Development of male flowers in Zizania aquatica (North American wild-rice;Gramineae)

We investigated the histology and developmental morphology of flowers of wi ld-rice (Zizania aquatica), a member of grass subfamily Oryzoideae, to dete rmine whether male flowers in this species develop in a manner similar to t hose 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 in dicates that many species within the subfamily Panicoideae develop male flo wers 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 d evelopment through the death of the subepidermal cells. Cell death in gynoe cia of maize is known to be controlled by the product of the gene tasselsee d2 (ts2), and an orthologue of ts2 has been shown to have the same effect i n the sister genus Tripsacum. It seems likely that ts2 orthologues mediate cell death throughout the Panicoideae, but the phylogenetic range of the ce ll 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 ont ogenetic timing of abortion that are characteristic of male flower formatio n in studied species of Panicoideae. This indicates that these unisexual fl owers 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 indepen dent origins of male flowers in Gramineae apparently do not have a common s ystem of genetic control.