Hybrids & Hybridization Sciences, 900 University Ave., Riverside, CA, 92521, USA2University Of Colorado - Boulder, Department Of Ecology And Evolutionary Biology, C127 Ramaley Hall, University Of Colorado-Boulder Campus Box 334, P.O. Box 400328, Boulder, CO, 80309, USA3University Of Colorado-Boulder, School Of Biological Sciences, Campus Box 334, Boulder, CO, 80309-0334, USA4University of California, Riverside, 900 University Avenue, Riverside, CA, 92521, United States
731
NIELSEN, TORA , CORTƒS, ANDRƒS* and KARRENBERG, SOPHIE
Origin and Distribution of a Putative Interploidy Hybrid Between Salix herbacea L. and Salix polaris Wahlenb. (Salicaceae)
H
ybridization is a major creative force in plant evolution and a primary cause of extensive morphological and genetic variation observed in many plant genera. Hybridization is also documented between taxa of different ploidy levels, but this process is not well understood. Different ploidy levels and frequent hybridizations make the genus Salix an excellent study system for evaluating evolutionary consequences of across-ploidy hybridization. In the current study, we investigated the arctic-alpine dwarf shrubs Salix herbacea L. (diploid), Salix polaris Wahlenb. (hexaploid) and their putative Scandinavian hybrid (tetraploid). We used morphological traits, flow cytometry and microsatellite marker analyses in 86 samples across 14 allopatric and sympatric sites along with controlled pollination experiments to infer patterns of hybridization and reproductive isolation. Hexaploids and diploids were found at allopatric sites, but only diploid S. herbacea and tetraploid putative hybrids were detected in the Scandinavian contact zone. It was evident that hybridization with and introgression from S. herbacea has influenced the present day morphology and genetic make-up of the Scandinavian tetraploid. Analysis of molecular variance (AMOVA) and ABC modelling suggested that the tetraploid probably arose before its range expansion into the Scandinavia. Crossing experiments demonstrated that reproductive barriers between diploids and tetraploids are asymmetrical and incomplete. Thus, on-going gene flow is probably occurring, and may mask signs of an ancient hybridization. Our investigation suggests that what has traditionally been believed to be a common Scandinavian hybrid between S. polaris and S. herbacea, is most likely a ploidy-reduced form of S. polaris. In turn, the observed morphological variation within this tetraploid form is likely to be introduced by rare hybridization with diploid S. herbacea. Results of this study highlight the importance of interploidy hybridization and introgression in arctic Salix.
Uppsala University, Department Of Ecology And Genetics, Norbyvagen 18D, Uppsala, 75236, Sweden
732
DE VOS, JURRIAAN 2 and JAMES, THOMSON 3
Reproductive barriers between heterostylous primroses
T
he interaction between floral traits and reproductive isolation is crucial to explaining the extraordinary diversity of angiosperms. Flowers enable the evolution of complex relationships with pollinators, promoting reproductive isolation and diversification. Distinct features of corollas (e.g., color, scent, texture, shape, tube length) and reproductive organs (e.g., position, form, pollen/stigma ultrastructure and proteins) can facilitate isolating mechanisms by attracting different pollinators, restricting interspecific pollen transfer, or rejecting interspecific pollen. Heterostyly, a complex floral polymorphism that optimizes outcrossing, evolved repeatedly in angiosperms and has been shown to accelerate diversification in primroses. However, no experimental study has evaluated its possible effects on reproductive isolation. We quantify multiple reproductive barriers between the heterostylous Primula elatior (oxlip) and P. vulgaris (primrose), elucidate the relative importance of pre- vs. postmating barriers, and test whether traits of heterostyly contribute to individual barriers. We find that premating isolation is key for both species, while postmating isolation is more pronounced in P. vulgaris. More specifically, ecogeographic isolation is important for both species, but stronger in P. elatior than in P. vulgaris, while phenological, seed developmental, and hybrid sterility barriers are stronger in P. vulgaris than in P. elatior. Consequently, reproductive isolation under sympatry is highly asymmetric between species, thus gene flow is expected to be higher from P. vulgaris into P. elatior than vice versa. Furthermore, we document for the first time that, in addition to widely occurring species-dependent asymmetries, morph-dependent asymmetries affect reproductive barriers between heterostylous species. Indeed, the interspecific decrease of reciprocity between high sexual organs of complementary morphs limits interspecific pollen transfer from S-morph anthers to L-morph stigmas, while higher reciprocity between low sexual organs favors introgression over isolation from L-morph anthers to S-morph stigmas. Therefore, patterns of pollen movement across species boundaries are likely affected by morph composition of adjacent populations. As small populations of heterostylous species often have unbalanced morph ratios, human-mediated habitat fragmentation that progressively reduces population sizes likely affects the permeability of species boundaries. Finally, intramorph incompatibility persists across species boundaries, but is weakened in L-flowers of P. elatior, opening a possible backdoor to gene flow through intra-morph pollen transfer between species. To summarize, our study highlights the general importance of premating isolation and newly illustrates that both morph- and species-dependent asymmetries shape boundaries between heterostylous species. 1
University of Zurich, Department of Systematic and Evolutionary Botany, Zürich, Switzerland2Royal Botanic Gardens, Kew, Richmond, UK3University of Toronto, Ecology and Evolutionary Biology Department, Toronto, Canada4University of Zurich, Department of Systematic and Evolutionary Botany, Zürich, 8008, Switzerland
301