Negative covariance suggests mutation bias in a two-locus microsatellite system in the fish Sparus aurata

Constraints on microsatellite length appear to vary in a species-specific manner. We know very little about the nature of these constraints and why they should vary among species. While surveying microsatellite variation in the Mediterranean gilthead sea bream, Sparus aurata, we discovered an unusual pattern of covariation between two closely linked microsatellite loci. One- and two-locus haplotypes were scored from PCR amplification products of each locus separately and both loci together. In a sample of 211 fish, there was a strong negative covariance in repeat number between the two loci, which suggests a mechanism that maintains the combined length below a constrained size. In addition, there were two clusters of the same combined haplotype length, one consisting of a long repeat array at one locus and a short array at the other and vice versa. We demonstrate that several models of biased mutation or natural selection, in theory, could generate this pattern of covariance. The common feature of all the models is the idea that tightly linked microsatellites do not evolve in complete independence, and that whatever size dependence there is to the process, it appears to "read" the combined size of the two loci.