Principles of Genetics and Breeding

3.1.4.1 Examples for pleiotropism in fishes

1. L, D, B and G colour genes in common carp have many pleiotropic effects. For e.g., Blue (bb) and gold (gg) common carp have lowered growth rate as a pleiotropic effect.
2. Scale patterns in common carp are important qualitative phenotypes when they are grown for food. The phenotypic effect of the S and N genes have been extensively studied. 17 pleiotropic effects were detected in mirror, line and leather carp. Some pleiotropic effects include, mean growth rate of the fish, mean number of soft rays in dorsal fin and in anal fin, mean number of rays in pelvic fin, erythrocyte count, etc.
3. The pleiotropic effect of gene N in common carp. One of the allelic pairs Nn (gene N) which was lethal in the homozygous state reduced the growth rate, causing a reduction in size of many organs and diminished the viability of the heterozygotes. Pleiotropic effect of gene N in linear carp with genotype Nn includes the reduction of hemoglobin content and erythrocyte number. These carps were less resistant to heating and oxygen deficiency. Gene S has a pleiotropic influence on a host of characters, in particular on the structure of swim bladder, but pleiotropy in this case is much weaker. Heterozygous for gene S are distinguished by a slightly elevated viability. The degree of expression of the scale genes is markedly dependent upon the presence of other modified genes.
4. The pleiotropic effect of gene L in Japanese carp. “Light-coloured” carp result from a dominant mutation frequently among the Japanese decorative carp. Carp, homozygous with respect to the mutant gene (LL) die at the stages of the larvae or fry, no living fishes with this genotype are found among the fingerlings; the heterozygotes (LI) survive but posses a lowered viability. The gene ‘L’ in a heterozygote state has a pronounced pleiotropic action, the pectoral fins undergo elongation, the posterior chamber of the air bladder becomes shorter, and the head dimensions increase. The light-coloured carp have larger intestine, the protein content in the blood serum is decreased. Their growth rate is characteristically accelerated to about 20% during the first year of life and their behaviour is quieter. The lighter pattern of the pigmentation throughout the trunk is due to the stable contraction of melanophores.
5. Dominant mutation found in koi is associated with the particular light-yellow pattern (the stripe on the back and the ornamental pattern on the head). Carp with such a pattern, both homo and hetero ones (DD and Dd) are rarely viable. The gene D is pleiotropic first like the gene L, the head size is increased, the posterior chamber of the air bladder is elongated, the number of vertebrae is increased.
6. The a allele in channel catfish produces albinism in the homozygous state (aa). Pleiotropy effects of the ‘aa’ genotype are that albinos spawn later, produce smaller egg mass; produce poorer quality eggs, have a poor percentage of hatching; produce progeny that are less viable and have a poor growth rate than normally pigmented channel catfish. Moreover, because of their brighter colour albinos are subject to more predation than normally pigmented channel catfish.
7. The S allele in T. aurea produces saddle back in the heterozygous state (S+). Pleiotropic effects of the S+ genotype are vertebral anomalis in vertebrae 1, 2 and 3; abnormal pelvic, pectoral or anal fin, abnormal caudal skeletons; lowered disease resistance and reduced viability.