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Measuring inbreeding depression in the wild: the old ways are the best

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Measuring inbreeding depression in normally outbreeding natural populations is an uphill task, because it requires inbreeding coefficients that are calculated from pedigrees. Instead of calculating inbreeding coefficients directly, several studies have reported the use of microsatellite-derived metrics, such as heterozygosity, to infer relative inbreeding among individuals. In two new papers, Slate et al. and Balloux et al. show that these molecular metrics are often only weakly correlated with inbreeding coefficients, and that correlations between heterozygosity and fitness require a new interpretation.

Section snippets

The bad news

There are surprisingly few published efforts to correlate microsatellite heterozygosity and f. Jon Slate and co-authors [6] examine an exceptional dataset for New Zealand domestic sheep Ovis aries of the Coopworth breed: the focal generation of 590 individuals was the result of a known pedigree of at least seven generations and, as part of a quantitative trait locus (QTL) mapping exercise, each individual was genotyped at an average of 101 evenly spaced microsatellite loci. Although the

The good news

Two useful practical points also emerge from the papers by Balloux et al. and by Slate et al. First, both papers remind us that, if average heterozygosity is to reflect f, then the heterozygosity of loci within an individual should be correlated. For example, a highly inbred individual should be homozygous across multiple loci throughout the genome. In principle, then, a sample of microsatellite genotypes from a population can be used to test whether average heterozygosity carries any

The future

Apart from encouraging us to validate molecular metrics better, these new papers highlight two further issues that must be addressed. First, how do we interpret the heterozygosity–fitness correlations involving microsatellites? Bengt Hansson and Lars Westerberg recently reviewed three possibilities [10]. First, because microsatellites are generally believed to be non-functional and neutral, direct effects of the microsatellites themselves seem unlikely. Second, if, as outlined above,

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