DEVELOPMENT OF MEISHAN AND YORKSHIRE LITTERMATE CONCEPTUSES IN EITHERA MEISHAN OR YORKSHIRE UTERINE ENVIRONMENT TO DAY 90 OF GESTATION ANDTO TERM

To investigate the impacts of uterine type and conceptus genotype on d evelopment through late gestation, Meishan and Yorkshire embryos were co-transferred into the uteri of either Meishan or Yorkshire recipient s that were subsequently slaughtered on Day 90 of gestation. At slaugh ter, regardless of conceptus genotype, fetuses and placentae were mark edly smaller when recovered from Meishan than from Yorkshire recipient s. Whereas Meishan and Yorkshire fetuses recovered from Meishan uteri were similar in weight, Meishan fetuses were markedly lighter than lit termate Yorkshire fetuses when recovered from Yorkshire uteri. Because of the marked differences between fetal weights observed in Yorkshire recipients on Day 90 of gestation, Meishan and Yorkshire embryos were co-transferred to Yorkshire recipients that were allowed to farrow. S urprisingly, Meishan and Yorkshire fetuses cogestated in Yorkshire rec ipients were born at similar weights, whereas Meishan placentae were m arkedly smaller. The weight of Meishan placentae were similar on Day 9 0 and at term, whereas the weight of Yorkshire placentae were markedly larger (similar to 70%) at term than on Day 90. The constant weight o f Meishan placentae from Day 90 to farrowing appears to result from an ability to increase their vascularity during this interval. In contra st, Yorkshire placentae may be forced to increase their weight to keep pace with fetal growth during this period. Because uterine capacity s ets the upper limit on litter size, the decreased endometrial surface area required per conceptus in the Meishan pig seems to explain its gr eater potential for increased litter size.