The genetics and evolution of subdioecy in Astilbe biternata

All populations of Astilbe biternata have different individuals which express male, female, and hermaphrodite phenotypes. This breeding system has been termed subdioecy by previous researchers. Because all other Astilbe species are hermaphroditic and A. biternata is a tetraploid which presumably arose from the hybridization of two unknown diploid Astilbe species, it is assumed that the ancestors of A. biternata were hermaphroditic. Both the pathway along which the breeding system is evolving and the trajectory of evolution in Astilbe biternata are assessed.;The distributions of the quantitative genders of individuals in populations at Coweeta Hydrological Laboratory and patterns of gender transitions of individuals at Coweeta and in the Duke University greenhouses were used to deduce that subdioecy in A. biternata is evolving along the gynodioecious pathway between hermaphroditism and dioecy. Furthermore, clonal repeatability studies suggest that there is genetic variation for the propensity to set fruit in hermaphrodites and crossing studies show that there is genetic variation for the propensity to produce a full complement of stamens in the inflorescence. These observations of genetic components to gender expression as well as field observations suggesting a negative genetic correlation between stamen production and fruit production support assumptions of theoretical models of the evolution along the gynodioecious pathway.;Fruit set of females was found to be over four times as high as fruit set in hermaphrodites. Moreover, outcross siring success of males was slightly higher than for hermaphrodites; however, when the evolutionary trajectory was placed in the context of theoretical models, the siring success of males was not sufficiently high compared to hermaphrodites to predict the maintenance of males in populations. Therefore, the relative reproductive outputs of males, females, and hermaphrodites suggests that future evolution of the breeding system will be towards gynodioecy. However, this conclusion must be viewed with caution because the inheritance of factors determining male sterility is complex. Future studies, some of which are already in progress, will be devoted to better understanding the genetics of sex-determination in A. biternata.;Genetic studies of the inheritance of allozyme markers suggest that A. biternata is an allotetraploid, formed from a hybridization event between two unknown diploid species of Astilbe. It is suggested that this hybridization event may have initiated the shift in breeding systems from an ancestral hermaphroditic condition to the present subdioecious breeding system.