecious species, it is not known whether it has plastic component. The question is interesting because males or females with an ability to enhance their leakiness plastically in the absence of mates would have an advantage of being able to produce progeny by self‐fertilization. Here, we demonstrate that leaky sex expression in the wind‐pollinated dioecious herb Mercurialis annua is plastically responsive to its mating context. We compared experimental populations of females growing either with or without males. Females growing in the absence of males were leakier in their sex expression than controls growing with males, producing more than twice as many male flowers. Our results thus provide a striking instance of plasticity in the reproductive behaviour of plants that is likely adaptive. We consider how females might sense their mating environment as a function of pollen availability, and we discuss possible constraints on the evolution of plasticity in sex expression when the environmental signals that individuals receive are unreliable.

Range-wide Phylogeography of a Nivicolous Protist Didymium nivicola Meyl. (Myxomycetes, Amoebozoa): Striking Contrasts Between the Northern and the Southern Hemisphere

Soil protists play a crucial role in terrestrial ecosystems and often show immense taxonomic diversity. However, for many groups, distribution patterns remain largely unknown. We investigated range-wide intraspecific diversity of a specialized airborne protist (Didymium nivicola Meyl.) that occupies a narrow ecological niche associated with long-lasting snow cover. We sampled 122 collections covering all areas where the species was recorded worldwide. We obtained 105 and 41 sequences of small ribosomal subunit rDNA (SSU) and elongation factor 1-alpha (EF1A), respectively. While the species is very diverse in the austral Andes, Southern Hemisphere (SH; 17 SSU ribotypes and 12 EF1A genotypes identified), its populations are genetically uniform across three continents of the Northern Hemisphere (NH; single ribotype, single genotype). Our results indicate the austral Andes as a possible diversification centre for D. nivicola where populations seem to reproduce sexually. Two main parts of the range display highly contrasting genetic patterns, thus biogeographical history and dynamics. Current distribution of D. nivicola in the NH is likely a result of a dispersal event from the SH and subsequent long-distance dispersal (LDD) that might be associated with a shift to asexual mode of reproduction.

The loss of self-incompatibility in a range expansion

It is commonly observed that plant species' range margins are enriched for increased selfing rates and, in otherwise self-incompatible species, for self-compatibility (SC). This has often been attributed to a response to selection under mate and/or pollinator limitation. However, range expansion can also cause reduced inbreeding depression, and this could facilitate the evolution of selfing in the absence of mate or pollinator limitation. Here, we explore this idea using spatially explicit individual-based simulations of a range expansion, in which inbreeding depression, variation in self-incompatibility (SI), and mate availability evolve. Under a wide range of conditions, the simulated range expansion brought about the evolution of selfing after the loss of SI in range-marginal populations. Under conditions of high recombination between the self-incompatibility locus (S-locus) and viability loci, SC remained marginal in the expanded metapopulation and could not invade the range core, which remained self-incompatible. In contrast, under low recombination and migration rates, SC was frequently able to displace SI in the range core by maintaining its association with a genomic background with purged genetic load. We conclude that the evolution of inbreeding depression during a range expansion promotes the evolution of SC at range margins, especially under high rates of recombination.‬.

Flower morphology is correlated with distribution and phylogeny in Bertolonia (Melastomataceae), an herbaceous genus endemic to the Atlantic Forest

Several centers of endemism have been proposed for Melastomataceae, particularly in Amazonia and Atlantic Forest. Despite the high degree of human-caused degradation in the last 500 years, the Atlantic Forest still presents some of the largest levels of diversity and endemism across all angiosperms. With several recently described species in the last decade, the knowledge on Bertolonia's distribution and morphological characterization has changed, with most new species found in northern Atlantic Forest and with different flower color patterns than the species from southern Atlantic Forest. We first tested the monophyly of the genus sampling over 85% of its species to generate a reliable phylogenetic hypothesis. Afterwards, we used Bertolonia as a model group to study distribution patterns and morphological evolution of lineages in the Atlantic Forest. Bertolonia is particularly interesting to address such questions because it is endemic to this domain, with species distributed either in the southern, central or northern portions of the Atlantic Forest. The second step of our work aimed to respond (1) Do closely related species endemic to Atlantic Forest tend to have similar distributions and share similar morphological traits? and (2) Are floral traits more conserved within clades than vegetative characters? We hypothesize that both (1) and (2) are true due to the low dispersal ability and consequent microendemic distribution of most species in the genus. Our results confirm the veracity of the proposed hypotheses. Three major groups were recovered in our analysis: marmorata, formosa and nymphaeifolia clades. Most species that occur in northern Atlantic Forest were recovered in the marmorata clade. Most species distributed in central Atlantic Forest were recovered in the formosa clade, and the ones from the southern Atlantic Forest in the nymphaeifolia clade. A similar pattern was recovered with reproductive characters reconstructed across the phylogeny. Generally, species with pink petals and cream-colored anthers with an extrorse pore occur in northern Atlantic Forest, and species with white petals and yellow anthers with an introrse or apical pore tend to occur in southern Atlantic Forest. Some vegetative characters also have evolutionary congruence and are restricted to one or few lineages, while other characters, such as the texture of the leaf blade surface, have a strong taxonomic value and are useful to identify species, but are not homologous. Our analysis also indicates that the division between southern and northern Atlantic Forest could also be related to ancient events, not only linked with recent phylogeographic patterns. Moreover, we suggest that the orientation of the anther pore in Bertolonia could be related with diversity of species and stability of its populations. In summary, we corroborate, based on the evolutionary history of Bertolonia, that closely related species endemic to Atlantic Forest tend to have similar distributions and share similar morphological floral traits.

Coloniality, clonality, and modularity in animals: The elephant in the room

Nearly half of the animal phyla contain species that propagate asexually via agametic reproduction, often forming colonies of genetically identical modules, that is, ramets, zooids, or polyps. Clonal reproduction, colony formation, and modular organization have important consequences for many aspects of organismal biology. Theories in ecology, evolution, and development are often based on unitary and, mainly, strictly sexually reproducing organisms, and though colonial animals dominate many marine ecosystems and habitats, recognized concepts for the study of clonal species are often lacking. In this review, we present an overview of the study of colonial and clonal animals, from the historic interests in this subject to modern research in a range of topics, including immunology, stem cell biology, aging, biogeography, and ecology. We attempt to portray the fundamental questions lying behind the biology of colonial animals, focusing on how colonial animals challenge several dogmas in biology as well as the remaining puzzles still to be answered, of which there are many.

A genome-wide investigation of the worldwide invader Sargassum muticum shows high success albeit (almost) no genetic diversity

Twenty years of genetic studies of marine invaders have shown that successful invaders are often characterized by native and introduced populations displaying similar levels of genetic diversity. This pattern is presumably due to high propagule pressure and repeated introductions. The opposite pattern is reported in this study of the brown seaweed, Sargassum muticum, an emblematic species for circumglobal invasions. Albeit demonstrating polymorphism in the native range, microsatellites failed to detect any genetic variation over 1,269 individuals sampled from 46 locations over the Pacific-Atlantic introduction range. Single-nucleotide polymorphisms (SNPs) obtained from ddRAD sequencing revealed some genetic variation, but confirmed severe founder events in both the Pacific and Atlantic introduction ranges. Our study thus exemplifies the need for extreme caution in interpreting neutral genetic diversity as a proxy for invasive potential. Our results confirm a previously hypothesized transoceanic secondary introduction from NE Pacific to Europe. However, the SNP panel unexpectedly revealed two additional distinct genetic origins of introductions. Also, conversely to scenarios based on historical records, southern rather than northern NE Pacific populations could have seeded most of the European populations. Finally, the most recently introduced populations showed the lowest selfing rates, suggesting higher levels of recombination might be beneficial at the early stage of the introduction process (i.e., facilitating evolutionary novelties), whereas uniparental reproduction might be favored later in sustainably established populations (i.e., sustaining local adaptation).

What's ploidy got to do with it? Understanding the evolutionary ecology of macroalgal invasions necessitates incorporating life cycle complexity

Biological invasions represent grave threats to terrestrial, aquatic, and marine ecosystems, but our understanding of the role of evolution during invasions remains rudimentary. In marine environments, macroalgae account for a large percentage of invaders, but their complicated life cycles render it difficult to move methodologies and predictions wholesale from species with a single, free-living ploidy stage, such as plants or animals. In haplodiplontic macroalgae, meiosis and fertilization are spatiotemporally separated by long-lived, multicellular haploid and diploid stages, and gametes are produced by mitosis, not meiosis. As a consequence, there are unique eco-evolutionary constraints that are not typically considered in invasions. First, selfing can occur in both monoicious (i.e., hermaphroditic) and dioicious (i.e., separate sexes) haplodiplontic macroalgae. In the former, fertilization between gametes produced by the same haploid thallus results in instantaneous, genome-wide homozygosity. In the latter, cross-fertilization between separate male and female haploids that share the same diploid parent is analogous to selfing in plants or animals. Separate sexes, therefore, cannot be used as a proxy for outcrossing. Second, selfing likely facilitates invasions (i.e., Baker's law) and the long-lived haploid stage may enable purging of deleterious mutations, further contributing to invasion success. Third, asexual reproduction will result in the dominance of one ploidy and/or sex and the loss of the other(s). Whether or not sexual reproduction can be recovered depends on which stage is maintained. Finally, fourth, haplodiplontic life cycles are predicted to be maintained through niche differentiation in the haploid and diploid stages. Empirical tests are rare, but fundamental to our understanding of macroalgal invasion dynamics. By highlighting these four phenomena, we can build a framework with which to empirically and theoretically address important gaps in the literature on marine evolutionary ecology, of which biological invasions can serve as unnatural laboratories.

Asexual parasites and their extraordinary host ranges

In diverse parasite taxa, from scale insects to root-knot nematodes, asexual lineages have exceptionally large host ranges, larger than those of their sexual relatives. Phylogenetic comparative studies of parasite taxa indicate that increases in host range and geographic range increase the probability of establishment of asexual lineages. At first pass, this convergence of traits appears counter-intuitive: intimate, antagonistic association with an enormous range of host taxa correlates with asexual reproduction, which should limit genetic variation within populations. Why would narrow host ranges favor sexual parasites and large host ranges favor asexual parasites? To take on this problem I link theory on ecological specialization to the two predominant hypotheses for the evolution of sex. I argue that both hypotheses predict a positive association between host range and the probability of invasion of asexual parasites, mediated either by variation in population size or in the strength of antagonistic coevolution. I also review hypotheses on colonization and the evolution of niche breadth in asexual lineages. I emphasize parasite taxa, with their diversity of reproductive modes and ecological strategies, as valuable assets in the hunt for solutions to the classic problems of the evolution of sex and geographic parthenogenesis.

Do selfing species have greater niche breadth? Support from ecological niche modeling

We explore the relationship between plant mating system (selfing or outcrossing) and niche breadth to gain new insights into processes that drive species distributions. Using a comparative approach with highly selfing versus highly outcrossing sister species, we test the extent to which: (1) species pairs have evolved significant niche divergence and less niche overlap, (2) selfers have wider niche breadths than outcrossers or vice versa, and (3) niches of selfers and outcrossers are defined by significant differences in environmental variables. We applied predictive ecological niche modeling approaches to estimate and contrast niche divergence, overlap and breadth, and to identify key environmental variables associated with each species' niche for seven sister species with divergent mating systems. Data from 4862 geo-referenced herbarium occurrence records were compiled for 14 species in Collinsia and Tonella (Plantaginaceae) and 19 environmental variables associated with each record. We found sister species display significant niche divergence, though not as a function of divergence time, and overall, selfers have significantly wider niche breadths compared to their outcrossing sisters. Our results suggest that a selfing mating system likely contributes to the greater capacity to reach, reproduce, establish, and adapt to new habitats, which increases niche breadth of selfers.

Reproductive transitions in plants and animals: selfing syndrome, sexual selection and speciation

The evolution of predominant self-fertilisation frequently coincides with the evolution of a collection of phenotypes that comprise the 'selfing syndrome', in both plants and animals. Genomic features also display a selfing syndrome. Selfing syndrome traits often involve changes to male and female reproductive characters that were subject to sexual selection and sexual conflict in the obligatorily outcrossing ancestor, including the gametic phase for both plants and animals. Rapid evolution of reproductive traits, due to both relaxed selection and directional selection under the new status of predominant selfing, lays the genetic groundwork for reproductive isolation. Consequently, shifts in sexual selection pressures coupled to transitions to selfing provide a powerful paradigm for investigating the speciation process. Plant and animal studies, however, emphasise distinct selective forces influencing reproductive-mode transitions: genetic transmission advantage to selfing or reproductive assurance outweighing the costs of inbreeding depression vs the costs of males and meiosis. Here, I synthesise links between sexual selection, evolution of selfing and speciation, with particular focus on identifying commonalities and differences between plant and animal systems and pointing to areas warranting further synergy.

Evolutionary history of the buildup and breakdown of the heterostylous syndrome in Plumbaginaceae

The evolutionary pathways leading to the heterostylous syndrome are not well understood, and models concerning the origins of distyly differ in the order in which reciprocal herkogamy and self-incompatibility evolve. We investigated the evolution and breakdown of distyly in Plumbaginaceae, a family with considerable diversity of floral traits and reproductive systems. Using Bayesian Markov chain Monte Carlo analyses and stochastic character mapping, we examined the evolutionary assembly and breakdown of the heterostylous syndrome based on a well-resolved phylogeny of 121 species of Plumbaginaceae and six outgroup taxa using five nuclear and plastid gene regions. We used the distribution of reproductive traits and reconstructed ancestral characters across phylogenies to evaluate competing models for the evolution of distyly. The most likely common ancestor of Plumbaginaceae was self-incompatible and monomorphic for sex-organ arrangement and pollen-stigma characters. Character state reconstructions indicated that reciprocal herkogamy evolved at least three times and that shifts to selfing and apomixis occurred on multiple occasions. Our results provide comparative support for the early ideas of H. G. Baker on evolutionary pathways in Plumbaginaceae, and the more recent selfing avoidance model by D. & B. Charlesworth in which distyly evolves from self-incompatible ancestors.