PHYSIOLOGICAL BASIS OF GENETIC VARIATION IN OVULATION RATE NIMALAJITH PATHIRAJA B.V.Sc (SRI-LANKA) THESIS SUBMITTED TO THE UNIVERSITY OF EDINBURGH IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

Improvement of litter size (lambs bprn/ewes lambing) in sheep is difficult for several reasons. The experimental results on direct selection for litter size, coupled with generally low estimates of genetic parameters of litter size led to widespread interest in the use of indirect selection criteria for genetic improvement of litter size. It has been argued that ovulation rate is the main limiting factor to improve reproduction rate in sheep. The understanding of the physiological basis of genetic variation in ovulation rate would allow development of physiological criteria for genetic improvement of ovulation rate and development of reliable means of phenotypic modification of gonadal function. Literature on the indirect methods of selecting for litter size is reviewed with emphasis on the ovulation rate. Postulated physiological methanism of between breed variation in ovulation rate in sheep, the variation of the sensitivity of the hypothalamus/pituitary to negative feed-back of gonadotrophin release, is further investigated and its genetic nature is examined. In passive immunization studies gonadotrophic releasing patterns were studied in 5 groups of Welsh Mountain sheep (10 each) before and after passive immunization against oestrone (E1), estradiol 17 (E2), androstenedione (A), testosterone (T) and Control (C). Immunization against all four steroids increased the / LH pulse frequency and mean LH levels. FSH levels were increased only in the group immunized against E1 and E2. However, after 12-15 hours the trophic and negative feed-back system tended to equilibrate Immunization against all four steroids increased the ovulation rate significantly. It also slightly changed the oestrous cycle length. Number of animals showing oestrus also were affected by the high antibody titres. The lambing percentage was increased after immunization against testosterone. The half life of the four antibodies in the circulation varied from 12-16 days. In the between breed study of the hypothalamic sensitivity to negative feed-back in Scottish Blackface and Finnish Landrace . sheep the--basal LH levels, LH pulse frequency and FSH levels were studied before and after insertion of silasticoestradial implants. The ovulation rate and the number of animals ovulating were studied after implant insertion. In this study the previous finding of between breed variation in hypothalamic/pituitary sensitivity to negative feed-back of gonadotrophin release was confirmed and extended the findings to LH pulse frequency, basal FSH levels and to intact animals. In the mouse study the direct effect of the oestradiol 17 antiserum on 'the ovulation rate was studied. The heritability of the hypothalamic pituitary sensitivity to negative feed-back was estimated by offspring parent regression. Passive immunization against E2 increased ovulation rate significantly. The heritability estimated was not significantly different from zero. In the concluding section the results are discussed and prospects for further research are outlined. It is argued that the / most fruitful approach for developing physiological selection criteria for increasing ovulation rate is to study the within breed or strain physiological basis of differences in ovulation rate.