Evolution of a Plant Homeotic Multigene Family: Toward Connecting Molecular Systematics Andmolecular Developmental Genetics

Abstract

Molecular developmental genetics and systematics appear to have much to offerone another, but the phylogenetic perspective of systematics has yet to be applied consistentlyto molecular developmental problems. Yet phylogenetic analysis of the genes involved in theregulation of development is an important approach to elucidating orthologous relationshipsamong the members of multigene families. Recognizing orthologues, in turn, is a prerequisitefor the identification of general development mechanisms underlying the diversity of observedmorphologies. Gene phylogenies also provide objectively based hypotheses for studying theevolution of gene structure and expression. This approach is explored with the MADS-box familyof plant genes, many of whose members are involved in the regulation of floral development. Topologies of reconstructed phylogenies are used to address questions concerning intron evolution and expression patterns in the family. Detailed understanding of the molecular basis ofdevelopmental pathways should offer systematists new criteria for assessing the homologies ofmorphological characters used in phylogenetic analysis. This approach suggests a future directionfor "molecular" systematics, complementing its current role in inferring organismal phylogeniesdirectly from gene trees. Such a research program is likely to be time consuming but justifiablefor certain classes of morphological characters, particularly those marking ancient origins ofmajor groups.