Abstract
Hybridization and polyploidy are major forces in the evolution of plant diversity and the study of these processes is of particular interest to understand how novel taxa are formed and how they maintain genetic integrity. Sorbus is an example of a genus where active diversification and speciation are ongoing and, as such, represents an ideal model to investigate the roles of hybridization, polyploidy and apomixis in a reticulate evolutionary process. To elucidate breeding systems and evolutionary origins of a complex of closely related Sorbus taxa, we assessed genotypic diversity and population structure within and among taxa, combining data from nuclear DNA microsatellite markers and flow cytometry. Clonal analysis and low genotypic diversity within the polyploid taxa suggest apomixis is obligate. However, genetic variation has led to groups of ‘clone-mates’ within apomictic taxa that strongly suggest mutation is responsible for the genotypic diversity of these apomictic lineages. In addition, microsatellite profiles and site demographics suggest hybridization events among apomictic polyploid Sorbus may have contributed to the extant diversity of recognized taxa in this region. This research demonstrates that both macro- and micro-evolutionary processes are active within this reticulate Sorbus complex. Conservation measures should be aimed at maintaining this process and should therefore be prioritized for those areas of Sorbus species richness where the potential for interspecific gene flow is greatest.
Highlights
Hybridization between species resulting in the formation of new polyploid populations that are distinct and reproductively isolated from the parental taxa is the most common mechanism for sympatric speciation (Grant, 1981; Mallet, 2007)
Hybridization, polyploidy and apomixis are all features of these and other complex genera and those groups that contain evolutionary young species represent good models to investigate the roles of these processes in plant speciation
The 10 core loci yielded a total of 202 alleles from 207 Sorbus samples, ranging between eight (MSS13 and SA03) and 24 (SA14) alleles per locus
Summary
Hybridization between species resulting in the formation of new polyploid populations that are distinct and reproductively isolated from the parental taxa is the most common mechanism for sympatric speciation (Grant, 1981; Mallet, 2007). The frequency and the main formation routes of polyploid taxa remain unclear (Soltis et al, 2010) and studies of hybridization processes in polyploid species complexes may help to understand this form of speciation. Apomixis (synonymous with agamospermy; asexual seed production) is often associated with polyploidy (Whitton et al, 2008) and effectively causes instant reproductive isolation of novel polyploids from sexual progenitors, enabling sympatric establishment while maintaining the heterozygosity associated with hybridization. Hybridization, polyploidy and apomixis are all features of these and other complex genera and those groups that contain evolutionary young species represent good models to investigate the roles of these processes in plant speciation
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