Evolution of embryonic pathways in Drosophila: patterns of constraint and positive selection support the hourglass model.

MENSCH JULIÁN; SERRA FRANCOIS; LAVAGNINO NICOLÁS; DOPAZO HERNÁN; HASSON ESTEBAN

San Diego

Congreso; 52° Drosophila Research Conference; 2011

The Genetics Society of America

Several lines of reasoning lead to the hypothesis that proteins participating together in a biological function will evolve at correlated rates. Also, similar rates of evolution among interacting genes are expected since related proteins are likely to be affected by common selective pressures, which either constraint or promote their divergence. Until recently, understanding developmental change and conservation has relied on embryological comparisons and analyses of single genes. The present study has taken a genomic approach to this classical problem, providing insights into how selection operates across early stages in Drosophila development. Two main molecular evo-devo hypotheses were tested: 1) according to Von Baer´s Law, early maternal expressing genes have lower rates of evolution compared to zygotic genes and 2) low incidence of positive selection will be observed since no divergence changes would occur in early stages of development. Our study of more than 2,000 embryonic genes demonstrate that genes expressed during middle embryogenesis (early zygotic genes) are more constrained (dN/dS<<1) than genes expressed earlier (maternal genes) or later in the egg development (late zygotic genes). This result points that middle embryogenesis, the stage at which the identity and position of body segments begins (segmental pattering), is the most conserved along Drosophila life cycle. Nevertheless, positively selected genes were found in all embryonic stages. Particularly, one positively selected network composed by nuclear pore proteins was identified within the maternal transcriptome. All in all, results concur with the idea that the origin of serially repeated segment formation seems to be the Drosophila phylotypic stage, but at the same time, we showed that early developmental modules are not limited to evolve by stabilizing selection.