Document Type

Honors Thesis


Evolutionary theory suggests that fitness decreases with hybridization as a result of negative epistatic interactions between alleles that rose independently in genetic backgrounds making hybrids incompatible with parents’ species. However, heterosis can occur as a result of the deleterious effects of negative epistatic interactions making hybrids fitter than their parents. To examine this, the fitness of first-generation (F1) hybrids of Tex and PA populations of Daphnia pulex was determined using body size as a life history trait. F1 hybrids obtained from crossing between seven Tex parental isolates and PA parental isolates were taken from stock and grown under standard conditions (18°C under 18:6 light: dark photoperiod; Scenedesmus obliquus concentration of 500,000 cells per ml) to measure their body size. The body size was measured using a Leica M125 microscope. The body size of F1 hybrids was significantly different from the parents in 37 of the 50 Tex hybrids while others performed equally well as the parents. The body size of F1 hybrids of the PA21 population was significantly different in 20 of the 29 hybrids examined. Variations were also found among F1 siblings and parental isolates. This experiment would be an addition to current research on hybridization to understand hybrid effects and predict genetic outcomes in hybrids in the field of agriculture and animal breeding.

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