Reproductive Biology of the Booroola Merino Sheep

Abstract

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.

Variations in patterns of follicle development in prolific breeds of sheep

“…Follicular response to LH is also acquired in smaller follicles in Booroola ewes compared to Merino ewes (Scaramuzzi, Turnbull, Downing & Bindon, 1981). Furthermore, decreased ovarian inhibin content has been found in Booroola Merino ewes compared to the prolific Merino and control Merino ewes (Cummins, O'Shea, Bindon, Lee & Findlay, 1983;Bindon, 1984). These between-breed and strain differences suggest that each breed might have a different mechanism by which ovulation rate is controlled.…”

“…There is considerable variation in ovulation rate between breeds of sheep and between strains of sheep within breeds. Despite numerous studies the exact mechanisms which control ovulation rate are still not known (see reviews by Scaramuzzi & Radford, 1983;Webb & Gauld, 1984;Bindon, 1984). A number of follicular characteristics have been associated with higher ovulation rate.…”

Plasma FSH, LH and immunoreactive inhibin concentrations in FecBB/FecBB and FecB+/FecB+ Booroola ewes and rams from birth to 12 months of age

“…In ram lambs, the effects of the FecBB gene on plasma hormone concentrations are equivocal (Seek el al, 1988;Montgomery et al, 1989;Purvis el al, 1991) and, to our (Bindon, 1984;McNatty et al, 1991aMcNatty et al, , 1992Montgomery el al, 1992 (McNatty et al, 1987a(McNatty et al, , 1989(McNatty et al, , 1993a. In rams, most studies have found no effect of the FecBB gene on plasma FSH and LH concentrations that are independent of sire effects (Bindon et al, 1985(Bindon et al, , 1989Montgomery et al, 1989;Hochereau-de Reviers and Seek, 1991;Price et al, 1991a, b, c;Isaacs et al, 1992 (1982).…”

LH, FSH and ovulation rate in ewes treated with exogenous oestradiol

“…Bindon (1984) compared fecund Booroola and normal Merino ewes and concluded that ovulation rate was independent of the amount and pattern of LH secreted in the preovulatory period. Similarly, some researchers examining the nutritional stimulation of ovulation rate have found no effects on pituitary LH content or basal plasma LH concentrations before or after the preovulatory LH surge (Haresign, 1981;Rhind et al, 1984).…”