MADS-box genes are involved in floral development and evolution

MADS-box genes encode transcription factors in all eukaryotic organisms thu s far studied. Plant MADS-box proteins contain a DNA-binding (M), an interv ening (I), a Keratin-like (K) and a C-terminal C-domain, thus plant MADS-bo x proteins are of the MIKC type. In higher plants most of the well-characte rized genes are involved in floral development. They control the transition from vegetative to generative growth and determine inflorescence meristem identity. They specify floral organ identity as outlined in the ABC model o f floral development. Moreover, in Antirrhinum majus the MADS-box gene prod ucts DEF/GLO and PLE control cell proliferation in the developing flower bu d. In this species the DEF/GLO and the SQUA proteins form a ternary complex which determines the overall "Bauplan" of the flower. Phylogenetic reconstructions of MADS-box sequences obtained from ferns, gym nosperms and higher eudicots reveal that, although ferns possess already MI KC type genes, these are not orthologous to the well characterized MADS-box genes from gymnosperms or angiosperms. Putative orthologs of floral homeot ic B- and C-function genes: have been identified in different gymnosperms s uggesting that these genes evolved some 300-400 million years ago. Both gym nosperms and angiosperms also contain a hitherto unknown sister clade of th e B-genes, which we termed Bsister. A novel hypothesis will be described su ggesting that B and Bsister might be involved in sex determination of male and female reproductive organs, respectively.