Reconstructing the evolution of novel developmental regulators

One of the central mysteries of the evolutionary process is how new organismal features emerge from ancestors lacking them. Shifts in reproduction are some of the most striking changes in the recent evolutionary histories of animals and plants. In Caenorhabditis nematodes, self-fertile hermaphrodites have repeatedly evolved from female ancestors by acquiring limited spermatogenesis. This novelty is an appealing subject of evolutionary developmental biology research. It involves a binary fate change (from egg to sperm) in the cells of a single tissue, the germ line. Germ cells are found across animals, making lessons learned here likely to have broader implications. Self-fertility also merits study because its appearance radically alters population genetics and the dynamics of sexual selection, with many potential downstream consequences. This project examines the changes to genes and proteins that enable self-fertility in two different species. By synthesizing molecular biology, developmental genetics, and evolution, one impact of this research is the identification of new molecular mechanisms using a comparative approach. The project is also an excellent platform for training in interdisciplinary science. This includes both the sustained work by research trainees at various career stages, as well as shorter interactions with teachers and transfer students. Several lines of evidence point to the evolution of a gene called fog-2 as a key step in the evolution of the C. elegans hermaphrodite. It is only found in C. elegans, and its inactivation by mutation reverts the XX sex to a functional female that must outcross to reproduce, like its ancestors. Previous research has found that the FOG-2 protein works with an ancient RNA-binding protein, GLD-1, to reduce the activity of a key female-promoting gene, tra-2. However, GLD-1 also regulates hundreds of other genes, and its loss produces a complex set of abnormalities not restricted to sex determination. A long-standing mystery has thus been why loss of FOG-2 only impacts sex determination. Recent studies indicate the specificity of FOG-2 for sex is due to a previously unrecognized interaction between FOG-2 and the newly formed TRA-2 protein. A second self-fertile species, C. briggsae, also relies on a recently evolved F-box protein (SHE-1). However, SHE-1 does not interact with C. briggsae GLD-1, nor with Cbr-TRA-2. As XX spermatogenesis evolved convergently in C. elegans and C. briggsae, sexual adaptation appears to be genetically flexible. The core scientific goal of this project is to reconstruct the steps that allowed FOG-2 and SHE-1 to emerge from a large family of related F-box proteins to become the essential regulators of hermaphrodite development they are today. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.