Evidence against a germ plasm in the milkweed bugOncopeltus fasciatus, a hemimetabolous insect

Abstract

SummaryPrimordial germ cell (PGC) formation in holometabolous insects like Drosophila melanogaster relies on maternally synthesised germ cell determinants that are asymmetrically localised to the oocyte posterior cortex. Embryonic nuclei that inherit this “germ plasm” acquire PGC fate. In contrast, historical studies of basally branching insects (Hemimetabola) suggest that a maternal requirement for germ line genes in PGC specification may be a derived character confined principally to Holometabola. However, there have been remarkably few investigations of germ line gene expression and function in hemimetabolous insects. Here we characterise PGC formation in the milkweed bug Oncopeltus fasciatus, a member of the sister group to Holometabola, thus providing an important evolutionary comparison to members of this clade. We examine the transcript distribution of orthologues of 19 Drosophila germ cell and/or germ plasm marker genes, and show that none of them localise asymmetrically within Oncopeltus oocytes or early embryos. Using multiple molecular and cytological criteria, we provide evidence that PGCs form after cellularisation at the site of gastrulation. Functional studies of vasa and tudor reveal that these genes are not required for germ cell formation, but that vasa is required in adult males for spermatogenesis. Taken together, our results provide evidence that Oncopeltus germ cells may form in the absence of germ plasm, consistent with the hypothesis that germ plasm is a derived strategy of germ cell specification in insects.

BMP signaling is required for the generation of primordial germ cells in an insect

“…We previously hypothesized that the inductive mode was ancestral among metazoans and that the inheritance mode had evolved independently in multiple derived lineages (1,2). Consistent with this hypothesis, multiple basally branching insects do not segregate maternally provided germ plasm, unlike the relatively derived Drosophila model (3,4). However, experimental evidence for the inductive mode was available only for salamanders (5,6) and mice (7)(8)(9)(10), and to date, inferences of induction outside of vertebrates have been based on gene expression and cytological data (1,(11)(12)(13)(14)(15)(16).…”

The evolution of insect germline specification strategies

“…More recently, the absence of maternal provision was demonstrated in the milkweed bug Oncopeltus fasciatus (a hemipteran member of the Paraneoptera, the sister group to the Holometabola) [31] (Fig. 2).…”

“…Instead, the first specific localization of germline genes such as vasa , tudor , nanos , and piwi occurs late in the cellular blastoderm stage at the posterior pole, just as the first signs of embryonic morphogenesis are taking place. So far the molecular basis of this zygotic induction of germ cell fate is not known [31]. …”

“…The honeybee germline is established very late in embryogenesis in association with the mesoderm [29,45,46], which is at least superficially reminiscent to the emergence of germ cells from among the mesoderm in Gryllus . In contrast, Tribolium germ cells form at the posterior pole at the onset of gastrulation and ingress to the interior of the embryo [41], in a manner very similar to Oncopeltus and other hemipterans ([31], and references therein).…”

Hexapoda: Comparative Aspects of Later Embryogenesis and Metamorphosis

“…A requirement for vasa in germ cell development also was not detected in the hemimetabolous milkweed bug, Oncopeltus fasciatus , using RNAi (EwenCampen et al 2013a ). In this species, vasa and nanos are ubiquitously expressed throughout the egg during oogenesis and early blastoderm stages and become concentrated in the posterior pole at the time of germ cell formation (Ewen-Campen et al 2013a ).…”