Understanding the role of ecological divergence in the evolution of isolated populations in the Arremonops rufivirgatus species complex across Mesoamerica

BACKGROUND

The topographic complexity and wide range of environmental conditions of the Neotropical region have allowed the evolution of the most diverse avifauna in the world. Distributional patterns within this avian diversity mirror this complexity, and many species show allopatric distributions in environmentally continuous regions. Here, we used environmental variables and historical presence records to understand the evolution of the distribution of three isolated groups (Gulf, Pacific, and Yucatan Peninsula) of the Olive Sparrow (Arremonops rufivirgatus) species complex. We assessed the role of environmental factors underlying geographic distribution patterns in the complex based on ecological niche modeling and performed paleoclimatic reconstructions to assess distributional changes based on suitable areas during the Late Pleistocene.

RESULTS

Niche similarity was not rejected in the Pacific/Yucatan comparison, but the Gulf/Pacific and Gulf/Yucatan comparisons showed niche differentiation. We found regions with low climatic suitability representing a biogeographic barrier for the Pacific and the Yucatan groups, but not for the Yucatan and the Gulf groups, suggesting that biotic factors, such as competition with ecologically similar species, may be involved in geographic isolation.

CONCLUSIONS

Our results suggest that allopatric distributions in the three groups within the A. rufivirgatus complex probably evolved due to biotic interactions with ecologically similar species in the relatively environmentally continuous areas across the Gulf Slope, but to range contractions leading to isolation in the Yucatan and the Pacific groups.

Uncovering the genetic architecture of parallel evolution

Identifying the genetic architecture underlying adaptive traits is exceptionally challenging in natural populations. This is because associations between traits not only mask the targets of selection but also create correlated patterns of genomic divergence that hinder our ability to isolate causal genetic effects. Here, we examine the repeated evolution of components of the auxin pathway that have contributed to the replicated loss of gravitropism (i.e. the ability of a plant to bend in response to gravity) in multiple populations of the Senecio lautus species complex in Australia. We use a powerful approach which combines parallel population genomics with association mapping in a Multiparent Advanced Generation Inter-Cross (MAGIC) population to break down genetic and trait correlations to reveal how adaptive traits evolve during replicated evolution. We sequenced auxin and shoot gravitropism-related gene regions in 80 individuals from six natural populations (three parallel divergence events) and 133 individuals from a MAGIC population derived from two of the recently diverged natural populations. We show that artificial tail selection on gravitropism in the MAGIC population recreates patterns of parallel divergence in the auxin pathway in the natural populations. We reveal a set of 55 auxin gene regions that have evolved repeatedly during the evolution of the species, of which 50 are directly associated with gravitropism divergence in the MAGIC population. Our work creates a strong link between patterns of genomic divergence and trait variation contributing to replicated evolution by natural selection, paving the way to understand the origin and maintenance of adaptations in natural populations.

Gene expression and genetic divergence in oak species highlight adaptive genes to soil water constraints

Drought and waterlogging impede tree growth and may even lead to tree death. Oaks, an emblematic group of tree species, have evolved a range of adaptations to cope with these constraints. The two most widely distributed European species, pedunculate (PO; Quercus robur L.) and sessile oak (SO; Quercus petraea Matt. Lieb), have overlapping ranges, but their respective distribution are highly constrained by local soil conditions. These contrasting ecological preferences between two closely related and frequently hybridizing species constitute a powerful model to explore the functional bases of the adaptive responses in oak. We exposed oak seedlings to waterlogging and drought, conditions typically encountered by the two species in their respective habitats, and studied changes in gene expression in roots using RNA-seq. We identified genes that change in expression between treatments differentially depending on species. These "species × environment"-responsive genes revealed adaptive molecular strategies involving adventitious and lateral root formation, aerenchyma formation in PO, and osmoregulation and ABA regulation in SO. With this experimental design, we also identified genes with different expression between species independently of water conditions imposed. Surprisingly, this category included genes with functions consistent with a role in intrinsic reproductive barriers. Finally, we compared our findings with those for a genome scan of species divergence and found that the expressional candidate genes included numerous highly differentiated genetic markers between the two species. By combining transcriptomic analysis, gene annotation, pathway analyses, as well as genome scan for genetic differentiation among species, we were able to highlight loci likely involved in adaptation of the two species to their respective ecological niches.

Evolutionary and Ecological Drivers of Local Adaptation and Speciation in a North American Avian Species Complex

Throughout the speciation process, genomic divergence can be differentially impacted by selective pressures, as well as gene flow and genetic drift. Disentangling the effects of these evolutionary mechanisms remains challenging, especially for non-model organisms. Accounting for complex evolutionary histories and contemporary population structure often requires sufficient sample sizes, for which the expense of full genomes remains prohibitive. Here, we demonstrate the utility of partial-genome sequence data for range-wide samples to shed light into the divergence process of two closely related ducks, the Mexican duck (Anas diazi) and mallard (A. platyrhynchos). We determine the role of selective and neutral processes during speciation of Mexican ducks by integrating evolutionary and demographic modelling with genotype-environment and genotype-phenotype association testing. First, evolutionary models and demographic analyses support the hypothesis that Mexican ducks originally diverged ~300,000 years ago in a climate refugia arising during a glacial period in in a southwestern North America, and that subsequent environmental selective pressures played a key role in divergence. Mexican ducks then showed cyclical demographic patterns that likely reflected repeated range expansions and contractions, along with bouts of gene flow with mallards during glacial cycles. Finally, we provide evidence that sexual selection acted on several phenotypic traits as a co-evolutionary process, facilitating the development of reproductive barriers that initially arose due to strong ecological selection. More broadly, this work reveals that the genomic and phenotypic patterns observed across species complexes are the result of myriad factors that contribute in dynamic ways to the evolutionary trajectories of a lineage.

Behavioral, morphological, and ecological trait evolution in two clades of New World Sparrows (Aimophila and Peucaea, Passerellidae)

The New World sparrows (Passerellidae) are a large, diverse group of songbirds that vary in morphology, behavior, and ecology. Thus, they are excellent for studying trait evolution in a phylogenetic framework. We examined lability versus conservatism in morphological and behavioral traits in two related clades of sparrows (Aimophila, Peucaea), and assessed whether habitat has played an important role in trait evolution. We first inferred a multi-locus phylogeny which we used to reconstruct ancestral states, and then quantified phylogenetic signal among morphological and behavioral traits in these clades and in New World sparrows more broadly. Behavioral traits have a stronger phylogenetic signal than morphological traits. Specifically, vocal duets and song structure are the most highly conserved traits, and nesting behavior appears to be maintained within clades. Furthermore, we found a strong correlation between open habitat and unpatterned plumage, complex song, and ground nesting. However, even within lineages that share the same habitat type, species vary in nesting, plumage pattern, song complexity, and duetting. Our findings highlight trade-offs between behavior, morphology, and ecology in sparrow diversification.

Genetic and morphological variation of metacercariae of Microphallus piriformes (Trematoda, Microphallidae): Effects of paraxenia and geographic location

Host organism offers an environment for a parasite, and this environment is heterogenous within the host, variable among individual as well as between the hosts, and changing during the host's lifetime. This heterogeneity may act as a prerequisite for parasite species divergence. Intraspecific variability related to a certain type of heterogeneity may indicate an initial stage of speciation, and thus poses an evolutionary importance. Here we analyzed genetic and morphologic variation of trematode metacercariae of Microphallus piriformes (Trematoda, Microphallidae). Genetic variability of trematodes was assessed from sequences of cytochrome c oxidase subunit 1 (COI) and internal transcribed spacer region (ITS-1). Morphological variation of metacercarial body shape was for the first time analyzed using geometric morphometrics. Parasites from the White Sea and the Barents Sea coasts demonstrated partial genetic divergence (according to COI sequence analysis) and had significantly different body shape. Neither genetic nor morphological variation of metacercariae was related to intermediate host species. We discuss possible causes of the observed genetic divergence of parasite populations in different geographic regions.

Insecticide resistance and fitness cost in Bemisia tabaci (Hemiptera: Aleyrodidae) invasive and resident species in La Réunion Island

BACKGROUND

Global and intensive use of insecticides has led to the emergence and rapid evolution of resistance in the major pest Bemisia tabaci (Gennadius). In La Réunion, an island of the South West Indian Ocean, three whitefly species coexist, two of which are predominant, the indigenous Indian Ocean (IO) and the invasive Middle East Asia Minor 1 (MEAM1) species. To assess the resistance level of both of these species to acetamiprid and pymetrozine, whitefly populations were sampled at 15 collection sites located all over the island in agroecosystems and natural areas, and tested using leaf-dip bioassays. We also investigated the potential cost of resistance to acetamiprid by measuring six fitness-related traits for MEAM1 populations that displayed different resistance levels.

RESULTS

IO was mainly found in natural areas and was susceptible to both acetamiprid and pymetrozine. MEAM1 populations displayed evidence of high resistance to pymetrozine, whereas resistance to acetamiprid was more variable. No fitness-related costs were associated with this resistance in MEAM1 populations.

CONCLUSION

This is the first assessment of the susceptibility to insecticides for B. tabaci IO species. For the time being, no resistance to the tested insecticides has evolved in this species despite (i) its presence in agroecosystems and their surroundings, and (ii) its close proximity to, and possible hybridization with, the MEAM1 species. In contrast, with continuous selection pressure of insecticide treatments and in the absence of fitness cost to resistance, the invasive exotic species MEAM1 will continue to threaten agriculture in La Réunion. © 2019 Society of Chemical Industry.

Nonlinear frequency-dependent selection promotes long-term coexistence between bacteria species

Negative frequency-dependent selection (NFDS) is an important mechanism for species coexistence and for the maintenance of genetic polymorphism. Long-term coexistence nevertheless requires NFDS interactions to be resilient to further evolution of the interacting species or genotypes. For closely related genotypes, NFDS interactions have been shown to be preserved through successive rounds of evolution in coexisting lineages. On the contrary, the evolution of NFDS interactions between distantly related species has received less attention. Here, we tracked the co-evolution of Escherichia coli and Citrobacter freundii that initially differ in their ecological characteristics. We showed that these two bacterial species engaged in an NFDS interaction particularly resilient to further evolution: despite a very strong asymmetric rate of adaptation, their coexistence was maintained owing to an NFDS pattern where fitness increases steeply as the frequency decreases towards zero. Using a model, we showed how and why such NFDS pattern can emerge. These findings provide a robust explanation for the long-term maintenance of species at very low frequencies.

Genomic mechanisms of sympatric ecological and sexual divergence in a model agricultural pest, the European corn borer

The European corn borer, Ostrinia nubilalis, is a model species for elucidating mechanisms underlying adaptively differentiated subpopulations in the face of reciprocal gene flow, and is a major pest of cultivated maize in North America and Eurasia. Strains are characterized by different pheromone communication systems in combination with voltinism strains that are adapted to distinct local climate and photoperiod through adjustments in diapause traits. However, only partial barriers to inter-strain hybridization exist in areas of sympatry. Recent research shows that genes governing important strain-specific isolating traits are disproportionately located on the Z-chromosome. Furthermore, co-adapted combinations of some of these genes are non-recombining due to location within a large chromosomal inversion, and assist in maintaining strain integrity despite hybridization.

The evolution of recombination rates in finite populations during ecological speciation

Recombination can impede ecological speciation with gene flow by mixing locally adapted genotypes with maladapted migrant genotypes from a divergent population. In such a scenario, suppression of recombination can be selectively favoured. However, in finite populations evolving under the influence of random genetic drift, recombination can also facilitate adaptation by reducing Hill-Robertson interference between loci under selection. In this case, increased recombination rates can be favoured. Although these two major effects on recombination have been studied individually, their joint effect on ecological speciation with gene flow remains unexplored. Using a mathematical model, we investigated the evolution of recombination rates in two finite populations that exchange migrants while adapting to contrasting environments. Our results indicate a two-step dynamic where increased recombination is first favoured (in response to the Hill-Robertson effect), and then disfavoured, as the cost of recombining locally with maladapted migrant genotypes increases over time (the maladaptive gene flow effect). In larger populations, a stronger initial benefit for recombination was observed, whereas high migration rates intensify the long-term cost of recombination. These dynamics may have important implications for our understanding of the conditions that facilitate incipient speciation with gene flow and the evolution of recombination in finite populations.

X exceptionalism in Caenorhabditis speciation

Speciation genetics research in diverse organisms shows the X-chromosome to be exceptional in how it contributes to "rules" of speciation. Until recently, however, the nematode phylum has been nearly silent on this issue, despite the model organism Caenorhabditis elegans having touched most other topics in biology. Studies of speciation with Caenorhabditis accelerated with the recent discovery of species pairs showing partial interfertility. The resulting genetic analyses of reproductive isolation in nematodes demonstrate key roles for the X-chromosome in hybrid male sterility and inviability, opening up new understanding of the genetic causes of Haldane's rule, Darwin's corollary to Haldane's rule, and enabling tests of the large-X effect hypothesis. Studies to date implicate improper chromatin regulation of the X-chromosome by small RNA pathways as integral to hybrid male dysfunction. Sexual transitions in reproductive mode to self-fertilizing hermaphroditism inject distinctive molecular evolutionary features into the speciation process for some species. Caenorhabditis also provides unique opportunities for analysis in a system with XO sex determination that lacks a Y-chromosome, sex chromosome-dependent sperm competition differences and mechanisms of gametic isolation, exceptional accessibility to the development process and rapid experimental evolution. As genetic analysis of reproductive isolation matures with investigation of multiple pairs of Caenorhabditis species and new species discovery, nematodes will provide a powerful complement to more established study organisms for deciphering the genetic basis of and rules to speciation.

Spatiotemporal heterogeneity in precipitation patterns explain population-level germination strategies in an edaphic specialist

BACKGROUND AND AIMS

Many locally endemic species in biodiversity hotspots are restricted to edaphic conditions that are fixed in the landscape, limiting their potential to track climate change through dispersal. Instead, such species experience strong selection for germination strategies that can track suitable conditions through time. Germination strategies were compared among populations across the geographic range of a California vernal pool annual, Lasthenia fremontii Local germination strategies were tested to determine the associations with geographic variation in precipitation patterns.

METHODS

This study evaluated patterns of seed germination, dormancy and mortality in response to simulated variation in the timing, amount and duration of the first autumn precipitation event using seeds from six populations that span a geographic gradient in precipitation. Next, it was tested whether the germination strategies of different populations can be predicted by historical precipitation patterns that characterize each site.

KEY RESULTS

A significant positive relationship was observed between the historical variability in autumn precipitation and the extent of dormancy in a population. Marginal populations, with histories of the most extreme but constant autumn precipitation levels, expressed the lowest dormancy levels. Populations from sites with historically higher levels of autumn precipitation tended to germinate faster, but this tendency was not statistically significant.

CONCLUSIONS

Germination in L. fremontii is cued by the onset of the first rains that characterize the beginning of winter in California's Great Central Valley. However, populations differ in how fast they germinate and the fraction of seeds that remain dormant when germination cues occur. The results suggest that seed dormancy may be a key trait for populations to track increasingly drier climates predicted by climate change models. However, the low dormancy and high mortality levels observed among seeds of the southernmost, driest populations make them most vulnerable to local extinction.

Adaptive signal coloration maintained in the face of gene flow in a Hispaniolan Anolis Lizard

Background

Studies of geographic variation can provide insight into the evolutionary processes involved in the early stages of biological diversification. In particular, multiple, replicated cases of geographic trait divergence present a powerful approach to study how patterns of introgression and adaptive divergence can vary with geographic space and time. In this study, we conduct replicated, fine-scaled molecular genetic analyses of striking geographic dewlap color variation of a Hispaniolan Anolis lizard, Anolis distichus, to investigate whether adaptive trait divergence is consistently associated with speciation, whereby genetic divergence is observed with neutral markers, or whether locally adapted traits are maintained in the face of continued gene flow.

Results

We find instances where shifts in adaptive dewlap coloration across short geographic distances are associated with reproductive isolation as well as maintained in the face of gene flow, suggesting the importance of both processes in maintaining geographic dewlap variation.

Conclusion

Our study suggests that adaptive dewlap color differences are maintained under strong divergent natural selection, but this divergence does not necessarily lead to anole speciation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0763-4) contains supplementary material, which is available to authorized users.

Post-glacial habitat release and incipient speciation in the genus Delphinus

The role of ecological and changing environmental factors in the radiation of species diversity is a fundamental question in evolutionary biology. Of particular interest is the potential for these factors to determine the boundary between what we would consider differentiation among populations and incipient speciation. Dolphins in the genus Delphinus provide a useful test case, exhibiting morphological variation in beak length, coloration and body size across their wide geographic distribution, and in particular among coastal and more pelagic habitats. Two species have been proposed, D. delphis and D. capensis, but morphologically similar allopatric populations are not monophyletic, indicating that the mostly coastal 'long-beaked' D. capensis form is not a single globally distributed species. However, the sympatric populations in the Eastern North Pacific currently designated as these two species are both morphologically and genetically differentiated. Here we use microsatellite DNA and mitochondrial DNA markers to investigate the evolutionary mechanisms that led to this incipient speciation event. We used coalescent and assignment methods to investigate the timing and extent of reproductive isolation. Our data indicate that although there is some level of on-going gene flow, the putative species found in the Eastern North Pacific are reciprocally monophyletic. The timing of isolation appears to be associated with regional changes in paleoceanographic conditions within the Holocene timeframe.

Speciation in peripheral populations: effects of drift load and mating systems

Speciation in peripheral populations has long been considered one of the most plausible scenarios for speciation with gene flow. In this study, however we identify two additional problems of peripatric speciation, as compared to the parapatric case, that may impede the completion of the speciation process for most parameter regions. First, with (predominantly) unidirectional gene flow, there is no selection pressure to evolve assortative mating on the continent. We discuss the implications of this for different mating schemes. Second, genetic load can build up in small populations. This can lead to extinction of the peripheral species, or generate selection pressure for lower assortative mating to avoid inbreeding. In this case, either a stable equilibrium with intermediate assortment evolves or there is cycling between phases of hybridization and phases of complete isolation.

Recombination Rate Evolution and the Origin of Species

A recipe for dissolving incipient species into a continuum of phenotypes is to recombine their genetic material. Therefore, students of speciation have become increasingly interested in the mechanisms by which recombination between locally adapted lineages is reduced. Evidence abounds that chromosomal rearrangements, via their suppression of recombination during meiosis in hybrids, play a major role in adaptation and speciation. By contrast, genic modifiers of recombination rates have been largely ignored in studies of speciation. We show how both types of reduction in recombination rates facilitate divergence in the face of gene flow, including the early stages of adaptive divergence, the persistence of species after secondary contact, and reinforcement.

'Becoming a species by becoming a pest' or how two maize pests of the genus Ostrinia possibly evolved through parallel ecological speciation events

New agricultural pest species attacking introduced crops may evolve from pre-existing local herbivores by ecological speciation, thereby becoming a species by becoming a pest. We compare the evolutionary pathways by which two maize pests (the Asian and the European corn borers, ACB and ECB) in the genus Ostrinia (Lepidoptera, Crambidae) probably diverged from an ancestral species close to the current Adzuki bean borer (ABB). We typed larval Ostrinia populations collected on maize and dicotyledons across China and eastern Siberia, at microsatellite and mitochondrial loci. We found only two clusters: one on maize (as expected) and a single one on dicotyledons despite differences in male mid-tibia morphology, suggesting that all individuals from dicotyledons belonged to the ABB. We found evidence for migrants and hybrids on both host plant types. Hybrids suggest that field reproductive isolation is incomplete between ACB and ABB. Interestingly, a few individuals with an 'ABB-like' microsatellite profile collected on dicotyledons had 'ACB' mtDNA rather than 'ABB-like' mtDNA, whereas the reverse was never found on maize. This suggests asymmetrical gene flow directed from the ACB towards the ABB. Hybrids and backcrosses in all directions were obtained in no-choice tests. In laboratory conditions, they survived as well as parental strain individuals. In Xinjiang, we found ACB and ECB in sympatry, but no hybrids. Altogether, our results suggest that reproductive isolation between ACB and ABB is incomplete and mostly prezygotic. This points to ecological speciation as a possible evolutionary scenario, as previously found for ECB and ABB in Europe.

Caenorhabditis evolution in the wild

Recent research has filled many gaps about Caenorhabditis natural history, simultaneously exposing how much remains to be discovered. This awareness now provides means of connecting ecological and evolutionary theory with diverse biological patterns within and among species in terms of adaptation, sexual selection, breeding systems, speciation, and other phenomena. Moreover, the heralded laboratory tractability of C. elegans, and Caenorhabditis species generally, provides a powerful case study for experimental hypothesis testing about evolutionary and ecological processes to levels of detail unparalleled by most study systems. Here, I synthesize pertinent theory with what we know and suspect about Caenorhabditis natural history for salient features of biodiversity, phenotypes, population dynamics, and interactions within and between species. I identify topics of pressing concern to advance Caenorhabditis biology and to study general evolutionary processes, including the key opportunities to tackle problems in dispersal dynamics, competition, and the dimensionality of niche space.

The bud midge Prodiplosis longifila: Damage characteristics, potential distribution and presence on a new crop host in Colombia

The Dipteran Prodiplosis longifila is a severe pest, mainly of Solanaceae, in South America and some years ago it damaged Tahiti lime crops in the United States. It is a potential invasive pest. Despite its presence in Colombia, nothing is known regarding the taxonomic identification of P. longifila or the characteristics of the damage it produces. Moreover, the current and potential distributions of this pest are unknown. To determine these factors, P. longifila was sampled in several Solanaceae- and Citrus (x) latifolia (Tahiti lime)-producing areas in Colombia. The larvae consumed tender foliage, flowers and fruits in tomato, fruits in sweet pepper, and buds in Tahiti lime. P. longifila was not found in asparagus or in potatoes. Its presence in Tahiti lime was previously unknown in Colombia. Adults recovered in the laboratory were taxonomically identified using male morphological characteristics such as the shapes of the genitalia, antenna and wing. P. longifila was found in the Andean region of Colombia. The ecological niche model for populations found in tomato suggests that P. longifila is limited in its distribution by altitude and variables associated with temperature and precipitation. The highest probability of occurrence is in areas where tomato, sweet pepper and the new host, Tahiti lime, are grown. Therefore, it is necessary to implement preventive measures, such as planting tomato materials free of P. longifila larvae, in areas where the pest is not yet present but where there is the potential for its development.

Gradient evolution of body colouration in surface- and cave-dwelling Poecilia mexicana and the role of phenotype-assortative female mate choice

Ecological speciation assumes reproductive isolation to be the product of ecologically based divergent selection. Beside natural selection, sexual selection via phenotype-assortative mating is thought to promote reproductive isolation. Using the neotropical fish Poecilia mexicana from a system that has been described to undergo incipient ecological speciation in adjacent, but ecologically divergent habitats characterized by the presence or absence of toxic H2S and darkness in cave habitats, we demonstrate a gradual change in male body colouration along the gradient of light/darkness, including a reduction of ornaments that are under both inter- and intrasexual selection in surface populations. In dichotomous choice tests using video-animated stimuli, we found surface females to prefer males from their own population over the cave phenotype. However, female cave fish, observed on site via infrared techniques, preferred to associate with surface males rather than size-matched cave males, likely reflecting the female preference for better-nourished (in this case: surface) males. Hence, divergent selection on body colouration indeed translates into phenotype-assortative mating in the surface ecotype, by selecting against potential migrant males. Female cave fish, by contrast, do not have a preference for the resident male phenotype, identifying natural selection against migrants imposed by the cave environment as the major driver of the observed reproductive isolation.

Evolutionary consequences of self-fertilization in plants

The transition from outcrossing to self-fertilization is one of the most common evolutionary changes in plants, yet only about 10-15% of flowering plants are predominantly selfing. To explain this phenomenon, Stebbins proposed that selfing may be an 'evolutionary dead end'. According to this hypothesis, transitions from outcrossing to selfing are irreversible, and selfing lineages suffer from an increased risk of extinction owing to a reduced potential for adaptation. Thus, although selfing can be advantageous in the short term, selfing lineages may be mostly short-lived owing to higher extinction rates. Here, we review recent results relevant to the 'dead-end hypothesis' of selfing and the maintenance of outcrossing over longer evolutionary time periods. In particular, we highlight recent results regarding diversification rates in self-incompatible and self-compatible taxa, and review evidence regarding the accumulation of deleterious mutations in selfing lineages. We conclude that while some aspects of the hypothesis of selfing as a dead end are supported by theory and empirical results, the evolutionary and ecological mechanisms remain unclear. We highlight the need for more studies on the effects of quantitative changes in outcrossing rates and on the potential for adaptation, particularly in selfing plants. In addition, there is growing evidence that transitions to selfing may themselves be drivers of speciation, and future studies of diversification and speciation should investigate this further.

Genetic differentiation and selection against migrants in evolutionarily replicated extreme environments

We investigated mechanisms of reproductive isolation in livebearing fishes (genus Poecilia) inhabiting sulfidic and nonsulfidic habitats in three replicate river drainages. Although sulfide spring fish convergently evolved divergent phenotypes, it was unclear if mechanisms of reproductive isolation also evolved convergently. Using microsatellites, we found strongly reduced gene flow between adjacent populations from different habitat types, suggesting that local adaptation to sulfidic habitats repeatedly caused the emergence of reproductive isolation. Reciprocal translocation experiments indicate strong selection against immigrants into sulfidic waters, but also variation among drainages in the strength of selection against immigrants into nonsulfidic waters. Mate choice experiments revealed the evolution of assortative mating preferences in females from nonsulfidic but not from sulfidic habitats. The inferred strength of sexual selection against immigrants (RI(s)) was negatively correlated with the strength of natural selection (RI(m)), a pattern that could be attributed to reinforcement, whereby natural selection strengthens behavioral isolation due to reduced hybrid fitness. Overall, reproductive isolation and genetic differentiation appear to be replicated and direct consequences of local adaptation to sulfide spring environments, but the relative contributions of different mechanisms of reproductive isolation vary across these evolutionarily independent replicates, highlighting both convergent and nonconvergent evolutionary trajectories of populations in each drainage.