Summary. Prolific breeds of sheep (Romanov, Finn and Booroola Romanov crosses heterozygous for the Booroola gene (F +) were compared with breeds of lower prolificacy (Ile-de-France, Finn × Scottish Blackface, Merino × Blackface and Booroola × Romanov not carrying a copy of Booroola gene (++)) by in-vivo monitoring of follicular kinetics by ink labelling during the late luteal phase and follicular phase of the oestrous cycle followed by histological examination of the ovaries or follicle dissection. At each of 3 successive laparotomies, the 3 largest follicles of each ovary were measured and ink labelled. At the final laparotomy, around the beginning of oestrus, all ewes were ovariectomized.
High ovulation rate was not associated with the total number of antral follicles in any of the breeds. However, there were more follicles > 2 mm in diameter in Romanov and Booroola × Romanov crosses (F +) compared to their respective controls. Such a feature was not observed in Finnish Landrace compared to Finn × Blackface and Merino × Blackface ewes. A more numerous population of recruitable follicles, together with a similar incidence of selection through atresia, were the features associated with the high ovulation rate of Romanov compared to Ile-de-France ewes. The high ovulatory potential of the Finn ewes resulted from a markedly reduced incidence of selection through atresia. Booroola × Romanov ewes carrying a copy of the Booroola gene (F +) appeared to possess features of both parental breeds, including high numbers of recruitable follicles, smaller follicular size when recruitment occurs and an extended time for recruitment. Booroola × Romanov (++) ewes, not carrying the gene, appeared to have lost part of the 'Romanov characteristics' of a more numerous population of recruitable follicles. The variability in the kinetics of preovulatory enlargement, seen in these breeds of sheep, demonstrates that there are a number of pathways through which high ovulation rate can be achieved and hence through which ovulation rate might be manipulated.
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