The use of phylogenetic data to address the importance of individual genes in a complex phenotype or in the evolution of a particular trait, is an approach that has received little attention. In collaboration with The Green Plant Phylogeny Research Coordination Group ("Deep Green"), we have initiated a study of the evolution of desiccation tolerance. Phylogenetic analyses suggest that desiccation tolerance was primitively present in the bryophytes (basal-most living clades of land plants), but was lost early in the evolution of tracheophytes. Desiccation tolerance has re-evolved in seeds and pollen and vegetatively in Selaginella, the ferns, and at least eight independent evolutions in the angiosperms. In the moss Tortula ruralis, we have identified several genes that appear to be intimately involved in desiccation tolerance. One of these genes, a rehydrin Tr 288, accumulates transcript at high levels in response to desiccation that are only used upon rehydration. This gene has physical properties similar to the stress induced dehydrin proteins. Tr 288 however, has little sequence similarity with dehydrins other than the presence of a rudimentary K-box sequence at its carboxy terminus. Our working hypothesis is that Tr288 is an ancestral dehydrin and as such may be a useful marker in a phylogenetic analysis of desiccation tolerance. We have devised a PCR based strategy, utilizing the highly repetitive nature of Tr288, to track the presence of Tr288 homologs in other species. Our studies indicate that we can find homologs within the Tortula ruralis complex and we are now expanding into other bryophyte groups and into the tracheophyte lineages. In particular we will be targeting the exemplar species identified by Deep Green in their synthesis of a phylogenetic tree for all land plants.

Key words: Tortula ruralis , bryophytes, desiccation tolerance, rehydrin, stress biology