Abstract

Many organisms require vitamins because they provide the precursors to essential enzyme cofactors important for metabolism. The requirement for vitamin B12 differs between diverse groups of organisms. For instance, higher plants and fungi do not require B12 , whereas animals do. Within the algal kingdom, the requirement for B12 differs between closely related species, so these organisms are an interesting group to focus on to try to understand how vitamin auxotrophy may have evolved. In algae vitamin B12 is an essential cofactor for methionine synthase (METH), which is involved in the synthesis of methionine. However, there is an alternative isoform of methionine synthase (METE), which catalyses the same reaction but without B12 . I set out to determine the cellular mechanisms underlying the evolution of vitamin B12 dependence in algae and have compared elements of B12 metabolism in 15 algal species with sequenced genomes. It appears that the methionine synthase isoform is a key determinant in the evolution of vitamin B12 auxotrophy in algae. Furthermore through studies of physiology and gene regulation, we have accumulated data that points to multiple losses of METE as being a key factor in the evolution of B12 dependence in algae. Evidence in support of this theory includes the identification of a METE unitary pseudogene in Volvox carteri, a close relative of C. reinhardtii, which suggests that B12 dependence evolved relatively recently in this lineage. Moreover, the observation that METE expression is repressed by vitamin B12 in a marine diatom, Phaeodactylum tricornutum, provides a mechanism for gene loss and helps to explain the likely cause for the evolution of B12 dependence in algae. Finally, I aim to transform V. carteri with METE from C. reinhardtii to see if this will rescue its B12 -dependent phenotype. Given that eukaryotes must obtain vitamin B12 from prokaryotes, the selective loss of METE in different algal species may have had important physiological and ecological consequences for these lineages.