The karyotypes of salmonid fishes including taxa in the three subfamilies Coregoninae, Thymallinae and
Salmoninae are described. This review is an update of the (Hartley, 1987) review of the chromosomes of
salmonid fishes. As described in the previous review, the karyotypes of salmonid fishes fall into two main
categories based on chromosome numbers: the type A karyotypes have diploid numbers close to 80 with
approximately 100 chromosome arms (2n = 80, NF = 100), and the type B karyotypes have diploid
numbers close to 60 with approximately 100 chromosome arms (2n = 60, NF = 100). In this paper we have
proposed additional sub categories based on variation in the number of chromosome arms: the A′ type with
NF = 110–120, the A″ type with NF greater than 140, and the B′ type with NF less than 80. Two modes of
chromosome evolution are found in the salmonids: in the Coregoninae and the Salmoninae the
chromosomes have evolved by centric fusions of the Robertsonian type decreasing chromosome numbers (2n)
while retaining chromosome arm numbers (NF) close to that found in the hypothetical tetraploid ancestor
so that most extant taxa have either type A or type B karyotypes. In the Thymallinae, the chromosomes have
evolved by inversions so that chromosome arm numbers (NF) have increased but chromosome numbers (2n)
close to the karyotype of the hypothetical tetraploid ancestor have been retained and all taxa have type A″
karyotypes. Most of the taxa with type B karyotypes in the Coregoninae and Salmoninae are members of
the genus Oncorhynchus, although at least one example of type B karyotypes is found in all of the other genera.
These taxa either have an anadromous life history or are found in specialized lacustrine environments.
Selection for increases or decreases in genetic recombination as proposed by Qumsiyeh, 1994 could have
been involved in the evolution of chromosome number in salmonid fishes.