Genetic control of the duration of gestation in sheep

Summary. Results of egg transfer and of reciprocal crossing among breeds of sheep with a range of 1 week in mean gestation period indicate that the genotype of the fetus is the major determinant of the duration of gestation, over a range of litter sizes from 1 to 5 and a wide range of combinations of size of donor and recipient breeds. It is concluded that the fetus accounts for at least two-thirds of the genetic variation in gestation period in this species. Evidence for the role of the fetus in determining the duration of gestation comes from two independent sources, namely endocrine studies of the fetal pituitary-adrenal axis (Liggins, Kennedy & Holm, 1967; Liggins, 1969), and genetic studies, with the most critical evidence provided by interbreed egg transfer (Dickinson, Hancock, Hovell, Taylor & Wiener, 1962; Moore, 1968). We present here the results of interbreed egg transfer and crossbreeding experiments involving breeds with a wider range of gestation periods than have been used in earlier experiments. They provide striking evidence of the role of the fetal genotype in determining the duration of gestation in this species. Descriptions of the animals involved in the egg transfer experiment and of the procedures used for synchronizing oestrus in donors and recipients have been reported by Bradford, Quirke & Hart (1971). The time of onset of oestrus was recorded twice daily, and the actual time of parturition was recorded to the nearest hour, except for ewes lambing unattended during the night for which parturition was listed as occurring at 07.00 hours. The duration of gestation was calculated to the nearest half day. Where donor and recipient ewes were not exactly synchronized, the time of onset of oestrus in the donor was used as the basis for computing the gestation period; all but seven of the fifty-three recipient ewes for which gestation periods are reported were synchronized within 12 hr of the donor. In addition to the pregnancies following egg transfer, data were obtained on seventy-seven control pregnancies, mostly of donor ewes, following intra-breed mating. The same rams, in most cases one or two per breed, were used at the oestrus from which eggs were collected and on the control group. Extensive data were also available on gestation periods from the parental flocks. Lambs in the experiment were born during a 3-month period, March to