Evolution of dicliny in Bouteloua (Poaceae: Chloridoideae)

The majority of angiosperms produce hermaphrodite (perfect) flowers that have functional male (stamens) and female (pistils) floral organs. A lesser number (20--30%) produce unisexual (imperfect) flowers in which only stamens or pistils reach functional maturity. The research in this dissertation examines both the patterns and processes involved in the evolution of unisexual flowers in the genus Bouteloua (Poaceae: Chloridoideae). Bouteloua is ideal for such a study, because it includes multiple unisexual-flowered (diclinous) species and a diversity of breeding systems. The patterns of distribution of dicliny and breeding systems in Bouteloua were analyzed in a phylogenetic context. When dicliny and breeding system were traced onto a published phylogeny of Bouteloua, three to six independent origins of dicliny were estimated. Also, dioecy in Bouteloua appears to have evolved most frequently from hermaphroditism and monoecy. Analysis of the development of unisexual flowers in diclinous Bouteloua dimorpha indicates that male flowers have a developmental mechanism similar to species in grass subfamily Panicoideae. Other authors have suggested that the maize sex-determining gene, TASSELSEED2 (Ts2), is involved in male flower development throughout Panicoideae, and the developmental similarities suggest that Ts2 may also be involved in male flower development in Bouteloua (Chloridoideae). Putative homologs of Ts2 were isolated from species of Bouteloua. DNA sequences of this Ts2 homolog were obtained for multiple individuals of diclinous B. dimorpha and hermaphrodite B. hirsuta. Tests for neutrality indicate that Ts2 is evolving differently in these two species, and Ts2 may be under purifying selection in B. hirsuta . One explanation is that Ts2 has greater functional diversity in B. dimorpha than in B. hirsuta. In B. dimorpha, Ts2 may be involved in unisexual flower development as well as other functions common to both species. The results of this research indicate a common mechanism for male flower development in Bouteloua (Chloridoideae) and species of Panicoideae. This points to a common genetic mechanism involving Ts2 in these two groups. Analysis of molecular variation of Ts2 in Bouteloua indicates different evolutionary processes in a diclinous and a hermaphrodite species. This finding could also support the hypothesis of the function of Ts2 in Bouteloua.