Plant diversification through polyploidy and diverging insect-plant interactions.

摘要

One of the major mechanisms of diversification in terrestrial ecosystems is the evolution of polyploidy. Polyploids frequently differ from their diploid progenitors in morphology, physiology and ecological tolerances. Such differences may provide variability upon which diversifying selection may act through interactions with other species. This possibility suggests that polyploidy may have its greatest diversifying effect after the fact of polyploidization, by providing the initial variability on which diversifying selection then can act. Here I focus on interactions of Heuchera grossulariifolia (Saxifragaceae), a rhizomatous perennial in which autotetraploidy has arisen multiple times. In two related field studies, I examine two opportunities for pollinators to apply diversifying selection to H. grossulariifolia in natural populations where the cytotypes are sympatric. First, I ask whether pollinators may apply diversifying selection to the cytotypes by visiting them differentially. Second, I ask whether pollinators have differential efficacies in effecting seed set among flowers of the two cytotypes. Finally, I evaluate whether there is evidence of diversifying selection in the population structure of H. grossulariifolia at those sites.; The results from the first two chapters indicate that because these plants are largely self-incompatible, they must rely on insects for pollination. Diploids and tetraploids attract quantitatively different suites of floral visitors, among which only a few make most of the visits. This is not consistent however, as suites of floral visitors also differ quantitatively among field sites. Further, some of these major visitors effect seed set differentially between flowers of the two cytotypes. Taken together these results indicate that insect pollinators are capable of exerting diversifying selection on H. grossulariifolia. The results from the third chapter, in which the population structure is investigated, show that H. grossulariifolia is significantly differentiated among the four single cytotype groups, representing the two ploidies at the two sites. Nevertheless, evolution of polyploidy has not led to clear patterns of differentiation of tetraploids from their diploid progenitors, either within or among sites.