Exogenous selection shapes germination behaviour and seedling traits of populations at different altitudes in a Senecio hybrid zone

Evidence of hybrid breakdown among invasive hybrid cattails (Typha × glauca)

Interspecific hybridization has varied consequences for offspring fitness, with implications for the maintenance of species integrity. Hybrid vigour, when it occurs, can peak in first-generation (F1) hybrids and then decline in advanced-generation (F2+) hybrids. This hybrid breakdown, together with the processes affecting patterns of hybridization and hybrid fitness, determine the evolutionary stability of hybrid zones. An extensive hybrid zone in North America involving the cattails Typha latifolia, T. angustifolia, and their invasive hybrid T. × glauca is characterized by hybrid vigour among F1s, but the fitness of advanced-generation hybrids has not been studied. We compared seed germination and plant growth of T. latifolia (parental L), F1 T. × glauca (F1), hybrid backcrosses to T. angustifolia (bcA) and T. latifolia (bcL), and advanced-generation (F2) hybrids. Consistent with expectations under hybrid breakdown, we found reduced plant growth for F2 hybrids in comparison with F1s (plant height and above-ground biomass) and parental Ls (above-ground biomass). Backcrossed hybrids had intermediate measures of plant growth and bcLs were characterized by reduced seed germination in comparison with parental Ls. Hybrid breakdown could make the formation of F1s in North America finite because (1) hybridization among cattails is asymmetric, with T. angustifolia but not T. latifolia subject to genetic swamping, and (2) T. angustifolia is less common and subject to competitive displacement by F1s. Hybrid breakdown is therefore expected to reduce hybrid frequencies over time, contributing to the long-term maintenance of T. latifolia - the only native cattail in the study region.

Analyses of Hybrid Viability across a Hybrid Zone between Two Alnus Species Using Microsatellites and cpDNA Markers

Diploid Alnus glutinosa s. str. and autotetraploid A. rohlenae form a narrow hybrid zone in a study area in southern Serbia, which results in triploid hybrid formation. The vast majority of previous studies have been focused on studies of maternal plants, but the offspring resulting from their crossing have not been much studied. Here, we use the variability of microsatellites and chloroplast DNA between these species and their putative hybrids to create an overall picture of the development of the hybrid zone and its predicted type. To elucidate the gene transfer within both species, the origins of individual ploidies and especially the role of triploid hybrids, a germination experiment was carried out linked with a flow cytometry study of the resulting seedlings. The tension zone model seems to offer the most adequate explanation of our observations, with selection against triploid hybrids and the spatial positioning of the hybrid zone. Despite selection against them, the triploid hybrids play an important role in the exchange of genes between the two species and therefore serve as a bridge for introgression. The presence of fertile triploids is essential for enriching the haplotype diversity between these species and for the development of new genetic lineages.

Completing the hybridization triangle: the inheritance of genetic incompatibilities during homoploid hybrid speciation in ragworts (Senecio)

A new homoploid hybrid lineage needs to establish a degree of reproductive isolation from its parent species if it is to persist as an independent entity, but the role hybridization plays in this process is known in only a handful of cases. The homoploid hybrid ragwort species, Senecio squalidus (Oxford ragwort), originated following the introduction of hybrid plants to the UK approximately 320 years ago. The source of the hybrid plants was from a naturally occurring hybrid zone between S. aethnensis and S. chrysanthemifolius on Mount Etna, Sicily. Previous studies of the parent species found evidence for multiple incompatibility loci causing transmission ratio distortion of genetic markers in their hybrid progeny. This study closes the hybridization triangle by reporting a genetic mapping analysis of the remaining two paired cross combinations between S. squalidus and its parents. Genetic maps produced from F₂ mapping families were generally collinear but with half of the linkage groups showing evidence of genomic reorganization between genetic maps. The new maps produced from crosses between S. squalidus and each parent showed multiple incompatibility loci distributed across the genome, some of which co-locate with previously reported incompatibility loci between the parents. These findings suggest that this young homoploid hybrid species has inherited a unique combination of genomic rearrangements and incompatibilities from its parents that contribute to its reproductive isolation.

Demographic history of speciation in a Senecio altitudinal hybrid zone on Mt. Etna

Hybrid zones typically form as a result of species coming into secondary contact, but can also be established in situ as an ecotonal hybrid zone, a situation which has been reported far less frequently. An altitudinal hybrid zone on Mount Etna between two ragwort species (the low elevation Senecio chrysanthemifolius and high elevation S. aethnensis) could potentially represent either of these possibilities. However, a scenario of secondary contact vs. speciation with gene flow has not been explicitly tested. Here, we test these alternatives and demonstrate that the data do not support secondary contact. Furthermore, we report that the previous analyses of speciation history of these species were based on admixed populations, which has led to inflated estimates of ongoing, interspecific gene flow. Our new analyses, based on 'pure' S. aethnensis and S. chrysanthemifolius populations, reveal gene exchange of less than one effective migrant per generation, a level low enough to allow the species to accumulate neutral, genomewide differences. Overall, our results are consistent with a scenario of speciation with gene flow and a divergence time which coincides with the rise of Mt. Etna to altitudes above 2000 m (~150 KY). Further work to quantify the role of adaptation to contrasting environments of high and low altitudes will be needed to support the scenario of recent ecological speciation in this system.

Maintenance of Species Boundaries Despite Ongoing Gene Flow in Ragworts

The role of hybridization between diversifying species has been the focus of a huge amount of recent evolutionary research. While gene flow can prevent speciation or initiate species collapse, it can also generate new hybrid species. Similarly, while adaptive divergence can be wiped out by gene flow, new adaptive variation can be introduced via introgression. The relative frequency of these outcomes, and indeed the frequency of hybridization and introgression in general are largely unknown. One group of closely-related species with several documented cases of hybridization is the Mediterranean ragwort (genus: Senecio) species-complex. Examples of both polyploid and homoploid hybrid speciation are known in the clade, although their evolutionary relationships and the general frequency of introgressive hybridization among them remain unknown. Using a whole genome gene-space dataset comprising eight Senecio species we fully resolve the phylogeny of these species for the first time despite phylogenetic incongruence across the genome. Using a D-statistic approach, we demonstrate previously unknown cases of introgressive hybridization between multiple pairs of taxa across the species tree. This is an important step in establishing these species as a study system for diversification with gene flow, and suggests that introgressive hybridization may be a widespread and important process in plant evolution.

Long-term persisting hybrid swarm and geographic difference in hybridization pattern: genetic consequences of secondary contact between two Vincetoxicum species (Apocynaceae-Asclepiadoideae)

BACKGROUND

During glacial periods, glacial advances caused temperate plant extirpation or retreat into localized warmer areas, and subsequent postglacial glacial retreats resulted in range expansions, which facilitated secondary contact of previously allopatric isolated lineages. The evolutionary outcomes of secondary contact, including hybrid zones, dynamic hybrid swarm, and resultant hybrid speciation, depends on the strengths of reproductive barriers that have arisen through epistatic and pleiotropic effects during allopatric isolation. The aim of this study was to demonstrate refugia isolation and subsequent secondary contact between two perennial Asclepioid species and to assess the genetic consequences of the secondary contact. We modeled the range shift of two ecologically distinct Vincetoxicum species using the species distribution model (SDM) and assessed the genetic consequences of secondary contact by combining morphological and genetic approaches. We performed morphometric analysis (592 individuals) and examined 10 nuclear microsatellites (671 individuals) in V. atratum, V. japonicum, and putative hybrid populations.

RESULTS

Multivariate analysis, model-based Bayesian analysis, and non-model-based discriminant analysis of principal components confirmed the hybridization between V. atratum and V. japonicum. High pollen fertility and a lack of linkage disequilibrium suggested that the hybrid populations may be self-sustaining and have persisted since V. atratum and V. japonicum came into contact during the post-glacial period. Moreover, our findings show that the pattern of hybridization between V. atratum and V. japonicum is unidirectional and differs among populations. Geographically-isolated hybrid populations exist as genetically distinct hybrid swarms that consist of V. atratum-like genotypes, V. japonicum-like genotypes, or admixed genotypes. In addition, Bayesian-based clustering analysis and coalescent-based estimates of long-term gene flow showed patterns of introgressive hybridization in three morphologically 'pure' V. japonicum populations.

CONCLUSION

In this study, we demonstrated that climatic oscillations during the Quaternary period likely led to species range shift and subsequently secondary contact. Hybrid populations may be self-sustaining and have persisted since V. atratum and V. japonicum came into contact during the post-glacial period. Pattern of hybridization between V. atratum and V. japonicum is unidirectional and differs among populations. We concluded that these differences in the genetic consequences of secondary contact are caused by historical colonization processes and/or natural selection.

Evidence for asymmetrical hybridization despite pre- and post-pollination reproductive barriers between two Silene species

Interspecific hybridization is widespread among plants; nevertheless, pre- and post-zygotic isolating mechanisms may maintain species integrity for interfertile species in sympatry despite some gene flow. Interspecific hybridization and potential isolating barriers were evaluated between co-flowering Silene asclepiadea and Silene yunnanensis in an alpine community in southwest China. We investigated morphological and molecular (nuclear microsatellites and chloroplast gene sequence) variation in sympatric populations of S. asclepiadea and S. yunnanensis. Additionally, we analyzed pollinator behaviour and compared reproductive success between the putative hybrids and their parental species. Both the molecular and morphological data indicate that there were putative natural hybrids in the field, with S. asclepiadae the ovule parent and S. yunnanensis the pollen parent. Bumblebees were the primary visitors to S. asclepiadae and putative hybrids, while butterflies were the primary visitors to S. yunnanensis Pollen production and viability were significantly lower in putative hybrids than the parental species. The direction of hybridization is quite asymmetric from S. yunnanensis to S. asclepiadea Protandry combined with later peak flowering of S. yunnanensis, and pollinator preference may have contributed to the asymmetric pattern of hybridization, but putative hybrids were rare. Our results thus suggest that despite gene flow, S. asclepiadea and S. yunnanensis can maintain species boundaries, perhaps as a result of floral isolation and low fecundity of the hybrids.

The effect of drought on photosynthetic plasticity in Marrubium vulgare plants growing at low and high altitudes

Photosynthesis is a biological process most affected by water deficit. Plants have various photosynthetic mechanisms that are matched to specific climatic zones. We studied the photosynthetic plasticity of C3 plants at water deficit using ecotypes of Marrubium vulgare L. from high (2,200 m) and low (1,100 m) elevation sites in the Mishou-Dagh Mountains of Iran. Under experimental drought, high-altitude plants showed more tolerance to water stress based on most of the parameters studied as compared to the low-altitude plants. Increased tolerance in high-altitude plants was achieved by lower levels of daytime stomatal conductance (g s) and reduced damaging effect on maximal quantum yield of photosystem II (PSII) (F v /F m ) coupled with higher levels of carotenoids and non-photochemical quenching (NPQ). High-altitude plants exhibited higher water use efficiency (WUE) than that in low-altitude plants depending on the presence of thick leaves and the reduced daytime stomatal conductance. Additionally, we have studied the oscillation in H(+) content and diel gas exchange patterns to determine the occurrence of C3 or weak CAM (Crassulacean acid metabolism) in M. vulgare through 15 days drought stress. Under water-stressed conditions, low-altitude plants exhibited stomatal conductance and acid fluctuations characteristic of C3 photosynthesis, though high-altitude plants exhibited more pronounced increases in nocturnal acidity and phosphoenolpyruvate carboxylase (PEPC) activity, suggesting photosynthetic flexibility. These results indicated that the regulation of carotenoids, NPQ, stomatal conductance and diel patterns of CO2 exchange presented the larger differences among studied plants at different altitudes and seem to be the protecting mechanisms controlling the photosynthetic performance of M. vulgare plants under drought conditions.

Altitudinal gradients, plant hybrid zones and evolutionary novelty

Altitudinal gradients are characterized by steep changes of the physical and biotic environment that present challenges to plant adaptation throughout large parts of the world. Hybrid zones may form where related species inhabit different neighbouring altitudes and can facilitate interspecific gene flow and potentially the breakdown of species barriers. Studies of such hybrid zones can reveal much about the genetic basis of adaptation to environmental differences stemming from changes in altitude and the maintenance of species divergence in the face of gene flow. Furthermore, owing to recombination and transgressive effects, such hybrid zones can be sources of evolutionary novelty. We document plant hybrid zones associated with altitudinal gradients and emphasize similarities and differences in their structure. We then focus on recent studies of a hybrid zone between two Senecio species that occur at high and low altitude on Mount Etna, Sicily, showing how adaptation to local environments and intrinsic selection against hybrids act to maintain it. Finally, we consider the potential of altitudinal hybrid zones for generating evolutionary novelty through adaptive introgression and hybrid speciation. Examples of homoploid hybrid species of Senecio and Pinus that originated from altitudinal hybrid zones are discussed.

Rapid speciation with gene flow following the formation of Mt. Etna

Environmental or geological changes can create new niches that drive ecological species divergence without the immediate cessation of gene flow. However, few such cases have been characterized. On a recently formed volcano, Mt. Etna, Senecio aethnensis and S. chrysanthemifolius inhabit contrasting environments of high and low altitude, respectively. They have very distinct phenotypes, despite hybridizing promiscuously, and thus may represent an important example of ecological speciation “in action,” possibly as a response to the rapid geological changes that Mt. Etna has recently undergone. To elucidate the species’ evolutionary history, and help establish the species as a study system for speciation genomics, we sequenced the transcriptomes of the two Etnean species, and the outgroup, S. vernalis, using Illumina sequencing. Despite the species’ substantial phenotypic divergence, synonymous divergence between the high- and low-altitude species was low (dS = 0.016 ± 0.017 [SD]). A comparison of species divergence models with and without gene flow provided unequivocal support in favor of the former and demonstrated a recent time of species divergence (153,080 ya ± 11,470 [SE]) that coincides with the growth of Mt. Etna to the altitudes that separate the species today. Analysis of dN/dS revealed wide variation in selective constraint between genes, and evidence that highly expressed genes, more “multifunctional” genes, and those with more paralogs were under elevated purifying selection. Taken together, these results are consistent with a model of ecological speciation, potentially as a response to the emergence of a new, high-altitude niche as the volcano grew.

Interspecific crossing and genetic mapping reveal intrinsic genomic incompatibility between two Senecio species that form a hybrid zone on Mount Etna, Sicily

Studies of hybridizing species can reveal much about the genetic basis and maintenance of species divergence in the face of gene flow. Here we report a genetic segregation and linkage analysis conducted on F2 progeny of a reciprocal cross between Senecio aethnensis and S. chrysanthemifolius that form a hybrid zone on Mount Etna, Sicily, aimed at determining the genetic basis of intrinsic hybrid barriers between them. Significant transmission ratio distortion (TRD) was detected at 34 (∼27%) of 127 marker loci located in nine distinct clusters across seven of the ten linkage groups detected, indicating genomic incompatibility between the species. TRD at these loci could not be attributed entirely to post-zygotic selective loss of F2 individuals that failed to germinate or flower (16.7%). At four loci tests indicated that pre-zygotic events, such as meiotic drive in F1 parents or gametophytic selection, contributed to TRD. Additional tests revealed that cytonuclear incompatibility contributed to TRD at five loci, Bateson-Dobzhansky-Muller (BDM) incompatibilities involving epistatic interactions between loci contributed to TRD at four loci, and underdominance (heterozygote disadvantage) was a possible cause of TRD at one locus. Major chromosomal rearrangements were probably not a cause of interspecific incompatibility at the scale that could be examined with current map marker density. Intrinsic genomic incompatibility between S. aethnensis and S. chrysanthemifolius revealed by TRD across multiple genomic regions in early-generation hybrids is likely to impact the genetic structure of the natural hybrid zone on Mount Etna by limiting introgression and promoting divergence across the genome.