Genome-Wide Introgression Revealed Pervasive Hybrid Incompatibilities (HI) between Caenorhabditis species

Systematic characterization of hybrid incompatibility (HI) between related species remains the key to understanding speciation. The genetic basis of HI has been intensively studied in Drosophila species, but remains largely unknown in other species, including nematodes. This is mainly due to the lack of a sister species with which C. elegans can mate and produce viable progeny. The recent discovery of a C. briggsae sister species, C. sp.9, opened up the possibility of dissecting the genetic basis of HI in nematode species. However, paucity of molecular and genetic tools has prevented the precise mapping of HI loci between the two species. To systematically isolate the HI loci between the nematode species pair, we first generated 96 chromosomally integrated, independent GFP insertions in the C. briggsae genome. We next mapped the GFP insertion site into defined locations using a method we had developed earlier. The dominant and visible markers facilitated the directional crossing of its linked genomic sequences into C. sp.9. We then backcrossed each individual marker into C. sp.9 for at least 15 generations and produced 111 independent introgression lines, which together represent most of the C. briggsae genome. We finally dissected the HI patterns by scoring embryonic lethality, larval arrest, sex ratio, fertility, male sterility and inviability in a subset of the introgression lines, and identified pervasive HIs between the two species. The study produced a genome-wide landscape of HI between nematode species for the first time. The initial crossing results confirmed the Haldane?s rule and the fertility data from homozygous introgressions supported the rule of large X effect. The large collection of introgression lines allows mapping of numerous HI loci into defined genomic regions between C. briggsae and C. sp.9, thus facilitating further characterization of their genetic and molecular mechanisms. Importantly, the study permits comparative analysis of speciation genetics between nematodes and other species.