Development of Male Flowers in Zizania aquatica (North American Wild-Rice; Gramineae)

We investigated the histology and developmental morphology of flowers of wild-rice (Zizania aquatica), a member of grass subfamily Oryzoideae, to determine whether male flowers in this species develop in a manner similar to those in the subfamily Panicoideae, a group that includes many species with unisexual flowers. Zizania has evolved unisexual flowers from hermaphrodite ancestors and is only distantly related to the Panicoideae; the origins of unisexual flowers are independent in the two taxa. Ontogenetic evidence indicates that many species within the subfamily Panicoideae develop male flowers by a process similar to that already described for maize (Zea mays), a panicoid grass. Unisexual male flowers in maize initiate both the stamen (androecium) and the pistil (gynoecium), but the gynoecium aborts early in development through the death of the subepidermal cells. Cell death in gynoecia of maize is known to be controlled by the product of the gene tasselseed2 (ts2), and an orthologue of ts2 has been shown to have the same effect in the sister genus Tripsacum. It seems likely that ts2 orthologues mediate cell death throughout the Panicoideae, but the phylogenetic range of the cell death mechanism is not known. In this study we show that male flowers of Z. aquatica show neither the distinctive pattern of cell death nor the ontogenetic timing of abortion that are characteristic of male flower formation in studied species of Panicoideae. This indicates that these unisexual flowers may be produced by an entirely different mechanism from that employed by the Panicoideae. Either ts2 does not control sex expression in Zizania, or it is deployed at a different time, and possibly in different tissues, with a different histological result. Our results indicate that the independent origins of male flowers in Gramineae apparently do not have a common system of genetic control.