Principles of Genetics and Breeding

1.3.7 Polyploidy in fishes

• Polyploid forms are found naturally in six different orders of fishes: Acipenseriformes, Salmoniformes, Cypriniformes, Siluriformes, Poecilloformes and in one species of Perciformes.
• Tetraploid fishes are, however, the most common among the polyploid forms. Some species of primitive fish, including the paddlefish, Polyodon spathula, the shovelnose sturgeon, Scaphirhynchus platorhynchus, the lung fish, Protopterus dolloi appear to be tetraploid.
• Three species of cyprinids, the barb (Barbus barbus), the carp (Cyprinus carpio), and the goldfish (Carassius auratus) appear to be teraploid. The yellow fish species of the genus Barbus (Cyprinidae) found in South Africa are hexaploids. The Acipensiform species Acipenser gueidenstaedti is an octaploid species having the chromosome number, 2n=250+8.
• The polyploid origin of Salmo salar (Salmonidae) is suggested on the basis of the presence of quadrivalents in the meiotic prophase, duplicate expression of isozyme genes and for possessing high C-value.
• Cyprinus carpio (Cyprinidae) is also believed to be a tetraploid species based on its high chromosome number and C-value although quadrivalent formation does not takes place during meiosis. Some isozyme loci manifest tetraploid expression pattern but a good number of genes in this species are expressed in diploid dosage. Thus, although the genome is polyploid, many gene loci behave as diploid in terms of gene expression. This has led to the belief that C. carpio is in the process of diploidization.
• Polyploidy is believed to have played an important role in the evolution of fishes. Allopolyploidy, which is formed by polyploidization of the hybrids of related species, might have played more important role than the autopolyploidy. The triploid poecilid fishes probably originated by allopolyploidy.