Format: ORAL

Gracy Buckholtz, Max W.R. Gray, Jeannette Whitton

Department of Botany, The University of British Columbia

Diverse factors have the potential to explain geographical parthenogenesis, the tendency for related sexual and apomictic forms to occupy distinct ranges. Apomicts are expected to have a colonization advantage relative to obligately outcrossing sexuals, but if this advantage is purely demographic, we might expect sexuals to catch up over time and colonize sites where apomicts have established. Where this is not the case, it could reflect ecological and/ or additional life history divergence that precludes coexistence. Asymmetrical reproductive interference is an additional or alternative mechanism that may prevent invasion by sexuals into apomictic sites, through the action of pollen from apomicts reducing the reproductive fitness of sexual outcrossers. Townsendia hookeri (Asteraceae) includes obligate apomictic autotriploid and sexual outcrossing diploid populations that display a classical pattern of geographical parthenogenesis along the eastern slope of the Rocky Mountains, from Colorado, USA to British Columbia, Canada. The two forms overlap in range in southern Wyoming, but co-occurrence is rare. Transplants across the range reveal that sexuals can persist in sites occupied by apomicts. We measured the strength of reproductive interference (RI) by analyzing progeny produced in a mixed, open-pollinated common garden located in the zone of range overlap. Seed set of sexuals declined as a function of their proximity to apomicts. We used genomic markers (SNPs) to infer the parentage of progeny arrays from sexual maternal plants, using the programs BORICE to estimate the mating system and HIPHOP to determine parentage. We detected low levels of hybrid formation, and, contrary to previous interpretations from controlled crosses, no evidence of induced selfing. Our findings point to significant negative impacts of co-occurrence via RI, even though sexuals were roughly equal in number to apomicts in our garden. This suggests that, in some cases, reproductive interference may be key to explaining geographical parthenogenesis.