Abstract

This work reveals the chromosomal characteristics of the five cultured strains of rainbow trout, Oncorhynchus mykiss. The five strains - Americana, Donaldson, Cofradex, Steelhead, and Kamloops - are those commonly used in the Chilean salmon farming industry. Chromosome characterization involved the determination of diploid chromosome number (2n), chromosome arm number (NF) and sex chromosome of individuals by Hoechst 33258/Actinomicyn D (H/AmD) fluorescent stain. Considering all of the strains studied, the 2n ranged from 58 to 63 chromosomes. All of the karyotypes had NF = 104. Within this range of variation, the strains exhibited low (Americana, 2n = 59-60; Steelhead and Kamloops, 2n = 58-61), intermediate (Cofradex, 2n = 59-62) or high chromosome numbers (Donaldson, 2n = 61-63). In strains with the most extreme diploid chromosome numbers, the most frequent 2n corresponded to 59 and 61 chromosomes. Sex chromosome determination revealed that 24.5% of specimens of both sexes, presented an unexpected sex chromosome. In males, "XX" specimens were observed, whereas in females, "XY" or "XXX" individuals were recorded. The subtelocentric (st) morphology and centromeric H/AmD banding pattern of the additional "X" chromosome in "XX males" and "XXX females" is similar to that observed in the regular X chromosome. The chromosome number variation in the strains studied is consistent with the Robertsonian-type polymorphism described for rainbow trout. The presence of different chromosome numbers among strains, could reflect the chromosome differences existing within ancestral trout populations, or the presence of a certain degree of mixing with other populations of different geographical origin. The discovery of specimens with an unexpected sex chromosome is suggestive of an incomplete differentiation of the sex chromosome pair in the rainbow trout. © 2001 Elsevier Science B.V.