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The Ped gene, a gene that influences the rate of embryonic cleavage division, birth weight, litter size and weaning weight, is at least partially encoded by gene(s) that specify the Qa-2 antigen. Two congenic strains of mice, B6.K1 (Qa-2 negative) and B6.K2 (Qa-2 positive), which differ only at the Q region of the mouse major histocompatibility complex (MHC), were tested for the effect of the presence or absence of Qa-2 antigen on litter size, duration of gestation and embryo survival. It was confirmed that B6.K1 (Qa-2 negative) mice have smaller litters than do B6.K2 (Qa-2 positive) mice. In addition, the duration of gestation for the B6.K1 mice was found to be longer than the duration of gestation for the B6.K2 mice. Finally, a comparison of the relative survival of Qa-2-positive and Qa-2-negative mice in a single uterine environment showed the preferential survival of mice expressing the Qa-2 antigen. Thus, the presence of Qa-2 antigen appears to be advantageous for reproductive success.

Summary. Embryonic cell number in miniature pigs inbred for specific SLA haplotypes (a, c, and d) was determined on Day 6 by nuclear staining and, on Days 9 and 11, by DNA analyses (first day of oestrus = Day 0). Pigs exhibiting first behavioural oestrus at 08:00 h were hand-mated to an SLA homozygous boar 12 and 24 h later. Numbers of embryos flushed from uteri at 08:00–10:00 h on Days 6, 9 and 11 were greater (P < 0·05) for SLA^d females than for SLA^a or SLA^c females, which did not differ (8·2 vs 6·8 and 6·2, respectively). Recovery rates (embryos recovered/CL number) were similar, averaging 75·8% for all three SLA haplotypes. Embryos from SLA^d dams contained fewer blastomeres (23 cells) on Day 6 than did embryos from SLA^a (89 cells) or SLA^c (79 cells) females. The reduced cell numbers of SLA^d vs SLA^a or SLA^c embryos continued to Day 9 (28 vs 107 and 67 ng DNA/embryo) and Day 11 (167 vs 674 and 586 ng DNA/embryo). These results suggest an effect of the SLA complex on preimplantation embryonic development.

Keywords: pig; SLA complex; preimplantation; embryonic development

Summary. Systemic blood was collected from and surgery performed on sows of 3 strains of miniature swine bred for specific SLA (swine MHC) haplotypes (a, c and d) from Day 2 to Day 6 after mating (first day of mating = Day 0). Ovulation rate was determined by counting corpora lutea and embryos were flushed from the uterus. Progesterone, oestradiol-17β and oestrone were quantitated in blood plasma and uterine flushings by RIA. SLA^d/d females had a higher ovulation rate than SLA^a/a or SLA^c/c females (11·50 ± 0·87 vs 9·11 ± 0·68 and 8·17 ± 0·83, respectively; P < 0·01). Oestrone was higher than oestradiol-17β in systemic plasma (56·5 ± 6·4 vs 33·0 ± 4·7 pg/ml, P < 0·01) while oestradiol-17β was higher than oestrone in uterine flushings (19·8 ± 1·4 vs 14·9 ± 1·5 pg/horn, P < 0·10). Systemic progesterone concentration was correlated with day after mating (r = 0·93, P < 0·01). There was no effect of haplotype on any of the hormone concentrations measured. Litter size was analysed from 99 matings amongst SLA^a/a, SLA^a/c, SLA^a/d, SLA^d/c and SLA^d/d sires and dams. Litter size from -/d and d/d sows or from d/d boars were larger (P < 0·05) than for all other matings. Although ovulation rate was higher in SLA^d/d sows, the significant effect of sire SLA genotype on litter size suggests an additional effect of the d haplotype on embryonic survival.

Keywords: pig; major histocompatibility complex; litter size