MADS evolution : insights into evolutionary changes in transcription factors and their binding sites

Although most flowers follow a conserved 'bauplan' consisting of sepals, petals, stamens and carpels, there is a remarkable amount of morphological diversity. Interestingly, all flowers are specified by the conserved (A)BCE-model. Most of the transcription factors in this model belong to the MADS-domain family. We examined how these transcription factors and their binding sites in the genome evolved, as a first step to elucidate how diversity in flower morphology has been created. We analyzed the evolution of transcription factor binding sites by comparing binding sites of the major floral regulator SEPALLATA3 between two closely related Arabidopsis species, as well as between A. thaliana ecotypes. We found substantial overlap in transcription factor binding profiles between ecotypes, but limited overlap between the related species. We also assessed how transcription factors themselves can change in their properties by analyzing the divergence between paralogs. We examined how the PISTILLATA paralogs in Tarenaya hassleriana diverged, as this species occupies an interesting position in the eudicot phylogeny. We also studied whether divergence of the APETALA3 paralogs in Aquilegia could explain the specification of an additional floral organ in this genus. In both cases, we conclude that the paralogs diverged from each other in their biochemical properties. In the future, it would be interesting to assess how these changes in transcription factors and their binding sites affect floral regulatory networks and ultimately floral shape.