INVESTIGATIONS INTO THE CONTROL OF LITTER SIZE IN SWINE .2. COMPARISONS OF MORPHOLOGICAL AND FUNCTIONAL EMBRYONIC DIVERSITY BETWEEN CHINESE AND AMERICAN BREEDS

It has been suggested previously that the increased litter size of the prolific Chinese Meishan pig may result from an increased littermate embryonic synchrony. This study compared embryonic diversity in Meishan and domestic (white line crossbred) sows. Third-parity Meishan sows (n = 14) and second-parity domestic sows (n = 15) were observed for estrus every 6 h and were hand-mated at 24 and 30 h after first observed estrus (d 0) to boars of the same breed. The sows were slaughtered on d 11.9 +/- .1 (mean +/- SEM). Embryos were flushed from each uterine horn and were individually sonicated, lyophilized, and frozen (-86-degrees-C) until they were assayed for estradiol-17beta (E2beta), DNA, and protein content. Ovulation rate was higher (P < .001) in Meishan (24.9 +/- 1.1) than in domestic sows (15.2 +/- .7). The average littermate embryonic diameter was smaller (P < .001) for Meishan than for domestic sows, although morphological embryonic diversity (standard deviation for diameter) did not differ (P > .10) between the two breeds. In addition, for embryos of the same diameter, no differences (P > .10) in number of embryonic cells (micrograms of DNA/embryo) or in cell size (protein:DNA ratio) were observed for either breed. The more advanced (> 6 mm) embryos from Meishan sows contained less (P < .001) E2beta (picograms/embryo) than did embryos of the same size from domestic sows. The prolificacy of the Meishan sow may result from both an increased ovulation rate and reduced E2beta production by Meishan embryos. The reduced E2beta production by Meishan embryos may result in more gradual alterations in uterine environment with an increased survival of less-developed littermates.