FLORAL HOMEOTIC GENES WERE RECRUITED FROM HOMOLOGOUS MADS-BOX GENES PREEXISTING IN THE COMMON ANCESTOR OF FERNS AND SEED PLANTS

Flowers sensu late are short, specialized axes bearing closely aggrega ted sporophylls. They are typical for seed plants (spermatophytes) and are prominent in flowering plants sensu stricto (angiosperms), where they often comprise an attractive perianth. There is evidence that spe rmatophytes evolved from gymnosperm-like plants with a fern-like mode of reproduction called progymnosperms. It seems plausible, therefore, that the stamens/carpels and pollen sacs/nucelli of spermatophytes are homologous to fern sporophylls and sporangia, respectively, However, the exact mode and molecular basis of early seed and flower evolution is not yet known. Comparing flower developmental control genes to thei r homologs from lower plants that do not flower may help to clarify th e issue. We have isolated and characterized MADS-box genes expressed i n gametophytes and sporophytes of the fern Ceratopteris. The data indi cate that at least two different MADS-box genes homologous to floral h omeotic genes existed in the last common ancestor of contemporary vasc ular plants, some descendants of which underwent multiple duplications and diversifications and were recruited into novel developmental netw orks during the evolution of floral organs.