The coexistence of polyploids in Arnica cordifolia (Asteraceae).

摘要

Polyploidy is a central force structuring genetic diversity in angiosperms, yet its ecological significance is poorly understood. Theoretically, multiple cytotypes should not coexist, yet they do in many species. I explored mechanisms creating and maintaining multiple cytotypes by investigating the molecular relatedness, reproductive strategies, ecological differentiation, and a host-insect interaction in Arnica cordifolia.; I established that both triploids and tetraploids occur throughout the range of the species with both cytotypes occurring together in the majority of sites. Molecular analyses supported multiple triploidy events but revealed a lack of variation in the tetraploids, providing support for the triploid bridge hypothesis. Additionally, sequence polymorphism in ITS1 suggested a hybridization event prior to polyploidization. Fine scale sampling in A. cordifolia suggested that, in general, triploids were not more likely to associate with roadsides than tetraploids. Environmental traits did not differ between cytotype locations, nor did cytotypes perform differently in triploid or tetraploid locations in the field. I showed that both triploids and tetraploids can be asexual and the majority are not psuedogamous apomicts as previously thought. I found that a seedhead feeder, Campiglossa footeorum (Diptera: Tephntidae), attacks A. cordifolia throughout Colorado, is more abundant in sites with more tetraploids, and attacks tetraploids more than triploids.; These results show that habitat differentiation is not driving the maintenance of sympatric cytotypes in this species. As habitat differentiation has long been assumed to play a central role in cases of cytotype coexistence, this was an unexpected result that is likely to strongly influence the way plant evolutionary biologists think about the evolution of polyploidy. Differential fitness caused by interactions with C. footeorum or other species could be important to the maintenance of sympatric cytotypes. Minority cytotype exclusion is unlikely to occur via reproductive disadvantage, and therefore asexual reproduction could be an important factor promoting the coexistence of cytotypes in this system. The greater molecular diversity in triploids, lack of ecological differentiation between cytotypes, asexual reproduction (but not pseudogamous apomixis), and lack of minority cytotype reproductive advantage all run counter to original predictions and assumptions of mechanisms governing polyploidy distributions and the reproductive biology of this species.