Principles of Genetics and Breeding

6.1.2.3.Polyploidy in fishes

Haploid individuals among fishes are non-viable.

• When the egg development is stimulated by spermatozoa with destroyed nuclei, almost all the developing embryos become haploid, the development proceeding with malformations (haploid syndrome) and resulting in embryonic death at later stages of embryogenesis.

Triploids in fishes appear quite frequently.

• Apparently, in most fish species the reductional division may be omitted during meiosis (predominantly in females), and the resulting gametes are diploid.
• The fertilization of such an egg by a normal spermatozoan (as well as the fertilization of the normal egg by a diploid spermatozoan) leads to the appearance of triploid organisms.
• They can be quite viable : for example, certain varieties of Carassius auratus gibelio and of the viviparous fishes Poeciliopsis and Poecilia are triploid.
• Triploid organisms have recently been found in the rainbow trout.
• The decreased fertility of the triploids in bisexual fishes (associated with the irregular segregation of chromosomes in meiosis) apparently explains the absence of such forms in the natural populations of most species.
• Triploids appear in fishes from time to time as a consequence of distant hybridization.
• Probably tetraploids also appear from time to time as a result of fusion of the diploid gametes, but there are no data on the frequency of these chromosomal mutations in nature.