Juxtaposed microsatellite systems as diagnostic markers for admixture: an empirical evaluation with brown trout (Salmo trutta) as model organism

A juxtaposed microsatellite system (JMS) is composed of two microsatellite repeat arrays separated by a sequence of less than 200 bp and more than 20 bp. This paper presents the first empirical evaluation of JMSs for the stud y of genetic admixture induced by man, with brown trout (Salmo trutta) as m odel organism. Two distinct admixture situations were studied: native popul ations from streams of the Atlantic basin and of the Mediterranean basin, r espectively, all stocked with domestic strains originating from the Atlanti c basin. For these two situations, we first evaluated by simulation the abi lity of JMSs to differentiate between alien alleles and naturally shared ho moplasious or ancestral alleles, and thus to behave as diagnostic markers f or admixture. Simulations indicated that JMSs are expected to be reliable d iagnostic markers in most divergent (i.e. Mediterranean) populations and no nreliable diagnostic markers in most closely related (i.e. Atlantic) popula tions. Three JMSs were genotyped in domestic strains as well as in nonstock ed and stocked populations of brown trout sampled in different rivers of th e Mediterranean and Atlantic basins. The observed distributions of JMS hapl otypes were consistent with simulation predictions confirming that JMSs wer e reliable diagnostic markers only over a given proportion of the species r ange, i.e. in substantially divergent populations. JMSs also reinforced the diagnostic character of three microsatellite sites for the studied Mediter ranean populations. This last result is consistent with our simulation resu lts which showed that, although much less frequently than at JMSs, diagnost ic markers are likely to be found at single site microsatellites provided t hat the native Mediterranean population has a sufficiently small effective population size. For each population of the Mediterranean basin admixture c oefficients did not differ significantly across JMSs and mean admixture coe fficients sometimes differ among populations. The interpretation of the ori gin of JMS haplotypes based on the allele length variants was supported by nucleotide sequence analysis.