July 29, 2022
Sequencing of Saccharomyces cerevisiae in the 1990s revealed blocks of duplicated genes, suggesting an ancient whole-genome duplication. The original duplication has been postulated as a means to restore fertility to an interspecies hybrid of ancestral yeast parents; however, the question remains as to why so many paralog doublets have been retained through eons of evolution. Are these paralogs the residual clutter that has yet to be selected away, or has it represented an opportunity that created positive selection? Said another way, if every gene once had a twin, why have some been retained and others lost?
To dig further into understanding doublet evolution, a study by Purkanti and Thattai in a recent issue of Scientific Reports looks at the functional significance of these yeast paralog doublets from an evolutionary perspective. To ask whether retention of doublets across sub-clades of the yeast phylogenic tree might suggest selective advantage, they looked at which Gene Ontology biological processes showed the greatest enrichment of observed over expected for the 887 genes with conserved doublets. The greatest enrichment for over-represented doublets was in the GO process of endocytosis, with its parent term of vesicle-mediated transport also showing significant enrichment.
The authors then assigned vesicle-traffic genes to function-specific and pathway-specific modules to ask about doublet retention. Among the modules, the greatest enrichment of doublets was observed for coat/adaptor genes and lipid control genes. Other modules, such as the endosomal sorting complex required for transport (ESCRT), had no doublets at all, i.e., the genes in this module have completely reverted to singletons.
Intriguingly, the classes with singletons and the classes with doublets can be broadly grouped, where doublets tend to be retained in secretory and early endocytic pathways, whereas singletons largely act in retrograde Golgi traffic and late endocytic steps.
The authors separated all 360 ancestral vesicle traffic doublets into two groups: those that are retained as doublets in present-day species and those that are not. They looked at the nucleotide sequence identity as a proxy for evolutionary rate and found that retention of doublets is strongly associated with lower evolutionary rates. From this they conclude that doublet retention is under significant selection, as the purifying selection that lowers evolution rates is well correlated with functional significance.
While it may be mere coincidence that budding yeast happened to undergo ancestral genome duplication, it appears that this event has proven fortuitous for making S. cerevisiae even more useful as a model. As revealed in this study, the ease of manipulation and the vast knowledge base in yeast makes it possible to ask these deeply important questions about evolutionary impact on paralog doublets.
Categories: Research Spotlight