The genetics and histology of red,blond and associated colour variants in Oreochromis niloticus

A single red mutant was observed in a pure strain of O. niloticus (Egyptian). This was crossed to 6 normal coloured females and the progeny were used to study the genetics and histology of this commercially important trait. Other potentially important colour variants were also studied. These included blond, blotch, syrup and red melanophore. Histology showed that melanophore development rather than a biochemical pathway mutation was responsible for the colour in red fish. Histology of red tilapia showed almost a complete absence of melanin granules in the skin, blotched individuals showed areas characteristic of red and normal skin. The histology of the skin of blond fish showed the incomplete development of melanophores and melanocytes. Genetic analysis of the red trait shows that red is an autosomal dominant, and confirms that blond is autosomal recessive as is syrup. The analysis of blotch and red melanophore is not complete but suggests that both are epistatic to red. The implications of these findings are discussed in relation to the improvement of existing red strains and the importance of these colour traits for the development of new colour variants in tilapia.