Principles of Genetics and Breeding

1.3.5 Sex chromosomes in fishes

• Sex chromosomes (allosomes) are those chromosomes that usually determine an individual’s sex and the pairs of sex chromosomes are often morphologically different in the two sexes. Scientists studying fish chromosomes were unable to find heterochromatin in them for quite a long time. True heterochromosome, differing in their size and structure, were found recently in a number of species. Almost all types of sex chromosomes indicating male heterogamety multiple sex elements have been reported in fishes. Autosomes are the other pairs of chromosomes and are morphologically the same in males and females.
• Male heterogamety, that is XX♀-XY♂ sex chromosomes, is more prevalent among fishes than the female heterogamety (ZZ♂ -ZW♀). Male heterogamety, for example, is found in some deepwater fishes and female heterogamety in several species of catfishes.
• The occurrence of multiple sex chromosomes was reported in few species like Chiondraco harmatus, an Antartic fish. The sex chromosomes in lake trout (Salvelinus namaycush) could be detected by banding techniques. The X and Y chromosomes in this species are distinguishable by the prominent C-band on the X-chromosomes, which is much reduced in case of the Y-chromosomes.
• The recognition of sex heterochromatins, including molecular approach with sex-specific DNA probes as done in humans and domestic mammals, will bring many new data about sex determination systems in fishes.
• The sex chromosomes of the different species of Xiphophorus are homologous with one another. Of karyotyped cichlids, Geophagus brasilliensis has been reported to possess distinct sex chromosomes. Salmo gairdneri and Xiphophorus maculatus possess cytologically distinct sex chromosomes.