Chromosome size in salmon and trout

The chromosomes of the Atlantic salmon, Salmo salar (2n=58) are, on average, larger than those of the trout, S. trutta (2n=80). If the difference in chromosome size represents a permanent change in chromosome structure as between the two species the expectation is that the size difference between salmon and trout chromosomes will be maintained in the hybrid. If, alternatively, the size difference between salmon and trout chromosomes is genotypically determined the difference will not be maintained in nuclei of hybrid genotype. Measurements of a specific chromosome, S, of the salmon complement and of another, S ¹, of the trout complement in nuclei of parent species and of the hybrid show that the difference in size is maintained in hybrid nuclei. It is concluded therefore that the size difference between salmon and trout chromosomes is due to structural change rather than to genotypic control.