<p>Haldane’s rule states “when in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous sex”. Many studies regarding hybrid male sterility and in-viability have been done in order to better understand the process of speciation in males. Yet the study of speciation in hybrid females has been largely ignored. In our study we re-examined the extent of fertility in F1 reciprocal females of hybridization between <em>Drosophila simulans and D. mauritiana</em> species<em>.</em> Hybridization between these species produces fertile females and sterile males. Our goal was to address the following questions: 1. Are F1 hybrid females fully fertile? 2. Are there any maternal effects observed in reciprocal female hybrids? 3. Are there significant differences in ovariole numbers between the reciprocal hybrids? and 4. What is the state of the hybrid ovaries as a function of age? In order to answer these questions we looked at the level of oviposition and egg hatchability and the differences in ovariole numbers in pure species and F1 females. Our results indicated that the reciprocal hybrid females are not only fully fertile but they also showed heterosis. The heterosis observed in the hybrids can be attributed in part to the presence of maternal effects. The reciprocal hybrids also showed differences in ovariole number compared to each other and to the parental species. Our findings signify the importance of maternal effects as a potentially powerful mechanism for moderating the rates of evolution of speciation in hybrid females.</p> / Master of Science (MSc)