Journal of Reproduct on and Fertility Supplement 49, 113-121 The search for the Booroola (FecB) mutation G. W. Montgomery, J. M. Penty, E. A. Lord and M. F. Broom AgResearch Molecular Biology Unit, Department of Biochemistry and Centrefor Gene Research, University of Otago, PO Box 56, Dunedin, New Zealand Sheep derived from the Booroola Merino strain carry an autosomal mutation (FecB) that increases ovulation rate and litter size. One approach to characterize the genetic mutation is to locate the gene using positional cloning. The locus has been mapped to a region between genes for secreted phosphoprotein 1 (SPPI) and epidermal growth factor (EGF) on sheep chromosome 6. Analysis of possible candidate genes have excluded a number of genes associated with control of reproduction including genes from chromosome 6. Attempts to define close flanking markers and clone the region of DNA containing the mutation are now in progress. We have cloned additional markers and developed a linkage map showing that the FecB locus maps towards the centromere on chromosome 6. We have developed a yeast artificial chromosome (YAC) library for the sheep and begun screening the library to identify large DNA clones spanning the FecB region. These will be used to locate the mutation and shed light on how the mutation increases ovulation rate in Booroola sheep. Introduction Sheep derived from the Booroola Merino strain provide an important model to study mechanisms controlling ovulation rate in mammals because they carry an autosomal mutation that increases ovulation rate and litter size (Davis et al., 1982; Piper and Bindon, 1982). The effect of the locus (FecB; COGNOSAG, 1989) is additive for ovulation rate, but partially dominant for litter size because of embryonic loss in homozygous carriers with high ovulation rates (Piper et al., 1985). Carriers of the FecBmutation can be identified only by phenotypic effects in females. One goal of research in Booroola sheep has been to identify a simple test for carriers of the FecB allele by finding a biochemical or genetic marker linked to the gene (McNatty et al., 1991; Montgomery et al., 1992a). A suitable marker would provide a rapid and accurate means of predicting the Booroola genotype of males, and of lambs before puberty. A linked genetic marker is also essential as the first step towards molecular characterization of the gene and its function using genetic studies. Strategies for finding and characterizing genes without any prior knowledge of the gene function are now well developed and referred to as positional cloning (Collins, 1992). Positional cloning is initiated by locating the gene to a chromosome. Successive narrowing of the location and cloning of expressed genes in the region will eventually result in the identification of the gene. Positional cloning provides a powerful alternative to functional studies and has been used successfully to identify a number of genes responsible for inherited disorders in humans and mice. The first step of finding markers linked to the FecBlocus has now been completed. The locus maps near the centromere on sheep chromosome 6 (Montgomery et al., 1993, 1994a). This article reviews studies to map the FecBlocus and progress towards locating and characterizing the mutation responsible for the increased ovulation rate in Booroola gene carriers. © 1995 Journals of Reproduction and Fertility Ltd