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Sohini GhoshroyM.S., University of Connecticut, 2000 Contact Information: Graduate Advisor: |
Current research interest: Evolution of nitrogen metabolism in diatoms. My research concerns the evolution of the gene glutamine synthetase in basal lineages of red algae. I am also interested in the evolution of this gene in select green alga and cryptophytes. Oxygenic photosynthesis is distributed among four of the five supergroups in the eukaryote tree of life. Oxygenic photosynthesis evolved within the cyanobacterial lineage and was gained by eukaryotes via primary and secondary endosymbiotic events. A number of primary and secondary endosymbiotic associations resulted in an enormous amount of genetic diversity on which natural selection could act and numerous gene duplications, replacements and losses followed the endosymbiotic events as the symbiont genomes became integrated into the genetic architecture of the host cell. Thus, the metabolic pathways that we observe today can be chimeras involving proteins encoded by genes derived from either host or endosymbiotic genomes. Glutamine synthetase (GS) is an essential enzyme involved in nitrogen assimilation and there are three gene families (GSI, GSII, and GSIII). In many photosynthetic eukaryotes, GS isoenzymes are compartmentalized in the chloroplast and the cytosol. These isoenzymes can be members of the same family (e.g. the isoenzymes of vascular plants are members of the GSII family) or from different families (e.g. the isoenzymes of diatoms are members of the GSII and GSIII families). The GS families appear to be robust molecular markers and may be useful in unraveling the endosymbiotic histories and phylogenetic relationships among diverse lineages of organisms. In addition to being ecologically and commercially important, rhodophytes hold a central position in endosymbiosis. Rhodophytes gained their plastid via primary endosymbiosis and contributed plastids via secondary endosymbiosis. Despite their importance, there is little information concerning the molecular evolution and regulation of nitrogen assimilation in this group. This study seeks to expand our knowledge of the distribution of GSII genes in rhodophytes, which will eventually shed light on endosymbiotic gene rearrangement events. The lineages that arose via secondary endosymbiosis involving a red algal endosymbiont are collectively known as the chromalveolates. The chromalveolates are divided into two broad groups, chromists and alveolates. Chromists are further divided into cryptophytes, haptophytes and the heterokonts, while alveolates comprise dinoflagellates, apicomplexans and ciliates. Along with photosynthetic lineages, this broad group also includes non photosynthetic and parasitic organisms. At present, there are two major hypotheses suggesting the rise of the chromalveolate lineage. The chromalveolate hypothesis proposes that a single event of secondary endosymbiosis involving a red alga and a heterotrophic eukaryote gave rise to this lineage. The second hypothesis, one of serial endosymbiosis, which postulates that the cryptophytes were the first in line among the chromalveolate lineage to acquire the red secondary plastid and contributed it to other groups, such as the haptophyte, heterokonts and dinoflagellates in a serial manner. The proposed research has four major goals:
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