The role of the BMP and Toll pathways in patterning the dorsal-ventral axis of the jewel wasp Nasonia vitripennis

Two conserved pathways (Toll and BMP) pattern the embryonic DV axis of Drosophila melanogaster. The Toll pathway dominates this process and is the source of all polarity and most of the patterning along the DV axis of the embryo. This is in contrast to other animals, where Toll signalling is not employed for DV patterning, and BMP signalling plays the major role. The early patterning role of Toll can therefore be considered as an evolutionary novelty, which appeared within the insect lineage. To further investigate the evolution of axial patterning in insects we analyzed the DV axis of the wasp Nasonia vitripennis, which is a member of the Hymenoptera, the most basally branching order of the Holometabola. A detailed analysis of the expression of DV marker genes that cover the entire embryonic axis revealed that the expression pattern just prior to the onset of gastrulation is almost identical in Nasonia and Drosophila. However, in Nasonia these patterns are initially very dynamic and evolve over time, which indicates that the upstream network for generating this pattern is highly diverged. In order to examine the functional basis of DV patterning in Nasonia we knocked down Toll and BMP signalling and showed that BMP is required for almost all DV patterning and that Toll has only a limited role. These results indicate that the ancestral role of Toll in insect embryos was only to induce mesoderm, while BMP played the dominant role. To further analyze and identify new target genes or possible new components of both pathways, we performed a transcriptome analysis of wild type, Toll knockdown, and BMP knockdown embryos. The outcome was a set of 262 genes that were significantly up or down regulated when compared among the different data sets. To analyze the expression pattern of those genes, we established a high throughput in situ technique. A first round of analysis revealed many interesting candidates that might play a role in DV patterning. However, further descriptive and functional analyses of all 262 identified genes are needed

The Phylogenetic Origin of oskar Coincided with the Origin of Maternally Provisioned Germ Plasm and Pole Cells at the Base of the Holometabola

The establishment of the germline is a critical, yet surprisingly evolutionarily labile, event in the development of sexually reproducing animals. In the fly Drosophila, germ cells acquire their fate early during development through the inheritance of the germ plasm, a specialized maternal cytoplasm localized at the posterior pole of the oocyte. The gene oskar (osk) is both necessary and sufficient for assembling this substance. Both maternal germ plasm and oskar are evolutionary novelties within the insects, as the germline is specified by zygotic induction in basally branching insects, and osk has until now only been detected in dipterans. In order to understand the origin of these evolutionary novelties, we used comparative genomics, parental RNAi, and gene expression analyses in multiple insect species. We have found that the origin of osk and its role in specifying the germline coincided with the innovation of maternal germ plasm and pole cells at the base of the holometabolous insects and that losses of osk are correlated with changes in germline determination strategies within the Holometabola. Our results indicate that the invention of the novel gene osk was a key innovation that allowed the transition from the ancestral late zygotic mode of germline induction to a maternally controlled establishment of the germline found in many holometabolous insect species. We propose that the ancestral role of osk was to connect an upstream network ancestrally involved in mRNA localization and translational control to a downstream regulatory network ancestrally involved in executing the germ cell program

Ancient and diverged TGF-β signaling components in Nasonia vitripennis

The transforming growth factor beta (TGF)-β signaling pathway and its modulators are involved in many aspects of cellular growth and differentiation in all metazoa. Although most of the core components of the pathway are highly conserved, many lineage-specific adaptations have been observed including changes regarding paralog number, presence and absence of modulators, and functional relevance for particular processes. In the parasitic jewel wasp Nasonia vitripennis, the bone morphogenetic proteins (BMPs), one of the major subgroups of the TGF-β superfamily, play a more fundamental role in dorsoventral (DV) patterning than in all other insects studied so far. However, Nasonia lacks the BMP antagonist Short gastrulation (Sog)/chordin, which is essential for polarizing the BMP gradient along the DV axis in most bilaterian animals. Here, we present a broad survey of TGF-β signaling in Nasonia with the aim to detect other lineage-specific peculiarities and to identify potential mechanisms, which explain how BMP-dependent DV pattering occurs in the early Nasonia embryo in the absence of Sog. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00427-014-0481-0) contains supplementary material, which is available to authorized users

Dorsoventral Polarity of the Nasonia Embryo Primarily Relies on a BMP Gradient Formed without Input from Toll

SummaryIn Drosophila, Toll signaling leads to a gradient of nuclear uptake of Dorsal with a peak at the ventral egg pole and is the source for dorsoventral (DV) patterning and polarity of the embryo [1–3]. In contrast, Toll signaling plays no role in embryonic patterning in most animals [4, 5], while BMP signaling plays the major role [6]. In order to understand the origin of the novelty of the Drosophila system, we have examined DV patterning in Nasonia vitripennis (Nv) [7], a representative of the Hymenoptera and thus the most ancient branch points within the Holometabola [8]. We have previously shown that while the expression of several conserved DV patterning genes is almost identical in Nasonia and Drosophila embryos at the onset of gastrulation, the ways these patterns evolve in early embryogenesis are very different from what is seen in Drosophila [7] or the beetle Tribolium [9]. In contrast to Drosophila or Tribolium, we find that wasp Toll has a very limited ventral role, whereas BMP is required for almost all DV polarity of the embryo, and these two signaling systems act independently of each other to generate DV polarity. This result gives insights into how the Toll pathway could have usurped a BMP-based DV patterning system in insects. In addition, our work strongly suggests that a novel system for BMP activity gradient formation must be employed in the wasp, since orthologs of crucial components of the fly system are either missing entirely or lack function in the embryo

Patterning the dorsal–ventral axis of the wasp Nasonia vitripennis

AbstractRegulatory networks composed of interacting genes are responsible for pattern formation and cell type specification in a wide variety of developmental contexts. Evolution must act on these regulatory networks in order to change the proportions, distribution, and characteristics of specified cells. Thus, understanding how these networks operate in homologous systems across multiple levels of phylogenetic divergence is critical for understanding the evolution of developmental systems. Among the most thoroughly characterized regulatory networks is the dorsal–ventral patterning system of the fly Drosophila melanogaster. Due to the thorough understanding of this system, it is an ideal starting point for comparative analyses. Here we report an analysis of the DV patterning system of the wasp, Nasonia vitripennis. This wasp undergoes a mode of long germ embryogenesis that is superficially nearly identical to that of Drosophila, but one that was likely independently derived. We have found that while the expression of genes just prior to the onset of gastrulation is almost identical in Nasonia and Drosophila, both the upstream network responsible for generating this pattern, and the downstream morphogenetic movements that it sets in motion, are significantly diverged. From this we conclude that many network structures are available to evolution to achieve particular developmental ends

Global analysis of dorsoventral patterning in the wasp Nasonia reveals extensive incorporation of novelty in a regulatory network

Gene expression cuffdiff output (gene_exp.diff) using Nasonia transcriptome annotation 1.2. This supplemented the main result given in Additional file 2, due to some discrepancies between the two annotations. See sections V–VI for how this result was produced. (XLSX 138 kb