Identification of Asymmetric Hybrid Incompatibility Loci by Systematic Introgression of C. nigoni Genomic Fragment into C. briggsae Background

Closely related species may mate with each other but produce hybrid progeny that often suffer from severe damages in fitness, including developmental arrest, sterility or reduced fertility, which is collectively referred to as hybrid incompatibility (HI). HI is widely observed across species and serves as a way of blocking gene exchanges between related species, therefore enabling relative stability of a given genome. Intriguingly, HI often demonstrates asymmetry. For example, viable progeny can be produced in a cross in one direction but not in the other. A global picture of HI will not emerge between a species pair until systematic crossings are performed in both directions.

Studies on HI mainly focus on model organisms, for which abundant tools have been developed. C. elegans as a model organism contributes little to the field due to lack of a sister species with which it can mate and produce viable progeny. Isolation of C. nigoni as a sister species for C. briggsae paves the way for studying the genetic and molecular mechanism of interspecific HI using nematodes. We have recently established a genome-wide HI landscape produced by C. briggsae genomic DNA in an otherwise C. nigoni background, the first of its type in nematode species. However, a complete picture of HI between the two is lacking because the HI is only mapped in a single direction. Importantly, asymmetric HIs are frequently observed between the two species, which were found not caused by the incompatibility between their mitochondrial and nuclear genomes, indicating a role of nuclear genome in forming the asymmetric HI.

To unravel the genetic bases of asymmetric HI between the two nematodes, we propose to develop random GFP labels in C. nigoni genome in a way similar to that we used in C. briggsae but using newly developed miniMos and CRISPR/Cas9 techniques. We plan to build 150 such strains and backcross all the GFP-linked C. nigoni genomic DNAs into C. briggsae for 15 generations followed by mapping of introgression boundaries. We will use the introgressions to be generated in the proposed study to map asymmetric HI loci mainly observed in F1 and F2 generations. Contrasting genome-wide HI landscapes produced by introgressions in opposite directions will allow us to draw a global picture of HI and its asymmetries between the two species. The transgenic and introgression strains as well as the genomic resources to be generated will facilitate the community to molecularly clone the identified HI loci. The proposed work combined with our previous work on HI study will establish the nematode pair as a powerful model for studying the molecular and genetic bases of HI between nematode species.