Sequence polymorphisms in coding genes and variability in quantitative trait loci (QTL)-linked markers can be used to uncover the evolutionary mechanisms of traits involved in adaptive processes. We studied sequence variation in the EDA gene and allelic variation in 18 microsatellites — one of which (Gac4174) is linked with the EDA QTL — in low, partially and completely plated morphs from eight threespine stickleback European populations. The results agree with previous studies in that EDA polymorphism is closely related to plate number variation: EDA sequences grouped populations into low and completely plated morphs, whereas microsatellites failed to do so. Furthermore, partially plated fish were heterozygous with respect to the distinctive EDA alleles for completely and low plated morphs, indicating that completely plated morph alleles are not entirely dominant in controlling the expression of lateral plate number. An examination of population differentiation in plate number with quantitative genetic methods revealed that the degree of differentiation exceeded that expected from genetic drift alone (QST > FST). Our results support the adaptive genetic differentiation of plate morphs and the view that distinctive EDA gene polymorphism occurs in similar sites across the distribution range of this species. Yet, allele frequency differentiation in the Gac4174 microsatellite locus, informative in experimental crosses for plate number variation, did not differ from that of neutral markers and, was therefore unable to detect the signature of natural selection responsible for population divergence.
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