Establishing the molecular identity of loci responsible for hybrid male sterility and its rescue between nematodes of Caenorhabditis briggsae and C. nigoni

Hybrid male sterility (HMS) is one of the most common hybrid incompatibilities
observed across species. It blocks gene flow between species or populations, eventually leading to speciation. However, the molecular mechanism of HMS remains elusive especially between animal species. The identification of genes/elements responsible for HMS across species, especially animal species, remains the central task to understanding the mechanisms of HMS and speciation.
Studies of HMS in animals have mostly focused on Drosophila species, in which
two pairs of genes responsible for HMS have so far been identified. The only gene responsible for HMS in vertebrates was identified between mouse subspecies. This gene regulates recombination and synapsis. The genes responsible for HMS in other animal species remain largely unknown. This is mainly due to the fact that identification of HMS genes is notoriously challenging, because these genes usually show little or no conservation between species. Therefore, it is urgent to establish new animal species models that can be used to rapidly identify, map and clone HMS loci. Efforts to sample nematode species have resulted in the discovery of Caenorhabditis nigoni that can mate and produce viable hybrid progeny with its close relative, C. briggsae. Both species are close relatives of the model organism, C. elegans. The F1 hybrid males between wild-type C. briggsae and C. nigoni are sterile when C. nigoni is the mother and inviable when C. briggsae is the mother. The presence of hybrid fertile females and sterile males paves the way for mapping and cloning the genes/elements underlying HMS in nematode species.
To facilitate the mapping and cloning of such genes/elements, we previously
generated approximately 100 independent green fluorescent protein (GFP) insertions in the C. briggsae genome, and these were individually backcrossed into the C. nigoni background for at least 15 generations. This allowed us to generate 110 strains, each of which carries a unique GFP-linked C. briggsae genomic fragment as an introgression fragment in an otherwise C. nigoni background. At least three genomic fragments from the right arm of the C. briggsae X chromosome produced complete HMS in C. nigoni when present as an introgression fragment, which provides a framework to identify the genes/elements underlying HMS. Despite the sterility of the hybrid F1 males between wild-type C. briggsae males and C. nigoni females, the hybrid F1 males
are fertile when C. nigoni females carrying either of the three introgression fragments are used as the mother. However, the molecular mechanisms of the HMS and its rescue remain unclear. We hypothesize that there is an epistatic interaction between the C. briggsae X-derived introgression and the C. nigoni genome that produces the HMS.
We propose to test this hypothesis by identifying the genes/elements responsible for the HMS and its rescue followed by functional characterization. To circumvent the recombination resistance between the two species due to unusually high sequence divergence and rearrangement, we will induce targeted recombination using the CRISPR/Cas9 system with dual guide RNAs. This will allow us to map the relevant loci within an interval of less than 50 Kbps. The genes/elements underlying the HMS and its rescue will be cloned through targeted recombination using CRISPR/Cas9 in combination with omplementation test and site-directed mutagenesis. The identification of such genes/elements will provide novel insights into HMS, and may shed light on the genetic mechanism of male sterility in humans.