Evolution of the Molecular Composition of the Nasonia Dorsoventral Patterning Gene Regulatory Network

Gene regulatory networks are vital for developmental processes. Modifications to theses networks allow for the emergence of novel developmental outputs, thus exploring how GRNs vary across phylogenies can provide insight on the evolution of development. The DV GRN of the Drosophila embryo is a well-characterized example of these networks, but is not representative of most insects. The wasp, Nasonia, undergoes a similar mode of embryogenesis, and prior to gastrulation has a nearly identical spatial expression of tissue specific marker genes, but the establishment of these patterns is quite divergent. To understand this evolutionary variation, we characterized Nasonia at a depth and resolution to conduct a meaningful comparative GRN analysis. The establishment of Toll signaling was examined using a candidate gene approach, and RNAi-RNAseq identified downstream differentially expressed genes. Comparison to Drosophila uncovered a set of conserved genes. However, a majority of the genes uncovered are unique to Nasonia. A subset of these novel genes is characterized by the presence of multiple Ankyrin repeats and a PRANC domain. These genes have no clear orthologs outside of Chalcidoidea, and are most similar to genes found in bacteria/viruses, suggesting acquisition by horizontal gene transfer. Furthermore, knockdown of these genes change gene expression, morphogenetic movements, and developmental timing, suggesting their functional integration into the GRN. To understand how the genomic incorporation of these genes led to their network incorporation, we developed tools to establish Melittobia as a model system. Initial results indicate that only a subset of Melittobia orthologs have conserved expression, indicating that developmental integration of these genes can occur relatively rapidly and specifically in different lineages. This work demonstrates that a conserved patterning output (blastoderm) can arise from GRNs that share little in terms of molecular composition. Networks can incorporate novel genes to increase robustness/repeatability of developmental processes. Even modest improvement of the network may be enough to allow genes to be maintained and fully integrated network components. However, certain network sections are highly resistant to change. Therefore, more studies like this need to be conducted in order to fully understand the process and pattern of GRN evolution.