Genetic variation at MHC, mitochondrial and microsatellite loci in isolated populations of Brown trout (Salmo trutta)
Campos, J.L., Posada, D. and Moran, P.
We have studied levels and distribution of genetic variation in nine isolated populations of Brown trout in
NW Spain. In the present study, we have tried to test the importance of preservation of genetic variability
for the survival of a set of isolated Brown trout (Salmo trutta) populations from the same river drainage.
We screened genetic variation in three different markers, mitochondrial, microsatellites and Major Histocompatibility
Complex (MHC), presumed to be under different selective pressures. Overall, genetic
diversity varied considerably across populations and the distribution of genetic variation was similar at
MHC and microsatellites; highly polymorphic populations at the microsatellite loci were also highly
polymorphic at the MHC. We also observed high levels of differentiation among populations. Although we
found evidence suggesting that balancing selection has influenced the long term evolution of the MHC,
genetic drift seems to have eroded the effect of selection, becoming the predominant evolutionary force
shaping genetic variation in some of the smaller populations. Despite current lack of variation at the MHC,
these small populations seem to have remained viable for a long time.