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C.S.I.R. O., Division of Animal Genetics, P.O. Box 90, Epping, New South Wales 2121, Australia

(Received 7th January 1975)

When ewes are injected with synthetic Gn-RH on the day of oestrus, the magnitude of the LH release is greater than that following injection at other stages of the oestrous cycle (Reeves, Arimura & Schally, 1971; Hooley, Baxter, Chamley, Cumming & Findlay, 1974; Thimonier, Terqui & Pelletier, 1974). It is not known, however, whether additional LH on the day of oestrus has any effect on the number of ovulations, especially in ewes with the genetic potential for high ovulation rates. This question was investigated in four flocks of Merino sheep, designated `C', `O', `T' and `B', which are, respectively, an unselected group and groups selected since 1954, for low, medium and high litter size. The genetic history of these flocks has been described by Turner (1969).

In May 1973, eighty-nine ewes

Summary.

Spacing of zygotes, loss of the zona pellucida, and appearance of localized areas of increased vascular permeability at the implantation sites in the uterus were observed in groups of mice examined at 2-hr intervals from 10.00 hours to midnight on Day 4 of pregnancy, and at 02.00, 08.30 and 15.00 hours on Day 5. All the events examined had a 6- to 8-hr variation in their time of occurrence.

Studies of interrelationships between the zygote and the uterus require a precise knowledge of the timing, variation and sequence of the events leading up to implantation in any one species. Previous studies of preimplantation phenomena in the rodent have been based on relatively small numbers of animals examined at irregular intervals during the periods of interest.

Summary.

One hundred and four anoestrous Merino ewes and 120 Romney Marsh ewes were treated in separate experiments of factorial design with combinations of a single injection of 6-chloro Δ⁶-17-acetoxyprogesterone (cap), followed some days later by the oral administration of 6-methyl-17-α-acetoxyprogesterone (map). An injection of pregnant mare's serum (pms) was then given at two different times in relation to the final map treatment.

Dose of cap, number of map treatments and time of pms injection all influenced the proportion of ewes exhibiting oestrus.

In the first experiment, twenty-six ewes lambed of the twenty-seven ewes served, in the second experiment only four ewes lambed of the twenty-four served.

Summary.

Progestagens were administered to ewes during the breeding season, and the effects on oestrus, ovulation and fertility are described.

Summary.

The uterine weight and ovulation responses of immature intact and hypophysectomized mice from two lighting regimes have been studied after injection of gonadotrophin. The results indicate diurnal variation in ovarian response: the response to gonadotrophin was greater when it was administered during the second half of the light phase of the daily light cycle than at any other time.

Pregnant mice were fasted for 36- to 48-hr periods beginning at 06.00 hours or 18.00 hours on the 2nd, 3rd or 4th day after mating. Diurnal variation in the proportion of pregnancies failing was demonstrated. The effect was not attributable to differences in loss of body weight during fasting, and could be prevented by injections of human chorionic gonadotrophin (hcg).

The numbers of eggs ovulated by adult di-oestrous mice treated with hcg were studied after 36- or 48-hr fasts commencing at 06.00 hours or 18.00 hours 2 days before an injection of hcg. Fasting reduced numbers of ovulations, but the effect was influenced by the time of day when the fast began. Reversal of the lighting regime resulted in a reversal of the pattern.

It is of interest to know whether genetic differences in fecundity arise as a result of changes in the sensitivity of the ovary to gonadotrophin or of increased secretion of gonadotrophin. This has a bearing on whether efforts to alter fecundity by genetic means should be concentrated at the level of the ovary, the pituitary gland, or both.

There are a number of studies in mice in which genetic differences in ovulation rate or litter size have been attributed to changes in ovarian sensitivity (McLaren, 1962; Land & Falconer, 1969; Wolfe, 1971). This conclusion has generally been based on the ovulatory response to exogenous gonadotrophin by intact mice of different genetic groups. Such tests, however, are influenced by the endogenous gonadotrophin levels of the intact test animal (Lamond & Emmens, 1959; Lamond & Bindon,

Genetic differences in fecundity have arisen as a result of selection for litter size in Merino sheep since 1954. The selection experiment involves three flocks designated 'O', 'T' and 'B' and their genetic history has been described by Turner (1969). In the 'O' flock, in which selection has been against multiple births, the ewes regularly have a low (0 to 10%) incidence of twin births. The 'T' and 'B' flocks have a medium (50 to 60%) and high (70 to 80%) incidence of multiple births. In the 'B' flock, about 30% of ewes have triplets and 10% have quadruplets or quintuplets each year.

Current research on these groups is designed to examine the endocrine changes that have accompanied the genetic divergence in fecundity. It is known that ewes from the flocks of high fecundity