Sex Determination in Nematode Germ Cells

Rewiring the Sex-Determination Pathway During the Evolution of Self-Fertility

Although evolution is driven by changes in how regulatory pathways control development, we know little about the molecular details underlying these transitions. The TRA-2 domain that mediates contact with TRA-1 is conserved in Caenorhabditis. By comparing the interaction of these proteins in two species, we identified a striking change in how sexual development is controlled. Identical mutations in this domain promote oogenesis in Caenorhabditis elegans but promote spermatogenesis in Caenorhabditis briggsae. Furthermore, the effects of these mutations involve the male-promoting gene fem-3 in C. elegans but are independent of fem-3 in C. briggsae. Finally, reciprocal mutations in these genes show that C. briggsae TRA-2 binds TRA-1 to prevent expression of spermatogenesis regulators. By contrast, in C. elegans TRA-1 sequesters TRA-2 in the germ line, allowing FEM-3 to initiate spermatogenesis. Thus, we propose that the flow of information within the sex determination pathway has switched directions during evolution. This result has important implications for how evolutionary change can occur.

COP9 signalosome component CSN-5 stabilizes PUF proteins FBF-1 and FBF-2 in Caenorhabditis elegans germline stem and progenitor cells

RNA-binding proteins FBF-1 and FBF-2 (FBFs) are required for germline stem cell maintenance and the sperm/oocyte switch in Caenorhabditis elegans, although the mechanisms controlling FBF protein levels remain unknown. We identified an interaction between both FBFs and CSN-5), a component of the constitutive photomorphogenesis 9 (COP9) signalosome best known for its role in regulating protein degradation. Here, we find that the Mpr1/Pad1 N-terminal metalloprotease domain of CSN-5 interacts with the Pumilio and FBF RNA-binding domain of FBFs and the interaction is conserved for human homologs CSN5 and PUM1. The interaction between FBF-2 and CSN-5 can be detected in vivo by proximity ligation. csn-5 mutation results in the destabilization of FBF proteins, which may explain previously observed decrease in the numbers of germline stem and progenitor cells, and disruption of oogenesis. The loss of csn-5 does not decrease the levels of a related PUF protein PUF-3, and csn-5(lf) phenotype is not enhanced by fbf-1/2 knockdown, suggesting that the effect is specific to FBFs. The effect of csn-5 on oogenesis is largely independent of the COP9 signalosome and is cell autonomous. Surprisingly, the regulation of FBF protein levels involves a combination of COP9-dependent and COP9-independent mechanisms differentially affecting FBF-1 and FBF-2. This work supports a previously unappreciated role for CSN-5 in the stabilization of germline stem cell regulatory proteins FBF-1 and FBF-2.

Orthologs of the Caenorhabditis elegans heterochronic genes have divergent functions in Caenorhabditis briggsae

The heterochronic genes of Caenorhabditis elegans comprise the best-studied pathway controlling the timing of tissue and organ formation in an animal. To begin to understand the evolution of this pathway and the significance of the relationships among its components, we characterized 11 Caenorhabditis briggsae orthologs of C. elegans heterochronic genes. Using CRISPR/Cas9, we made a variety of alleles and found that several mutant phenotypes differ in significant ways from those of C. elegans. Although most mutant orthologs displayed defects in developmental timing, their phenotypes could differ in which stages were affected, the penetrance and expressivity of the phenotypes, or by having additional pleiotropies that were not obviously connected to developmental timing. However, when examining pairwise epistasis and synergistic relationships, we found those paralleled the known relationships between their C. elegans orthologs, suggesting that the arrangements of these genes in functional modules are conserved, but the modules' relationships to each other and/or to their targets has drifted since the time of the species' last common ancestor. Furthermore, our investigation has revealed a relationship between this pathway to other aspects of the animal's growth and development, including gonad development, which is relevant to both species.

The Caenorhabditis elegans cullin-RING ubiquitin ligase CRL4DCAF-1 is required for proper germline nucleolus morphology and male development

Cullin-RING ubiquitin ligases (CRLs) are the largest class of ubiquitin ligases with diverse functions encompassing hundreds of cellular processes. Inactivation of core components of the CRL4 ubiquitin ligase produces a germ cell defect in Caenorhabditis elegans that is marked by abnormal globular morphology of the nucleolus and fewer germ cells. We identified DDB1 Cullin4 associated factor (DCAF)-1 as the CRL4 substrate receptor that ensures proper germ cell nucleolus morphology. We demonstrate that the dcaf-1 gene is the ncl-2 (abnormal nucleoli) gene, whose molecular identity was not previously known. We also observed that CRL4DCAF-1 is required for male tail development. Additionally, the inactivation of CRL4DCAF-1 results in a male-specific lethality in which a percentage of male progeny arrest as embryos or larvae. Analysis of the germ cell nucleolus defect using transmission electron microscopy revealed that dcaf-1 mutant germ cells possess significantly fewer ribosomes, suggesting a defect in ribosome biogenesis. We discovered that inactivation of the sperm-fate specification gene fog-1 (feminization of the germ line-1) or its protein-interacting partner, fog-3, rescues the dcaf-1 nucleolus morphology defect. Epitope-tagged versions of both FOG-1 and FOG-3 proteins are aberrantly present in adult dcaf-1(RNAi) animals, suggesting that DCAF-1 negatively regulates FOG-1 and FOG-3 expression. Murine CRL4DCAF-1 targets the degradation of the ribosome assembly factor periodic trptophan protein 1 (PWP1). We observed that the inactivation of Caenorhabditis elegansDCAF-1 increases the nucleolar levels of PWP1 in the germ line, intestine, and hypodermis. Reducing the level of PWP-1 rescues the dcaf-1 mutant defects of fewer germ cell numbers and abnormal nucleolus morphology, suggesting that the increase in PWP-1 levels contributes to the dcaf-1 germline defect. Our results suggest that CRL4DCAF-1 has an evolutionarily ancient role in regulating ribosome biogenesis including a conserved target in PWP1.

A case of hermaphroditism in the gonochoristic sea urchin, Strongylocentrotus purpuratus, reveals key mechanisms of sex determination†

Sea urchins are usually gonochoristic, with all of their five gonads either testes or ovaries. Here, we report an unusual case of hermaphroditism in the purple sea urchin, Strongylocentrotus purpuratus. The hermaphrodite is self-fertile, and one of the gonads is an ovotestis; it is largely an ovary with a small segment containing fully mature sperm. Molecular analysis demonstrated that each gonad producedviable gametes, and we identified for the first time a somatic sex-specific marker in this phylum: Doublesex and mab-3 related transcription factor 1 (DMRT1). This finding also enabled us to analyze the somatic tissues of the hermaphrodite, and we found that the oral tissues (including gut) were out of register with the aboral tissues (including tube feet) enabling a genetic lineage analysis. Results from this study support a genetic basis of sex determination in sea urchins, the viability of hermaphroditism, and distinguish gonad determination from somatic tissue organization in the adult.