INTROGRESSION BETWEEN TWO CUTTHROAT TROUT SUBSPECIES WITH SUBSTANTIAL KARYOTYPIC, NUCLEAR AND MITOCHONDRIAL GENOMIC DIVERGENCE

Ulf Gyllensten ¹, Robb F. Leary ², Fred W. Allendorf ², and Allan C. Wilson ³

¹ Department of Genetics, University of Stockholm, S-10691, Stockholm, Sweden, Department of Biochemistry, University of California, Berkeley, California 94720
² Department of Zoology, University of Montana, Missoula, Montana 59812
³ Department of Biochemistry, University of California, Berkeley, California 94720

The authors used allozymes encoded by nuclear genes and restriction enzyme analysis of mitochondrial DNA (mtDNA) to study secondary contact between westslope (Salmo clarki lewisi) and Yellowstone cutthroat trout ( Salmo clarki bouvieri) in Forest Lake, Montana. Eleven diagnostic allozyme loci identified this as a random-mating hybrid swarm. No parental, first-generation hybrid or backcross genotypes were detected in the sample (N = 33), and genotype distributions at all the variable loci conform to binomial expectations. There is little linkage disequilibrium between the diagnostic loci, indicating that the nuclear genomes of the two subspecies are largely randomly associated. The allozymes and mtDNA give identical estimates of the proportional genetic contribution of each subspecies. Thus, males and females from both subspecies have contributed equally to this hybrid swarm. Although these subspecies have accumulated substantial genetic divergence between their nuclear (Nei's D = 0.34) and mitochondrial (2% sequence divergence) genomes, this has not resulted in a genetic barrier to exchange between them.