Origin and evolution of floral homeotic genes in green plants

摘要

Land plants have reproductive organs in both sporophyte and gametophyte generations. A flower is a sporophytic reproductive organ of angiosperms, composed of sepals, petals, stamens, and gynoecium (s). Haploid cells, mega and microspores are formedby meiosis in a stamen and a gynoecium. Microspores are enclosed in a multicellular male sporangium. Megaspores develop in a nucellus covered with inner and outer integuments. Both male and female gametophytic reproductive organs are reduced to few cells.Gymnosperms have simpler reproductive organs than angiosperms, and lack petals and sepals. A nucellus of a gymnosperm is covered with only one integument, and archegonia. gametophytic female reproductive organs enclosing egg cells are formed in the nucellus. Microspores are similar to angiosperms. In homosporous ferns, a sporophytic reproductive organ, a sporangium does not differentiate into males and female. Haploid spores formed in sporangia are dispersed and freeliving gametophytes develop on whichgametophytic reproductive organs, archegonia and antheridia are formed. Bryophytes have similar reproductive organs as ferns, but their sporophytes are more reduced than other land plants, and grow on gametophytes. The closest relative of land plants isCharophytes, freshwater green algae. In Charophytes, gametophytes with archegonia and antheridia are dominant, and they lack a sporophytic generation except the fertilized egg, whose first cell division is meiosis. In addition to the evolution of reproductive organs in each generation, evolution of morphological differences in sporophytic and gametophytic generations has not been well studied.