Mating system variation in hybrid zones: Facilitation, barriers and asymmetries to gene flow

Plant mating systems play a key role in structuring genetic variation both within and between species. In hybrid zones, the outcomes and dynamics of hybridization are usually interpreted as the balance between gene flow and selection against hybrids. Yet, mating systems can introduce selective forces that alter these expectations; with diverse outcomes for the level and direction of gene flow depending on variation in outcrossing and whether the mating systems of the species pair are the same or divergent. We present a survey of hybridization in 133 species pairs from 41 plant families and examine how patterns of hybridization vary with mating system. We examine if hybrid zone mode, level of gene flow, asymmetries in gene flow and the frequency of reproductive isolating barriers vary in relation to mating system/s of the species pair. We combine these results with a simulation model and examples from the literature to address two general themes: (i) the two‐way interaction between introgression and the evolution of reproductive systems, and (ii) how mating system can facilitate or restrict interspecific gene flow. We conclude that examining mating system with hybridization provides unique opportunities to understand divergence and the processes underlying reproductive isolation

Mating system variation in hybrid zones: Facilitation, barriers and asymmetries to gene flow

Plant mating systems play a key role in structuring genetic variation both within and between species. In hybrid zones, the outcomes and dynamics of hybridization are usually interpreted as the balance between gene flow and selection against hybrids. Yet, mating systems can introduce selective forces that alter these expectations; with diverse outcomes for the level and direction of gene flow depending on variation in outcrossing and whether the mating systems of the species pair are the same or divergent. We present a survey of hybridization in 133 species pairs from 41 plant families and examine how patterns of hybridization vary with mating system. We examine if hybrid zone mode, level of gene flow, asymmetries in gene flow and the frequency of reproductive isolating barriers vary in relation to mating system/s of the species pair. We combine these results with a simulation model and examples from the literature to address two general themes: (1) the two-way interaction between introgression and the evolution of reproductive systems, and (2) how mating system can facilitate or restrict interspecific gene flow. We conclude that examining mating system with hybridization provides unique opportunities to understand divergence and the processes underlying reproductive isolation

Gender Strategies and Sex-ratio Evolution in the Clonal Aquatic Plant: Sagittaria latifolia (Alismataceae)

Flowering plants display diverse reproductive systems, including a variety of gender strategies and mechanisms of clonal propagation. Here, I investigate gender strategies, sex-ratio evolution, and sexual dimorphism in the North American clonal aquatic, Sagittaria latifolia (Alismataceae), which exhibits three sex phenotypes (hermaphrodites, females, males) and two modal sexual systems (monoecy, dioecy). This provides an outstanding opportunity to examine the costs and benefits of combined versus separate sexes. My research focused on the northern range limit in eastern N. America, and on disjunct populations in western N. America. I developed microsatellite (SSR) markers to investigate population genetic structure at several spatial scales, including the clonal structure of local populations to continental patterns. These analyses provided insights on the roles of historical, ecological and reproductive factors in the evolution and maintenance of sexual system diversity. Phenotypic sex ratios varied near continuously from monoecy through subdioecy (three sex phenotypes) to dioecy. A comparison of phenotypic and genotypic sex ratios in dioecious populations demonstrated close correspondence. The northern range limit was characterized by a decline in female frequency and an increased incidence of subdioecy. I evaluated two hypotheses to explain this pattern: (1) increased sex inconstancy in dioecious populations; (2) hybridization between monoecious and dioecious populations. I found support for both hypotheses, although hybridization appears to be the more common pathway to subdioecy. I parameterized a model predicting female frequency and hermaphrodite sex allocation; observed and predicted values were correlated suggesting that subdioecious populations are closer to equilibrium than expected for a clonal perennial. A comparison of eastern and western populations indicated genetic differentiation between monoecy and dioecy in the east, but in the west, due to habitat isolation, geography plays a more important role in genetic differentiation. Evidence from cpDNA haplotype variation indicated that the western range was established following long-distance colonization from the east involving a genetic bottleneck. The discovery of gynodioecious populations of S. latifolia in the west, and the absence of ecological and genetic differentiation between monoecious and dioecious populations, raise the possibility that dioecy may have evolved autochthonously in the west, and more recently than in the eastern range.Ph

Data from: Clonal genetic structure and diversity in populations of an aquatic plant with combined versus separate sexes

Clonality is often implicated in models of the evolution of dioecy, but few studies have explicitly compared clonal structure between plant sexual systems, or between the sexes in dioecious populations. Here, we exploit the occurrence of monoecy and dioecy in clonal Sagittaria latifola (Alismataceae) to evaluate two main hypotheses: (1) clone sizes are smaller in monoecious than dioecious populations, because of constraints imposed on clone size by costs associated with geitonogamy; (2) in dioecious populations, male clones are larger and flower more often than female clones because of sex-differential reproductive costs. Differences in clone size and flowering could result in discordance between ramet- and genet-based sex ratios. We used spatially explicit sampling to address these hypotheses in 10 monoecious and 11 dioecious populations of S. latifolia at the northern range limit in eastern N. America. In contrast to our predictions, monoecious clones were significantly larger than dioecious clones, probably due to their higher rates of vegetative growth and corm production, and in dioecious populations there was no difference in clone size between females and males; ramet- and genet-based sex ratios were therefore highly correlated. Genotypic diversity declined with latitude for both sexual systems, but monoecious populations exhibited lower genotypic richness. Differences in life history between the sexual systems of S. latifolia appear to be the most important determinants of clonal structure and diversity

SexCloneSummary

This file summarizes phenotypic and genotypic sex ratio for 11 dioecious populations. It is used for the R analysis of correlation between phenotypic and genotypic sex ratio. Popn – population code. Lat – Latitude in degrees N. Long – Longitude in degrees W. Ph.Num.f – number of ramets phenotyped as female (in field). Ph.Num.h - number of ramets phenotyped as hermaphrodite (in field). Ph.Num.m - number of ramets phenotyped as male (in field). Ph.Num.hm - number of ramets phenotyped as hermaphrodite or male (in field). Ph.Num.fh - number of ramets phenotyped as female of hermaphrodite (in field). Ph.Total – total number of ramets sex phenotyped (in field). Ph.Freq.f - proportion of ramets phenotyped as female (in field). Ph.Freq.h - proportion of ramets phenotyped as hermaphrodite (in field). Ph.Freq.m - proportion of ramets phenotyped as male (in field). G.Num.f - number of female multilocus genotypes. G.Num.h - number of hermaphrodite multilocus genotypes. G.Num.m - number of male multilocus genotypes. G.Total – total number of multilocus genotypes with known sex phenotype. G.Freq.f - proportion of multilocus genotypes female. G.Freq.h - proportion of multilocus genotypes hermaphrodite. G.freq.m - proportion of multilocus genotypes male. Gcorr.Num.f.corr - number of female multilocus genotypes (corrected for by scoring error / mutations). G.Num.m.corr - number of male multilocus genotypes (corrected for by scoring error / mutations). G.Total.corr – total number of multilocus genotypes (corrected for by scoring error / mutations). G.Freq.f.corr - proportion of multilocus genotypes female (corrected for by scoring error / mutations). G.freq.m.corr – proportion of multilocus genotypes male (corrected for by scoring error / mutations)

SexCloneSummary

This file summarizes the sex of each clone from Clonality.Sagittaria.Sexclones.xls. Within each population’s worksheet each line represents a clone for which sex phenotype is known. Genotypic sex ratio is calculated based on these assignments. The final column “Clone.Size.Area” is the estimate of area covered (m2) by each clone (calculated by GenAlEx, see Clonality.Sagittaria.SexclonesArea.xls)

Spatial_MvsD

Popn – population ID. Sex.System = M for monoecious, D for dioecious. Lat – latitude in degrees N. Long – longitude in degrees W. Clonal.subrange – mean clonal subrange calculated by GenClone. Ac – Aggregation index (GenClone). P.Ac – P value associated with Ac. Total.Locations – total number of spatial locations surveyed in field. Num.Occupied – number of spatial locations occupied by a ramet. Num.Not.Occupied – number of spatial location not occupied by a ramet. Ppn.Occupied – proportion of spatial positions surveyed occupied by a rame

CloneSize_byClone_GenoDive_flwfreq

CloneSize_byClone_GenoDive_flwfre

CloneFlwFreq

SampleNo – original sample number. Popn – population code. Lat – latitude in degrees N. Long – longitude in degrees W. Genotype. Label – clone ID. Clone.Sex. Num.Ramets.Flw - number of ramets associated with multilocus genotype observed flowering (in field). Num.Ramets - number of ramets associated with multilocus genotype. Num.Ramets.NonFlw - number of ramets associated with multilocus genotype not observed flowering (in field). (The following are same as above but NA for genotypes associated with single ramet). Num.Ramets.Flw.1. Num.Ramets.NonFlw.1. Ppn.Ramets.Flw.1. Num.Ramets – number of ramets associated with multilocus genotype. Num.Ramets.Flw – number of ramets associated with multilocus genotype observed flowering (in field). Num.Ramets.NonFlw- number of ramets associated with multilocus genotype not observed flowering (in field). Ppn.Ramets.Flw – proportion of ramets associatied with multilocus genotype observed flowering (in field