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

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.