Theses and Dissertations from UMD
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Item INVESTIGATIONS INTO THE EVOLUTION OF SELF-FERTILE HERMAPHRODITISM AND REPRODUCTIVE ISOLATION USING CAENORHABDITIS NEMATODES(2013) Woodruff, Gavin; Haag, Eric; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.Item Reproductive isolation and X chromosome meiotic drive in Cyrtodiopsis stalk-eyed flies(2008-04-24) Christianson, Sarah Jane; Wilkinson, Gerald S; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Haldane's rule states that when one sex of hybrids shows sterility or inviability it tends to be the heterogametic sex. This pattern is considered a fundamental law of speciation, and is known to be caused by several separate mechanisms. One such mechanism may be the divergence at loci affecting sex chromosome meiotic drive. Cyrtodiopsis dalmanni and C. whitei are sister species of stalk-eyed fly found in Southeast Asia and in which a sizable proportion of wild-caught males produce female-biased broods due to X chromosome meiotic drive. I cross multiple populations of these two species and use QTL mapping and DNA sequencing techniques to test three predictions of the meiotic drive hypothesis of Haldane's rule. In chapter 1, my results show that male hybrid sterility is the fastest-evolving form of reproductive isolation in these species, consistent with Haldane's rule. In chapter 2, I show that DNA sequence evolution is consistent with a pattern of repeated selective sweeps associated with X-linked meiotic drive. In chapter 3, I report the discovery of autosomal suppressors of drive, a Y-linked suppressor, a cryptic (suppressed) driver, an inviability effect of drive, and several QTL for sterility and one QTL for male-biased progeny sex ratios. The suppressors and cryptic driver support the conclusion, from chapter 2, that meiotic drive and suppressors have evolved repeatedly in response to each other. The sterility and sex ratio QTL did not map to the same genetic marker interval, which is not necessarily inconsistent with the drive hypothesis if sufficient time has elapsed to allow additional sterility loci to evolve. Overall, these results provide several lines of support for the meiotic drive hypothesis of Haldane's rule, which is rapidly gaining traction among researchers in the field of speciation. This conclusion suggests that attempts should be made to identify specific genes affecting meiotic drive and male hybrid sterility in Cyrtodiopsis.