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 m anner. We know very little about the nature of these constraints and why th ey should vary among species. While sure-eying microsatellite variation in the Mediterranean gilthead sea bream, Sparus aurata, we discovered an unusu al pattern of covariation between two closely linked microsatellite loci. O ne- and two-locus haplotypes were scored from PCR amplification products of each locus separately and both loci together. In a sample of 211 fish, the re was a strong negative covariance in repeat number between the two loci, which suggests a mechanism that maintains the combined length below a const rained size. In addition, there were two clusters of the same combined hapl otype length, one consisting of a long repeat array at one locus and a shor t array at the other and vice versa. We demonstrate that several models of biased mutation or natural selection, in theory, could generate this patter n of covariance. The common feature of all the models is the idea that tigh tly 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.