Population estimators or progeny tests: what is the best method to assess null allele frequencies at SSR loci?
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Nuclear SSRs are notorious for having relatively high frequencies of null alleles, i.e. alleles that fail to amplify and are thus recessive and undetected in heterozygotes. In this paper, we compare two kinds of approaches for estimating null allele frequencies at seven nuclear microsatellite markers in three French Fagus sylvatica populations: (1) maximum likelihood methods that compare observed and expected homozygote frequencies in the population under the assumption of Hardy-Weinberg equilibrium and (2) direct null allele frequency estimates from progeny where parent genotypes are known. We show that null allele frequencies are high in F. sylvatica (7.0% on average with the population method, 5.1% with the progeny method), and that estimates are consistent between the two approaches, especially when the number of sampled maternal half-sib progeny arrays is large. With null allele frequencies ranging between 5% and 8% on average across loci, population genetic parameters such as genetic differentiation (F ST) may be mostly unbiased. However, using markers with such average prevalence of null alleles (up to 15% for some loci) can be seriously misleading in fine scale population studies and parentage analysis.
KeywordsMicrosatellites Fagus sylvatica Null alleles Progeny Population genetics
Data used for this paper were generated in the course of the European project DYNABEECH (Contract QLRT-1999-01210) and of a national project supported by the French Bureau des Ressources Génétiques (project BRG No. 88-2003-2004, AIP 223). A meeting for co-writing of this paper was supported by the European Commission through the Network of Excellence EVOLTREE (contract number: 016322) under the Sixth Framework Programme. We thank B. Jouaud, A. Roig (INRA, Avignon, France) and R. Pastorelli (CNR, Firenze, Italy) for laboratory work, as well as J. Glaubitz, and one anonymous reviewer for detailed comments on a previous version of the manuscript.
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