Effects of selection for postweaning gain on testicular function in mice.
Changes in testicular and epididymal function and selected endocrine organ size were examined in a line of mice selected for rapid postweaning gain (M16) and in reciprocal crosses with an unselected pedigree control line (ICR). The larger body weight of the M16 line was accompanied by larger testes, epididymides, seminal vesicle, pituitary, thyroid and adrenal weights (P smaller than .01) although the testes, epididymides, seminal vesicle and adrenal weights of M16 mice, expressed per gram body weight, actually decreased (P smaller than .01) relative to the ICR line. Testicular and epididymal sperm reserves were higher in M16 mice but the difference was not significant. However, when adjusted for gland size, testicular and epididymal sperm reserves were lower in the M16 line (P smaller than .01). Absolute and relative weights of testes, epididymides and pituitary were larger (P smaller than .01) in M16 male times ICR female crosses than in ICR male times M16 female crosses. Although testicular and epididymal sperm reserves were higher in M16 male times ICR female males the reciprocal difference was significant only for testicular sperm (P smaller than .05). Heterotic effects were significant for both absolute and relative weights of testes, epididymides (P smaller than.01), pituitary (P smaller than .05) and relative weight of siminal vesicles (P smaller than .05). Although percent heterosis was 8.5 for testicular and epididymal sperm reserves, significant heterotic effects were found only for epididymal sperm (P smaller than .05).