Developmental expression of Ap-Vas proteins in aphids and flies reveals their evolutionary roles in insects.

Abstract

Germ cells depend on germline genes for their specification, migration and differentiation during development. Among these, the ATP-dependent RNA helicase gene vasa (vas) is one of the most conserved germline genes across animal phyla. Most organisms have a single copy of vas, but duplications are observed in some lineages. The evolutionary basis for this duplication remains unclear. Most insects, such as the fly Drosophila melanogaster, possess a single vas gene, but the pea aphid Acyrthosiphon pisum and related species have four paralogs. Previously, we identified Ap-vas1 as the germline-specific gene in aphids. Here, we examine the developmental expression of Ap-vas2, Ap-vas3 and Ap-vas4 at both mRNA and protein levels, compare their patterns with those of Ap-vas1 and analyse their developmental roles. Using antibodies specific to each Ap-Vas protein for immunostaining, we find that Ap-Vas2-4 proteins are not enriched in germ cells but exhibit distinct somatic expression patterns, suggesting roles in somatic development, such as chromosome segregation and nuage formation. We further assessed the divergent N-terminal regions of Ap-Vas proteins using chimeric constructs in Drosophila oocytes. Only the N-terminus of Ap-Vas1 partially mediated germ-plasm localisation, whereas those of Ap-Vas2-4 significantly impaired posterior localisation. These findings suggest how divergence in the N-terminal regions of Vas proteins may underlie their functional diversification in germline and somatic cell development across insects.