Evolution of the monosaccharide transporter gene family in land plants

Monosaccharide transporters (MSTs) are integral membrane proteins that transport sugars across the plasma membrane. In the model flowering plant Arabidopsis thaliana, there are 53 MSTs that comprise seven subfamilies. In this dissertation, I first characterized the subfamily structure of the greater MST gene family in the major land plant lineages by searching available expressed sequence tag (EST) databases for orthologs of each subfamily. Results revealed differential subfamily expression and expansion across lineages with six of the seven Arabidopsis subfamilies present in the early divergent mosses, with all seven subfamilies present in the seed plants.; A search of the rice (Oryza sativa) genome revealed 65 putative MSTs. Combined phylogenetic analysis and chromosome mapping revealed three subfamilies to be greatly expanded by segmental duplications. Estimation of gene duplicate divergence dates using a protein molecular clock revealed divergence of most subfamilies occurred in the Proterozoic, with expansion of the large subfamilies continuing through the Cenozoic. Estimation of protein sequence divergence and positive selection at codon sites in 24 paralog pairs revealed a complex picture, with some fraction of duplicate pairs showing divergence patterns consistent with the neofunctionalization, subfunctionalization, and subneofunctionalization models of gene duplicate divergence.; An analysis of the relationships between protein sequence evolutionary rate and factors that may constrain evolution revealed significant, moderate negative correlations with gene expression level, gene expression breadth, and gene age. The strongest (positive) correlation was with the number of duplications in a gene's history, giving support to the allelic divergence model and the role of adaptive evolution in gene family expansion. Analysis of gene expression patterns in phylogenetic context revealed a complex picture consistent with several models of gene duplicate divergence, including subfunctionalization and subneofunctionalization.; Last, to evaluate specific mechanisms of sugar transport in non-seed plants, an analysis of sugar uptake in gametophytes of the homosporous fern Ceratopteris richardii revealed that they utilize exogenous sugars for significantly enhanced growth rate. There was no evidence for expression of disaccharide transporters. A review of previous functional studies of MSTs in higher plants suggests that members of the AZT subfamily are likely candidates for expression at the gametophyte-sporophyte boundary.