Genetic control of ovulation rate and embryo survival in mice. II. Effects of crossing selected lines.

ETEROSIS in characteristics which are part of the reproductive process is a commonly observed result of crossing unrelated stocks in many species (e.g., GOWEN 1952). In mammals, the observed heterosis in litter size could be due to superiority of the crossbred in ovulation rate, in embryo survival, or both. The second of these parameters can be further subdivided into preand postimplantation stages. If these components were found to exhibit different degrees of heterosis, the information could be used in developing more efficient animal crossbreeding programs. Such information would also contribute to a better understanding of the nature of the genetic control of litter size. Analyses by FALCONER and ROBERTS (19'60) of crosses of inbred lines of mice indicated that there was directional dominance of genes affecting preimplantation losses but not the other two components. BOSHIER (1968) obtained generally similar results. MCCARTHY (1967) on the other hand, concluded from an inbreeding experiment with a slightly different strain of mice from that used by FALCONER and ROBERTS that genes affecting ovulation rate as well as preimplantation losses showed directional dominance. Again, postimplantation losses appeared to be affected by genes acting additively. In swine, SQUIERS, DICKERSON and MAYER (1952) reported that ovulation rate and embryo mortality, primarily preimplantation but including a small proportion of early postimplantation losses, both showed heterosis or evidence of directional dominance, with ovulation rate showing the greater effect of the two. KING and YOUNG (1957) reached the same conclusion from a comparison of inbred and outbred sows. In this paper we present the results of crossing lines of mice differing in ovulation rate and in embryo survival, as a result of artificial selection, and we suggest that embryo survival, particularly at the preimplantation stage, exhibits more heterosis than does ovulation rate.