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Abstract
Improving reproduction in swine has been accomplished through selection for increased ovulation rate and litter size. Quantitative methodology relies upon estimating the cumulative effect of all loci in the genome. Identification of individual loci affecting reproduction will allow for increased accuracy of phenotypic selection. Fifty-eight microsatellite markers were scored in an F2 population of pigs developed at the University of Nebraska. The population was produced by crossing a line previously selected for ten generations using an index of ovulation rate and embryonic survival and a line selected at random. The lines were from a common base population and differed by 6.7 eggs ovulated and 3.1 fetuses at 50 d gestation following ten generations of index selection. Three replicates of F1 matings were made, but only the first replicate was used in this analysis (n = 114 F2 females). Measurements were made for ovulation rate, number of fully-formed pigs and number of mummified pigs. Data were analyzed using the method developed by Haley et al. (1994) with a modification to include a random animal effect in the statistical model. Likelihood-ratio tests were performed to determine significance of a quantitative trait locus (QTL) effect using the approximate method described by Haley et al. (1994) and by deviating the log-likelihood for the full model from the log-likelihood for the reduced model. A QTL for ovulation rate was found on chromosome eight (P$<$.001). Evidence was found that QTL affecting ovulation rate are on chromosomes 4 (P$<$.10), 13 and 15 (P$<$.05). These results were not significant for an experiment-wise threshold value of P$<$.001. Results for number of fully-formed pigs and number of mummified pigs were not consistent between the two methods of computing the likelihood-ratio making the interpretation of these results unclear. The addition of the second and third replicates will increase the F2 population size to approximately 400 females and should provide better evidence for presence of QTL affecting reproductive traits in this population.
