INVESTIGATIONS INTO THE EVOLUTION OF SELF-FERTILE HERMAPHRODITISM AND REPRODUCTIVE ISOLATION USING CAENORHABDITIS NEMATODES
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Phenotypic variation, reproductive isolation, and the consequences of selfing are important issues in biology. The nematode genus Caenorhabditis includes both gonochoristic and androdioecious species. However, the natural genetic variants distinguishing reproductive mode remain unknown, and nothing is known about their reproductive isolation. Here, multiple facets with respect to the hybridization of the gonochoristic C. sp. 9 and the androdioecious C. briggsae are described. A large fraction of interspecies F1 arrest during embryogenesis, but a viable subset develops into fertile females and sterile males. Reciprocal parental crosses reveal asymmetry in male-specific viability, female fertility, and backcross viability. C. sp. 9 animals and F1 hybrids are inviable at cool temperatures that C. briggsae can tolerate, which may reflect their geographic distribution. The segregation of the selfing trait was evaluated in multiple hybrid generations using organismal, cellular, and molecular definitions of the trait. Selfing is recessive in F1 hybrids and was extremely rare in hybrid animals. All hybrid self-progeny are inviable. Multiple techniques were utilized to enrich the incidence of selfers in hybrid generations, and the genotyping of hybrid animals revealed segregation distortion at multiple loci. Additionally, it was found that C. briggsae hermaphrodites have lower numbers of self-progeny and reduced lifespan when mated with C. sp. 9 males. Fluorescent microscopy revealed that C. briggsae hermaphrodites previously mated with C. sp. 9 males accumulate germ line defects, and vital staining revealed that C. sp. 9 sperm are capable of ectopically invading C. briggsae hermaphrodite tissues. C. sp. 9 males with feminized germ lines were incapable of promoting the mating-dependent sterilization and lifespan reduction in C. briggsae hermaphrodites, although they were able to mate. This suggests that the degradation of mating-related traits in hermaphroditic lineages may lead to gonochoristic male mating-induced sterility and mortality in hermaphrodites, and therefore potentially to reproductive isolation between these lineages. Presumably, this is due to relaxed sexually antagonistic selection on traits associated with sperm competition. Collectively, these investigations are among the first in a C. briggsae/C. sp. 9 system that will likely prove fruitful for future studies in reproductive isolation and the evolution of self-fertile hermaphroditism.