Glutamine synthetase gene family evolution in select lineages of photosynthetic eukaryotes

Glutamine synthetase (GS; EC 6.3.1.2), an essential enzyme is a link between carbon and nitrogen metabolism. It consists of three gene families (GSI, GSII and GSIII) with a broad but sporadic distribution among the three domains of life. Endosymbiotic gene transfer (EGT), horizontal gene transfer (HGT) and evolutionary associations of organisms have been uncovered using this gene. I have explored the evolutionary relationships of photosynthetic eukaryotes from the supergroup Archaeplastida (mainly rhodophyte and green algae) and Chromalveolata, using phylogenetic analyses of GS genes.;An interesting association observed between a green algal GS gene with eubacterial genes, merited further exploration. The implications of the association were; the gene representing a phylogenetic anomaly, a HGT event or a contamination. I explored this problem by increased taxon sampling within green algal and early diverging plant lineages and was able to identify numerous GS genes similar to Eubacteria within these lineages. Phylogenetic analyses confirmed a HGT from Eubacteria to the green algal-land plant lineages after the divergence of the primary photosynthetic lineages. Interestingly this gene has been lost in the vascular plants and replaced in function by genes arising from recent gene duplication.;Primary producers in the ocean are dominated by members of supergroup Chromalveolata. Evolutionary history of this group is complex and undergoing current revisions. I studied the distribution and evolutionary association of the GS genes within this group with the intension of expanding our knowledge of nitrogen metabolism and evolutionary relationships. GSII gene analyses identified three major groups indicating presence of orthologous and paralogous genes. A strong support was recovered for chromist monophyly in GSIII phylogeny, a first for a nuclear gene studied so far.;Importance of Rhodophytes in ecology, economics and evolution is well recognized. They are the donor of the secondary plastid of the chromalveolates. Within rhodophytes I explored GS diversity with the purpose of identifying the secondary plastid donor lineage/s. Increased diversity of GS is reported here. GS analyses supported the endosymbiotic origin of chromalveolate GS genes from rhodophytes. An evidence for chloroplast targeted GS is reported, which is the first of its kind.