An ancestral MADS-box gene duplication occurred before the divergence of plants and animals

Changes in genes encoding transcriptional regulators can alter development and are important components of the molecular mechanisms of morphological e volution. MADS-box genes encode transcriptional regulators of diverse and i mportant biological functions. In plants, MADS-box genes regulate flower, f ruit, leaf, and root development. Recent sequencing efforts in Arabidopsis have allowed a nearly complete sampling of the MADS-box gene family from a single plant, something that was lacking in previous phylogenetic studies. To test the long-suspected parallel between the evolution of the MADS-box g ene family and the evolution of plant form, a polarized gene phylogeny is n ecessary. Here we suggest that a gene duplication ancestral to the divergen ce of plants and animals gave rise to two main lineages of MADS-box genes: Typel and Typell. We locate the root of the eukaryotic MADS-box gene family between these two lineages. A novel monophyletic group of plant MADS domai ns (AGL34 like) seems to be more closely related to previously identified a nimal SRF-like MADS domains to form Typel lineage. Most other plant sequenc es form a clear monophyletic group with animal MEF2-like domains to form Ty peII lineage. Only plant TypeII members have a K domain that is downstream of the MADS domain in most plant members previously identified. This sugges ts that the K domain evolved after the duplication that gave rise to the tw o lineages. Finally, a group of intermediate plant sequences could be the r esult of recombination events. These analyses may guide the search for MADS -box sequences in basal eukaryotes and the phylogenetic placement of new ge nes from other plant species.