Sex determination and gender expression in the angiosperm Urtica dioica L. (Urticaceae)

Urtica dioica L. (Urticaceae) is one of very few angiosperm species known to exhibit both monoecy and dioecy. The research presented in this dissertation had three major components: ^ First, an examination of developmental patterns of gender expression at the level of flowering nodes and entire shoots in monoecious individuals of subsp. dioica, subsp. gracilis, and dioica × gracilis hybrids (Chapter 2), from a combination of field and greenhouse data. Considerable variation occurred, both within and among taxa, in such characters as gender of initial and last flowering node(s), gender bias, relative proportions of mixed-gender and single-gender nodes, and number of times gender expression at flowering nodes changed between male, female, or a mixture of male and female as the plant developed. Clear differences between subspecies were evident in many traits. ^ Second, an investigation into the genetic mechanisms underlying sex determination in this species, using traditional (Mendelian) crosses and examining the segregation of sex forms (male, female, and monoecious) in the progeny (Chapter 3). Several crosses between U. dioica subsp. dioica and subsp. gracilis, as well as within subsp. dioica were conducted, including F2 and backcross generations for some crosses. Results from these crosses include evidence that the genetic basis of monoecy is different in subsp. dioica and subsp. gracilis, that monoecious individuals are clearly genetically different from females, and that there are differences among females, and/or among males, depending on whether their parents were unisexual or monoecious. The most important finding from the crossing studies, is the evidence of a nucleo-cytoplasmic system of sex determination in this species, based on the strongly female-biased sex ratios in the progeny of certain crosses and the asymmetrical progeny sex ratios in certain reciprocal crosses. ^ Finally, a brief chapter on gender expression of individual flowers (Chapter 4). Although the genus Urtica is characterized as having unisexual flowers, hermaphroditic flowers have been reported a few times. Descriptions and photographs of hermaphroditic flowers that were found in greenhouse cultures of U. dioica subsp. dioica and dioica × gracilis hybrids are provided.