Mating systems of diploid and allotetraploid populations of Tragopogon (Asteraceae). I. Natural populations

Although polyploidy is a significant force in the diversification of plants , the evolutionary consequences of polyploidization are not thoroughly unde rstood. One possible consequence of polyploidy predicted by most population genetic theories is that the newly synthesized polyploid will self-fertili ze at a greater rate than its diploid progenitors. To test for increased se lfing rates in a polyploid, the mating systems of the allotetraploid Tragop ogon mirus and one of its diploid progenitors, T. dubius, were compared. Tr agopogon mirus is a recently derived species that arose sometime in the las t 80 years and thus provides an opportunity to probe how quickly a shift in outcrossing rates might occur. Based on analyses of variation in maternal plants and their progeny arrays, the two tetraploid populations surveyed ha ve higher outcrossing rates than the two diploid populations. This result i s the opposite of that predicted by population genetic theory. This discrep ancy between theoretical and empirical results may result from bias in the genetic sample, traits in the natural histories of the taxa involved or a l ack of sufficient time since the formation of the polyploid (80 years or 40 -80 generations) for a shift towards increased selfing to have occurred. Al ternatively, the partial dominance model of inbreeding depression typically applied to polyploids may not be appropriate; the overdominance model pred icts outcrossing rates in diploids and their tetraploid derivatives that ar e consistent with those observed in T. dubius and T. mirus.