tailieunhanh - Báo cáo khoa hoc:" Genetic components of litter size variability in sheep"

Tuyển tập các báo cáo nghiên cứu về sinh học được đăng trên tạp chí sinh học thế giới đề tài: Genetic components of litter size variability in sheep | 249 Genet. Sel. Evol. 33 2001 249-271 INRA EDP Sciences 2001 Original article Genetic components of litter size variability in sheep Magali SanCristobal-Gaudy3 Loys BODINb Jean-Michel ELSENb Claude Chevalet3 a Laboratoire de génétique cellulaire Institut national de la recherche agronomique Bp 27 31326 Castanet-Tolosan France b Station d amélioration génétique des animaux Institut national de la recherche agronomique BP 27 31326 C astanet-Tolosan. France Received 6 June 2000 accepted 11 December 2000 Abstract - Classical selection for increasing prolificacy in sheep leads to a concomitant increase in its variability even though the objective of the breeder is to maximise the frequency of an intermediate litter size rather than the frequency of high litter sizes. For instance in the Lacaune sheep breed raised in semi-intensive conditions ewes lambing twins represent the economic optimum. Data for this breed obtained from the national recording scheme were analysed. Variance components were estimated in an infinitesimal model involving genes controlling the mean level as well as its environmental variability. Large heritability was found for the mean prolificacy but a high potential for increasing the percentage of twins at lambing while reducing the environmental variability of prolificacy is also suspected. Quantification of the response to such a canalising selection was achieved. canalising selection threshold trait heterogeneous variances litter size sheep 1. INTRODUCTION Selection for increasing prolificacy in sheep although leading to a better average litter size in selected lines also leads to an increase in prolificacy variability. This phenomenon is well known for qualitative traits where mean and variance are linked. Extreme litters are encountered in prolific ewes Romanov Finnish with five or even more lambs per lambing which is obviously unacceptable for ewe and lamb viability. Breeders would like to have litter sizes of two exactly - and not on average