Print PageBiodiversity is a direct result of the process of speciation, and species form when populations that were once interbreeding become reproductively isolated from each other via pre- and/or postzygotic reproductive barriers. Despite the importance of studying reproductive barriers between closely related plant taxa, biologists still have only a rudimentary understanding of the relative importance of different isolating mechanisms for plant speciation, especially with regard to diploid-polyploid species pairs. Our study assesses the contributions of multiple pre- and postzygotic reproductive barriers in the maintenance of species boundaries between two lily species native to eastern and central North America—diploid Erythronium mesochoreum and tetraploid Erythronium albidum. ITS sequence data indicates that E. mesochoreum and E. albidum are likely sister species (Allen et al. 2003), and previous karyotype data demonstrates that these species hybridize at low frequencies at sites where they are syntopic.
Results/Conclusions
Our results indicate that while E. mesochoreum and E. albidum occupy different types of habitats within their shared geographic range, overlaps in flowering phenology and the presence of at least 2 shared pollinator species are sufficient to facilitate interspecific pollen transfer in areas where the plant taxa co-occur. Further, our hand pollination experiment indicates that seed set in hybrid crosses is reduced by 31-53%, depending on the direction of the cross. This demonstrates that while hybrid seed production is possible, late prezygotic and early postzygotic barriers act to significantly reduce hybrid seed set. However, despite the incomplete nature of the isolating mechanisms we evaluated, the paucity of mature hybrids observed thus far (as determined by flow cytometry) likely indicates that additional postzygotic barriers, such as hybrid inviability or sterility, are responsible for maintaining nearly complete reproductive isolation between E. albidum and E. mesochoreum.
