Introgression of distant mitochondria into the genome of Gyrodactylus salaris: Nuclear and mitochondrial markers are necessary to identify parasite strains

Novel combinations of mitochondrial DNA (CO1) and internal transcribed spacers of nuclear ribosomal DNA ( ITS) were detected among Gyrodactylus parasites on brown trout ( Salmo trutta L.), rainbow trout ( Oncorhynchus mykiss (Walbaum)), and Ohrid trout ( Salmo letnica (Karaman)) from salmonid farms in Poland and Macedonia. Some clones differed from standard ITS only by <= 4 nucleotides, but they belonged to a mtDNA clade that differed from the Northern European lineages of G. salaris by d(MCL) = 0.266 +/- 0.108 (maximum composite likelihood distance). The divergence of d(MCL) = 0.013 +/- 0.005 within the alien mtDNA clade suggested introgression from an unknown maternal ancestor into the G. salaris Malmberg genome 137 to 57 kyr ago ( or, less probably, repeated introgression). A comparable modern hybrid was detected based on permanently heterozygous ITS (28 bp/1264 = 2.2%) in a clone that is widespread throughout Finnish rainbow trout farms. This was a F 1 hybrid of maternal G. pomeraniae Kuusela, Zietara et Lumme ( on roach, Rutilus rutilus ( L.)) and G. lavareti Malmberg ( on whitefish, Coregonus lavaretus (L.)). The mtDNA of the parental species differed by d(MCL) = 0.290 +/- 0.130. The observations emphasize that both nuclear and maternally-inherited DNA markers are necessary to characterize sexually-produced lineages or clones of Gyrodactylus. The hybridization events demonstrated predict incongruence of mitochondrial vs. nuclear gene trees, i.e., reticulate evolution in species trees.