Scott Baird (Advisor)
Master of Science (MS)
Reproductive isolation results when members of the same species cannot reproduce due to either prezygotic or postzygotic mechanisms, therefore restricting gene flow between populations. A leading model describing reproductive isolation developed by J.B. Haldane (1922) states, "When offspring from two different animal species have one sex that is rare, absent, or sterile, that sex is the heterozygous (XO) or heterogametic (XY) sex". Haldane's rule is illustrated among various animal taxa regardless of which sex is heterogametic. To date, Caenorhabditis mating tests are the only example of Haldane's rule that is caused by sexual transformation and not gender specific lethality or sterility. Crosses between C. briggsae strain AF16 males and C. remanei strain EM464 females resulted in all female F1 hybrids. The F1 hybrids were all phenotypically female even though some hybrids were genetically XO (Baird et al., 1992, Baird, 2002). The absence of male hybrids resulted from sex reversal rather than male specific lethality. This suggested that perhaps dysgenic interactions within the sex determination pathway of Caenorhabditis may serve as the mechanism for reproductive isolation. Utilizing the previously identified C. briggsae tra-2 gene as a candidate gene for sexual transformation, haplotype crosses were set up between C. remanei males and C. briggsae females from temperate and tropical strains. Results indicated that the sex reversal phenotype did not map to both the tropical and temperate clades. AF16, a tropical strain, was the only strain that exhibited the sex reversal phenotype. The tra-2 gene was also analyzed for selection pressure by comparing nonsynonymous and synonymous substitutions in the available C. briggsae tra-2 sequences. Data showed no evidence of natural selection acting on the gene region, indicating that perhaps the selective pressures have relaxed since the time of speciation.
Department or Program
Department of Biological Sciences
Year Degree Awarded
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