From the Environmental Sciences Division, Oak Ridge National Laboratory, P. Box 2008, Oak Ridge, TN 37831–6422 (Gunter, Roberts, Lee, and Tuskan) and the Life Sciences Division, Oak Ridge National Laboratory, P. Despite the absence of such structures in the majority of dioecious plants, gender seems to be under relatively strict genetic control in some species. Most studies of sex determination systems in plants involve dioecious annuals that have known sex chromosomes.However, it is unclear what direct effect such genes may have on sex determination in monoecious and dioecious species.One model proposed for the evolution of dioecy from hermaphroditic ancestors suggests that two simultaneous but independent mutations, which cause both male and female sterility and also suppress recombination between these mutations to prevent the formation of hermaphrodites, could be responsible for such a state (Lewis 1942; Ross 1978).A more reasonable hypothesis is that gynodioecy or androdioecy resulting from a single mutation may have provided a likely intermediary step between hermaphroditism and dioecy (Ainsworth 2000; Charlesworth and Gutman 1999). Do one or a few primary sex determination loci activate sex differentiation genes, or does a series of organ suppression genes activated by environmental clues control this process?Furthermore, despite evidence that sex determination is under genetic control in dioecious species with heteromorphic sex chromosomes (e.g., ), it is doubtful that the same mechanism would control sex in individual flowers in monoecious plants or in dioecious plants without sex chromosomes (Ainsworth et al. Are sex determination mechanisms so diverse that no one mechanistic model can account for sexual differentiation among all plant species?There is no robust evidence of sex chromosomes in Salicaceae, yet both species are represented by fairly stable sexually dimorphic systems not readily influenced by plant growth substances (Ainsworth et al.1998), although some level of hermaphroditic and female-biased sex ratios has been reported for both as a short-rotation energy crop (Åhman 1997; Alström-Rapaport et al. The development of molecular markers linked to sex has been attempted in a number of dioecious species through genetic mapping or breeding work (reviewed in Ainsworth 2000).
Family 961 was subsequently chosen because it was produced from the same maternal parent as families 960 and 962.(1998) using standard agarose gel electrophoresis separation.