The ancestral developmental tool kit of land plants

The origin of land plants from aquatic ancestors and their subsequent evolutionary radiation was due to major modifications in an ancestral developmental program allowing for plants of increasing complexity and stature. By comparing the developmental programs in different kinds of plants, we can understand how developmental differences lead to morphological differences. The ability to dissect the developmental genetic pathways of model angiosperm taxa such as Arabidopsis and maize has revealed many important developmental gene families that comprise the tool kit for flowering plant growth, patterning, and differentiation. This knowledge has been used to a limited extent as the basis for candidate gene approaches to explore the distribution and expression/function of a few developmental gene families in other flowering plants, gymnosperms, and to some extent, more distantly related plants. With the addition of nearly complete sequenced genomes for the moss Physcomitrella and the lycophyte Selaginella, comparisons of many more gene families are becoming possible. We performed searches of the unassembled Physcomitrella and Selaginella genomic sequences for some key developmental families. We combine these new data with information available in the literature from candidate gene approaches and present a first estimate of a suite of developmental genes that may have comprised the ancestral patterning tool kit of land plants. We conclude that the genome of the earliest embryophytes encoded homologues of many of the important developmental genes that have been identified in model angiosperm taxa, but the roles of these were probably somewhat different than the roles attributed to them in Arabidopsis and maize. Developmental gene families have diversified during land plant evolution, and genome complexity may correlate well with phylogenetic position.