Santucci F, Ibrahim K M, Bruzzone A, Hewit G M
School of Biological Sciences, University of East Anglia, Norwich, UK.
Heredity (Edinb). 2007 Sep;99(3):340-8. doi: 10.1038/sj.hdy.6801006. Epub 2007 May 23.
Microsatellites within the major histocompatibility complex (MHC) region have received increasing attention as proxy measures of the level of polymorphism at the Mhc genes themselves. We assessed the diversity of microsatellite loci within or in close proximity of the Mhc genes in several breeds of domestic sheep (Ovis aries) and the wild Mouflon (Ovis orientalis musimon). This was compared to variation at other microsatellite loci scattered throughout the sheep genome. Significantly higher number of alleles were observed at the MHC microsatellites. The sheep breeds studied fell into high- and low-diversity group. This grouping is not related to the agricultural use of the breeds, whether for milk, meat or wool. It is, however, correlated with the geographic origins of the breeds. Southern breeds are genetically more diverse than northern breeds. The observed heterozygosity was in most cases lower than Hardy-Weinberg expectations. The potential impact of selective breeding by man on this is discussed. Neutrality tests indicated that for most of the breeds, the distribution of alleles at the MHC-linked microsatellites are more even than would be expected if the genes were neutral and sampled from populations under drift-mutation equilibrium. Hitchhiking due to tight linkage with alleles at the MHC loci that are under balancing selection is proposed as a possible explanation for this pattern.
主要组织相容性复合体（MHC）区域内的微卫星作为Mhc基因本身多态性水平的替代指标受到了越来越多的关注。我们评估了几个家养绵羊品种（绵羊属）和野生摩弗伦羊（东方盘羊摩弗伦亚种）中Mhc基因内部或其附近微卫星位点的多样性。并将其与分布在绵羊基因组中的其他微卫星位点的变异进行了比较。在MHC微卫星上观察到的等位基因数量显著更多。所研究的绵羊品种分为高多样性组和低多样性组。这种分组与品种的农业用途无关，无论其用于产奶、产肉还是产毛。然而，它与品种的地理起源相关。南方品种在遗传上比北方品种更多样化。在大多数情况下，观察到的杂合度低于哈迪-温伯格预期。文中讨论了人类选择性育种对此的潜在影响。中性检验表明，对于大多数品种来说，如果这些基因是中性的且是从处于漂变-突变平衡的群体中抽样，那么与MHC连锁的微卫星上等位基因的分布比预期的更加均匀。文中提出，由于与处于平衡选择下的MHC位点上等位基因紧密连锁而导致的搭便车效应可能是这种模式的一种解释。
