INFLUENCE OF SEX ON EARLY GROWTH OF PIG CONCEPTUSES

Nineteen gilts were used in an experiment to examine the relationship between rate of development and embryonic sex on day 10 of pregnancy. All gilts were mated to the same boar approximately 24 h after detecti on of second oestrus. They were individually housed and fed similar di ets until slaughter on day 10 of gestation (day 0 = day of inseminatio n) for subsequent recovery of the conceptus. All conceptuses were phot ographed and their surface areas (mm(2)) measured by tracing outlines on a digitized tablet interfaced with a computer program. Within each litter, individuals were categorized as small, medium or large by thre e equal divisions of the size range between the smallest and largest m ember. Conceptuses were individually cultured in Medium 199 with 1% co lcemid and stained with 4% Giemsa. Metaphase spreads were located and sex was determined by presence or absence of the Y chromosome in at le ast two spreads from each specimen. A total of 214 conceptuses were re covered but only 125 (58%) were successfully karyotyped. The overall s ex ratio was not significantly different from 1:1 (57 males and 68 fem ales; P > 0.25). Sex was determined in 51 of 88 small embryos, 22 of 4 4 medium embryos and 52 of 82 large embryos and males represented 9 (1 7.6%), 10 (45.5%) and 38 (73%), respectively. Logistic analysis indica ted significantly more females in the small and significantly more mal es in the large groups (P < 0.001). Results demonstrate that most male conceptuses grow faster than females before commencement of attachmen t to the uterine lining.