Ap-Vas1 distribution unveils new insights into germline development in the parthenogenetic and viviparous pea aphid: from germ-plasm assembly to germ-cell clustering

Targeting the distribution of germ-cell markers is a widely used strategy for investigating germline development in animals. Among these markers, the <i>vasa</i> (<i>vas</i>) orthologues, which encode ATP-dependent RNA helicases, are highly conserved. Previous studies have examined asexual (parthenogenetic) and viviparous embryos of the pea aphid <i>Acyrthosiphon pisum</i> using a cross-reacting Vas antibody. This study utilized a specific antibody against Ap-Vas1, a Vas orthologue in the pea aphid, to gain new insights into germline development. The Ap-Vas1-specific antibody facilitates earlier detection of germ-plasm assembly at the oocyte posterior, challenging the previous assumption that germ-plasm assembly begins only at the onset of embryogenesis. Treatment of oocytes and early embryos with cytoskeleton inhibitors suggests that germ-plasm assembly primarily depends on actin, in contrast to the fly <i>Drosophila melanogaster</i>, where both actin and microtubules are essential. Since pea aphids lack an orthologue of <i>osk</i>, which encodes the protein Osk responsible for anchoring Vas to the germ plasm in <i>Drosophila</i>, this suggests that pea aphids employ distinct mechanisms for <i>osk</i>- and microtubule-independent formation of the germ plasm. Moreover, the clustering of germ cells into germarium-like structures in the extraembryonic region before entering the embryos suggests a gonad formation process different from that in <i>Drosophila</i>, where germ cells begin to cluster into germaria after settling within the embryonic gonads. Therefore, the analysis of the Ap-Vas1 distribution provides a deeper understanding of germline development in asexual pea aphids, uncovering novel aspects of parthenogenetic and viviparous reproduction in insects.