Topology-based analysis of genes that regulate ovariole number in D. melanogaster reveals putative functional gene modules.
The anatomical organization and physiology of the gonad regulates how many gametes will be produced, released, become fertilized, and develop into embryos. Insect ovaries are subdivided into egg-producing units called ovarioles, which range from 1 to over 100 across Drosophila species, and whose mean number can positively predict egg-laying capacity. Ovariole number is thus a quantitative phenotype we can use to empirically determine the genetic basis of this dimension of fitness. We use different approaches to find genes responsible for making sure ovarioles are made correctly, and that the right number of them are made. D. melanogaster is the primary model organism that we use for this line of work.
We found that ovariole number is predicted by the number of cells in a specific precursor pool within the developing ovary, called terminal filament precursor cells (TFPCs). We showed that the conserved Hippo pathway, which regulates cellular proliferation in all animals and whose dysfunction is a major contributor to all known cancers, is a crucial regulator of TFPC number throughout development. Germ cells and soma use the Hippo pathway to communicate during ovarian development to maintain homeostatic proportions of the two cell types.
Our current and ongoing work on this system uses forward genetics and cell-type-specific transcriptomics to perform genome-wide determination of the genetic regulatory networks controlling fecundity. By applying topology-based network analysis of the Drosophila protein-protein interactome to the results of our RNAi screen of the nearly 800 members of all known animal signalling pathways, we realized that all conserved signalling pathways regulate ovariole number and egg latying by interacting in non-random, separable sub-networks that control distinct aspects of ovarian function.
We developed a dissociation protocol for the developing D. melanogaster ovary that separates the germ line from the soma at different stages of ovariole development, and are using this approach to understand the complete transcriptional response of each cell type under different genetic backgrounds that lead to altered ovariole numbers.
We believe that other insects form their ovarioles and establish ovariole number using cellular and genetic mechanisms that are different from those we have identified in Drosophila, and are establishing suitable study systems so that we can understand the evolution of ovary development at a broader scale.
RNA-seq analysis of isolated germ cells (pink) and isolated somatic cells (blue) from developing D. melanogaster ovaries at different stages of ovariole formation.
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