Candidate gene analysis for loci affecting litter size and ovulation rate in swine

A candidate gene approach was used to determine whether specific loci expla in responses in ovulation rate (OR) and number of fully formed (FF), live ( NBA), stillborn, and mummified pigs at birth observed in two lines selected for ovulation rate and litter size compared with a randomly selected contr ol line. Line IOL was selected for an index of OR and embryonic survival fo r eight generations, followed by eight generations of two-stage selection f or OR and litter size. Line C was selected at random for 16 generations. Li ne COL, derived from line C at Generation 8, underwent eight generations of two-stage selection. Lines IOL and C differed in mean EBV by 6.1 ova and 4 .7 FF, whereas lines COL and C differed by 2.2 ova and 2.9 FF. Pigs of Gene ration 7 of two-stage selection lines were genotyped for the retinol bindin g protein 4 (RBP4, n = 190) and epidermal growth factor (EGF, n = 189) loci , whereas pigs of Generations 7 and 8 were genotyped for the estrogen recep tor (ESR, n = 523), prolactin receptor (PRLR, n = 524), follicle-stimulatin g hormone beta (FSH beta, n = 520), and prostaglandin-endoperoxide synthase 2 (PTGS2, n = 523) loci. Based on chi-square analysis for homogeneity of g enotypic frequencies, distributions for PRLR, FSH beta, and PTGS2 were diff erent among lines (P < 0.005). Differences in gene frequencies between IOL vs C and COL vs C were 0.33 +/- 0.25 and 0.16 +/- 0.26 for PRLR, 0.35 +/- 0 .20 and 0.15 +/- 0.24 for FSH<beta>, and 0.16 +/- 0.16 and 0.08 +/- 0.18 fo r PTCS2. Although these differences are consistent with a model of selectio n acting on these loci, estimates of additive and dominance effects at thes e loci did not differ from zero (P > 0.05), and several of them had signs i nconsistent with the changes in allele frequencies. We were not able to fin d significant associations between the polymorphic markers and phenotypes s tudied; however, we cannot rule out that other genetic variation within the se candidate genes has an effect on the traits studied.