US-AUSTRALIAN ACADEMIES JOINT WORKSHOP
US-AUSTRALIAN ACADEMIES JOINT WORKSHOP ON VERTEBRATE COMPARATIVE GENOMICS
Beckman Conference Centre, Irvine, California, 23-25 May 2007
Linkage mapping the sheep genome
by Jill Maddox
Dr Jill Maddox received her PhD in Sheep Immunology from the University of Melbourne in 1985. She has since held research positions at the Imperial Cancer Research Fund (UK), and the University of Melbourne where she has been a Senior Research Fellow since 1992. She has a graduate diploma in Information Technology and has also worked as a computer programmer/analyst. Her research interests include identifying genes that contribute to resistance to gastro-intestinal nematodes in sheep, using linkage analysis to map traits of economic importance in sheep, sheep immunogenetics, and using genetic selection to improve the productivity of meat sheep in Maharashtra. She is currently the chairperson of the ISAG Committee for Applied Genetics in Sheep and Goats.
The sheep genomics community has relatively limited resources when compared to the genomics communities of many other mammals. As a consequence the genome maps of sheep are relatively primitive when compared to the maps of species such as man, mouse, dogs, horses and cattle. Despite the lack of a sequence map, and the availability of only a sparse linkage map, genome scans are currently being conducted in sheep for a large range of economically and medically important traits. The sheep is also an important experimental model for humans for studying normal physiological mechanisms and for many inherited and acquired diseases.
The sheep linkage map has been developed as an international collaboration, and is primarily based on a three-generation full-sibling mapping pedigree, the International Mapping Flock (IMF), that was developed by AgResearch in the early 1990s. The current sheep linkage map, version 4.7, comprises 1,425 markers representing 1,381 loci and spans approximately 3,600 cM (sex averaged). As a consequence of the limited nature of sheep resources, the sheep mapping community has exploited both the resources from closely related species such as cattle and goats and more distantly related species such as humans. Most of the markers on the sheep map are microsatellites, and just over half of the markers are based on cattle sequence.
The sheep is unusual amongst mammals in that it is the only known eutherian mammal where the male map is longer than the female map. Other mammals with male maps that are longer than female maps include the Tammar wallaby and the opossum. About 50% of the male map expansion in sheep occurs at the centromeric and telomeric ends of chromosomes. It is unknown whether the longer male maps are genuine or an artefact that is peculiar to the IMF pedigree structure as most mapping studies in sheep utilise male maps.
The International Sheep Genomics Consortium (ISGC, www.sheephapmap.org) is currently undertaking a SNP discovery exercise with the aim of producing a 60K SNP chip and constructing a sheep HapMap. These resources will be extremely useful for trait mapping, and should also be useful for investigating the differences between male and female recombination in sheep.
Contact details:
Department of Veterinary Science
University of Melbourne
Victoria 3010
Tel: (+61) 3 8344 5736
Fax: (+61) 3 8344 7374
E-mail: jillm@rubens.its.unimelb.edu.au
Web: rubens.its.unimelb.edu.au/~jillm/jill.htm
