Aquaculture, Vol. 198 (1-2) (2001) pp. 63-77
©2001 Elsevier Science B.V. All rights reserved.
PII: S0044-8486(00)00581-0 
Diploid chromosome number variations and sex chromosome polymorphism in five cultured strains of rainbow trout (Oncorhynchus mykiss)

Nelson Colibueque1, Patricia Iturra2, Francisco Estay3 and Nelson F.Díaz3 

1.- Laboratorio de Genética y Acuicultura, Departamento de Ciencias Básicas, Universidad de Los Lagos, P.O. Box 933, Osomo, Chile 
2.- Programa de Genética Humana, Instituto de Ciencias Biomédicas, Facultad de Medicina,Universidad de Chile, Santiago, Chile.
3.- Departamento de Ciencias Ecológicas, Facultad de Ciencias, Universidad de Chile, Santiago,Chile 

Received 24 July 2000; received in revised form 29 August 2000; accepted 13 November 2000


This work reveals the chromosomal characteristics of the five cultured strains of rainbow trout, Oncorhynchus mykiss. The five strains Americana, Donaldson, Cofradex, Steelhead, and Kamloopsare 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-761-63). 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, presentes 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 pattem 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 describes 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 incompleto differentiation of the sex chromosome pair in the rainbow trout.

Keywords: Chromosome; Rainbow trout; Cultured strains; Robertsonian polymorphism; Sex chromosome