Differences between nuclear and mitochondrial introgressions of brown trout populations from a restocked main river and its unrestocked tributary

The introgression of domestic genes in wild trout populations as a result of intensive restocking has been observed to be low in numerous rivers of the Mediterranean watershed. With the aim of examining the process of introgression and improving our understanding the reasons for its possible limitations, we analysed the percentage of domestic genes for three protein-coding loci and for mitochondrial haplotypes. Two samples were analysed: a restocked river and a tributary which is no longer stocked. In order to follow the introgression over time, trout were separated into two age groups. The mitochondrial locus showed the same tendencies as those obtained with the nuclear loci, but with generally higher introgression values. A differential introgression among protein-coding loci was also observed. From these results, we discuss the possibility of selective factors acting against the domestic genes in river. The hypothesis of a difference in reproductive success between domestic males and females is also presented to explain the higher introgression of mtDNA.