APPEARANCE OF RED EYES IN KOI IS CONTROLLED BY DOMINANT MUTATION WHICH CAUSES ONLY PARTIAL SKIN DEMELANIZATION IN WILD-TYPE COLOR COMMON CARP
Albinism is the absence of black pigment, melanin, both in the skin and in the eyes. Albino fish usually have a yellowish body and red eyes. In many fish species, albinism is controlled by an autosomal recessive mutation of one gene. The initial purpose of this study was to investigate the inheritance of albinism in koi. However, the results of the study showed that the appearance of red-eyed koi results from the action of not an albino but another demelanization mutation.
An F1 progeny was obtained by crossing a yellow red-eyed koi male (akame kigoi according to Japanese classification) with a black-eyed white-red (Kohaku) female. The F1 progeny consisted of both red-eyed (Figure 1) and black-eyed fish with segregation close to the 1:1 Mendelian ratio. It was noticed that red-eyed koi seemed to have darker eyes than typical albino fish in other species. F2 progenies were obtained by crossing of F1 fish. Segregations red-eyed : black-eyed in F2 were recorded at both larval and juvenile stages. At the larval stage, future red-eyed fish had a light eye lens (Figure 2) while the retinal epithelium was dark which is not typical for albino larvae. The crosses black-eyed x black-eyed resulted in appearance of black-eyed fish only in F2. In progenies obtained by crosses red-eyed x red-eyed the segregations red-eyed : black-eyed were close to the 3:1 Mendelian ratio while crosses red-eyed x black-eyed resulted in segregations close to the 1:1 Mendelian ratio in F2. Based on the obtained data it was suggested that the appearance of red eyes in koi is controlled by a dominant mutation of one gene (A/a); koi with genotypes AA and Aa have red eyes while koi with genotype aa have black eyes.
One red-eyed koi female from the F1 progeny was also crossed with a dark wild-type color common carp male. At the larval stage the 1:1 segregation (dark lens : light lens) was observed in obtained progeny. However, all juveniles in this progeny had black eyes, and segregation with regard to fish body color was observed. About fifty percent of juveniles had dark body color typical for wild-type common carp while the other fifty percent of juveniles had yellowish body color, which was lighter than the wild-type color type but fish from this group had some melanin in the skin. This shows that the investigated mutation causes appearance of red eyes only in koi, which do not have melanin in the skin. In wild-type color common carp this mutation causes only partial demelanization of the fish body.