This paper reviews the genetic and physiological characteristics of the Booroola Merino, one of the four most prolific sheep breeds in the world, and which was acquired by CSIRO in 1958 from a commercial sheep property, 'Booroola', Cooma, N.S.W. The exceptional prolificacy of this genotypee.g. mean flock ovulation rate in 1982 of 4·2 (range 1-10) and mean litter size of 2· 5 (range 1-7)-is largely attributable to a single gene (F) of uncertain origin which increases ovulation rate. Crosses of the Booroola with other Merinos produce progeny which have a 47-87% increase in ovulation rate, a 45-56% increase in litter size at birth, and a 1-33% reduction in lamb survival relative to control Merinos. This represents a 16-37% increase in the number of lambs weaned per ewe joined in favour of the Booroola crosses.The exact site of action of the F gene is not well established, although it is expressed primarily at the ovary, where more than the normal number of follicles mature and ovulate each oestrous cycle. This may result from some abnormality of the Booroola follicle itself or it may reflect differences in Booroola gonadotrophin secretion. There is some evidence that Booroola ewes have elevated plasma concentrations of follicle stimulating hormone (FSH) early in life and during the oestrous cycle, and that FSH concentrations in the pituitary gland and urine of the adult ewe are also high. These elevated FSH levels in the adult are attributed to an ovarian feedback deficiency, probably because the inhibin content of the Booroola ovary is only one-third that of normal Merino ovaries. The low inhibin content appears to be due to Booroola follicles having significantly fewer granulosa cells than control Merinos. Analogous studies of the prolific D'man sheep of Morocco point to FSH as the main correlate of prolificacy.The testis growth rate, testis size and total daily production of spermatozoa of the Booroola ram are similar to those of normal Merinos, as also are the endocrine characteristics of adult rams. The Booroola gene's expression is evidently sex-limited. Several theories concerning the mode of action of the F gene are being tested.

Pregnant and non� pregnant ewes were killed at various times during the period up to 15 days after service. The purpose of the study was to examine a number of parameters related to ovarian and uterine activity with a view to establishing the point at which the embryo first has measurable effects on the physiology of the ewe.

Medium-wool Merino ewes, from groups that, as a result of selection since 1954 for this character, have a low (O group) and high (T group) incidence of multiple births, were injected with 0,375,750, or 1500 i.u. pregnant mare serum gonadotrophin (PMSG) on day 13 or 15 of the oestrous cycle (day 1 is the day of oestrus). The two groups were compared on the basis of the occurrence of oestrus, number of ovulations, ovarian follicle score, plasma progesterone level, and weight of luteal tissue. Conception rate in the two groups was compared by examining the reproductive tracts on day 20 of pregnancy, when the number and size of embryos were assessed. Injection of PMSG on days 13 and 15 gave similar results, so these data were pooled for all subsequent comparisons. Dose response lines for PMSG were constructed, the above ovarian characters being used as response types, and the curves were compared by relative potency analysis. On the basis of number of ovulations, plasma progesterone, and luteal tissue weight, PMSG was approximately three times as potent in T ewes as in O ewes. That is, for a given dose of PMSG, the T ewes produced an ovarian response three times as great as that of O ewes. At low doses of PMSG (4 1500 i.u.) the T ewes also produced more follicles in the ovary than 0 ewes. With 1500 i.u. PMSG this was reversed, since the O ewes produced many follicles that did not ovulate. The relative loss of potential embryos by day 20 was similar in the O and T ewes examined, and increased with increasing dose of PMSG. In ewes given 1500 i.u. PMSG only 20-35% of potential embryos were recovered. Embryo size on day 20 was not significantly influenced by either group or dose of PMSG. In untreated ewes the normal ovulation rate differed significantly between the O group (1.00) and T group (1.50). Ovarian data collected from untreated adult ewes of the O and T groups substantiate the differences observed in the PMSG study. It is not yet known whether the between-group differences in fecundity have arisen through changes in pituitary gonadotrophin secretion, changes in ovarian sensitivity, or both.

SummaryMerino ewes were ovariectomized on days 4, 8, or 12 of pregnancy and injected daily from that time with 1, 4, or 16 mg progesterone in oil. The ewes were killed on day 20 and examined for embryos. There was a clear effect of dose of progesterone on the number and viability of embryos. Of the ewes receiving 1, 4, and 16 mg progesterone there were 0 out of 12, 2 out of 11, and 11 out of 14 with viable embryos on day 20. The effect of increasing progesterone was also seen in significantly increased glandular and luminal epithelial cell heights in ewes receiving 16 mg per day.Plasma progesterone was measured daily during days 1-5 and twice daily during days 6-15 of pregnancy and of the oestrous cycle. The numbers of corpora lutea and embryos in these animals were verified at laparotomy on days 15-18. The difference between pregnant and non-pregnant ewes with regard to plasma progesterone was not significant until days 16-17. Both pregnant and non-pregnant ewes had highest levels of progesterone during the period day 10 to day 14. Different ewes had peak values at different times so that the mean progesterone values took the form of a variable plateau, lasting several days, of 2-3 ng/ml.Peak levels of progesterone appeared in the plasma at times consistent with the idea that this hormone is responsible for the rapid preimplantation growth phase of the embryo during days 11-15.

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