SEX-CHROMOSOME COMPLEMENT AND DEVELOPMENTAL DIVERSITY IN PREHATCHING AND POSTHATCHING PORCINE EMBRYOS

To examine sex and development relationships in porcine embryos in ear ly gestation, 10 gilts were killed on Day 4, 5, or 6 post mating (firs t day of standing estrus = Day 0). Embryos recovered immediately after slaughter were cultured in Medium 199 with colcemid (0.05 mu g/ml), f ixed on slides, and stained with 4% Giemsa. The number of cells in eac h specimen was counted from the slides, and, whenever cell dispersion allowed, sex was determined by presence or absence of the Y-chromosome in at least 2 spreads from each embryo. Three gilts slaughtered on Da y 4 yielded 2- and 4-cell stage embryos (n = 38), but no data on sex c ould be obtained due to lack of mitosis or readable metaphase spreads. Three Day 5 litters had individual specimens ranging from 8 to 14 cel ls (n = 8), 32 to 64 cells (n = 10), and 13 to 31 cells (n = 11), with the sex determined in 15 of these. Cell numbers ranged from 18 to 165 (n = 14), 16 to 32 (n = 9), 36 to 82 (n = 12), and 16 to 30 (n = 9) i n the 4 gilts slaughtered on Day 6, with the sex determined in 26 of t hese. Embryos within each litter were divided into low, medium and hig h cell numbers by 3 equal divisions of the range of cell numbers. Thre e Day-5 embryos and 1 Day-6 embryo were lost during preparation; neith er the cell numbers nor the sex could be determined in 4 Day-5 and in 3 Day-6 embryos. The overall sex ratio approximated 1:1, but on Day 5, the ratios for males to females were 0:5, 1:3 and 6:0 for the low, me dium and high cell number groups, respectively. Embryos of undetermine d sex in these same groups numbered 3, 1 and 3, respectively. On Day 6 the distribution was 1:11, 4:2 and 8:0 in favor of the males, while e mbryos of undetermined sex in the low, medium and high cell number gro ups numbered 5, 7 and 2, respectively. Chi-square analysis of the comb ined Day-5 and Day-6 results indicated the presence of significantly m ore females among embryos with low cell numbers and more males in the high cell number group (P < 0.01).