Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 5, 2008
Publication Date: September 1, 2008
Citation: Miles, J.R., Freking, B.A., Blomberg, L., Vallet, J.L., Zuelke, K.A. 2008. Conceptus development during blastocyst elongation in lines of pigs selected for increased uterine capacity or ovulation rate. Journal of Animal Science. 86(9):2126-2134. Interpretive Summary: Litter size is a key component to sow productivity, which has a significant economic impact for the swine industry. The two primary factors that have been shown to limit litter size include ovulation rate and uterine capacity. Selection lines for these component traits of litter size have been developed at the US Meat Animal Research Center. An analysis of prenatal survival rates in these lines has demonstrated that pigs selected for increased uterine capacity have improved prenatal survival while pigs selected for increased ovulation rate have decreased prenatal survival. During early pregnancy in the pig, the shape of the embryo changes from a sphere to a thin filament that is essential for proper establishment of pregnancy. Therefore, it is possible that deviations in embryo development during this change may be the cause of differences in prenatal survival rates in these lines. The objective of the present study was to evaluate embryo development during early pregnancy as a potential contributing factor to differences in prenatal survival rates observed between these selection-line pigs. For evaluation of embryo development, this study compared embryo mortality, shape, and diversity, as well as various other biochemical indicators of embryo health and development in these lines. There were no significant differences detected for any of the variables compared between the selection lines. Taken together, these results demonstrate limited deviations in embryo development during early gestation in these selection lines and suggest that the mechanisms involved in generating line differences in prenatal survival rates likely take place during later pregnancy.
Technical Abstract: Lines of pigs selected for increased uterine capacity (UC) have improved conceptus survival while pigs selected for increased ovulation rate (OR) have decreased conceptus survival relative to an unselected control (CO) line. The objective of this study was to evaluate conceptus development during embryo elongation as a potential contributing factor to differences in conceptus survival rate among these pig lines. Conceptuses were recovered from pregnant CO, UC, and OR line gilts at Day 10 and 12 of gestation. At Day 10 of gestation, conceptus morphologic diversity was assessed by comparing within-litter average conceptus diameter and the standard deviation of conceptus diameters. At Day 12 of gestation, conceptus morphologic diversity was assessed by comparing embryo populations obtained from individual gilts. Real-time PCR analyses for transcripts involved in steroidogenesis, cellular differentiation, and immune responsiveness were performed on spherical, ovoid, and filamentous conceptuses recovered from these selection lines. Uterine flushings were also assayed for total protein and estradiol-17ß levels at Day 10 and 12 of gestation. Morphological data were analyzed using ANOVA with the fixed effects of line, season, and their interaction. Conceptus mortality, uterine flushing, and real-time PCR data were analyzed using ANOVA with the fixed effects of line, day or embryo morphology, season, and their interactions. Conceptus mortality, measured as the ratio of conceptus recovery to ovulation rate, was not different between the lines on Day 10 and 12 of gestation. There were no significant line effects for conceptus morphologic diversity at Day 10 and 12 of gestation. Expression of transcripts associated with steroidogenesis (STAR, CYP11A1, and CYP19A1), cellular differentiation (KRT18 and VIM), and immune responsiveness (IL1B) in spherical, ovoid, and filamentous conceptuses were not different between the lines. Furthermore, total protein and estradiol-17ß levels in uterine flushings at Day 10 and 12 of gestation were not different between the selection lines. These findings indicate limited, if any, deviations between these lines of pigs in conceptus development during embryo elongation and suggest that mechanisms involved in generating line differences in survival rate likely are manifested later in gestation.