UBC Theses and Dissertations
Evidence for reproductive interference between sexual and apomictic populations of the Easter Daisy (Townsendia hookeri, Asteraceae) Garani, Alice
Townsendia hookeri (Asteraceae) plants can reproduce sexually or via apomixis (i.e. asexual reproduction through seeds), and the breeding system is tightly linked with ploidy level, so that sexual outcrossers are exclusively diploid while apomictic plants are polyploid. The species grows from central Colorado to British Columbia, with a disjunct distribution in Yukon Territory. Outside Yukon, sexual populations are restricted to the southernmost portion of the range (Colorado and southern Wyoming) and apomictic populations occur from Wyoming to Canada, a pattern consistent with geographical parthenogenesis. The major objective of this study was to expand our understanding of the factors that have shaped and maintain this distribution, conferring an apparent advantage to apomictic lineages over their sexual progenitors. Having documented that polyploid plants retain the ability to produce some functional pollen, I hypothesized that if sexual forms spread into an apomictic population, and thus receive mostly heterospecific pollen, they would have reduced reproductive success, because the progeny sired in sexual-asexual crosses are predicted to be weak or inviable, and to include hybrid apomicts. This asymmetric reproductive interference could help explain why diploids have failed to spread into the territory dominated by apomicts. To test this hypothesis I performed a cross-pollination experiment in the field. I showed that diploid and polyploid cytotypes have comparable reproductive success (at least, in the area where the experiment was conducted), and I also confirmed that diploids are outcrossers while polyploids are apomictic. The crossing experiment indicates that, despite its low viability, pollen produced by apomicts can fertilize and negatively affect diploid seed parents. As predicted, when sexual plants received heterospecific rather than conspecific pollen, seed set and germination rate were reduced, and seedlings had a lower survival rate. Flow cytometric analysis of offspring from sexual-asexual crosses revealed the presence of putative euploid (diploid, triploid and tetraploid) as well as aneuploid cytotypes. Based on ploidy level it is reasonable to expect that polyploid hybrids inherited apomictic genes and that at least some of them might be fully apomictic.
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