13. Inheritance of Main Color Varieties and Their Chromatophores

Body colors of the medaka, Oryzias latipes are manifested by chromatophores in its skin. The chromatophores responsible for body colors of this fish are melanophores, xanthophores and leucophores. All of these seem to be dermal.

In Japan, the wild type (brown) can be found and easily collected in small streams or rice fields. The domesticated medaka collectively called "Himedaka" have long been kept by goldfish breeders in Japan. The majority of them are genuine Himedaka which is xanthic in color. Since Aida (1921) called them orange-red in his classical work, this name has been followed by Japanese workers. Only a few white and other mutants are included in the cultivated populations. All these color races seem to have appeared by mutation from the wild medaka in some days. We do not know when and where they first appeared.

The first breeding experiment was performed by Ishikawa (1913). He found that brown (wild) is dominant to orange-red. The F1 offspring were all brown and in the F2 brown and orange-red appeared in a 3 : 1 ratio. On the other hand, since 1908 Toyama has been performing breeding experiments of the medaka as well as the carp and the goldfish in the fish farm of Kichigoro Akiyama, a goldfish dealer at Fukagawa, Tokyo. His result was published in 1916. In his experiment, three color varieties, viz. brown, orange-red and white (males only) were used.

At the same time, Ishiwara reported the inheritance of body colors of the medaka in 1916 and 1917. Both Toyama and Ishiwara arrived at the conclusion that orange-red is dominant to white and that the F1 of cross brown and white are all brown and in the F2 brown, blue, orange-red and white appear in a 9 : 3 : 3 : 1 ratio. Consequently, it follows that the brown has two dominant genes that control black and yellow pigments and white is double recessive.

However, the most extensive breeding experiments were performed by Aida (192l). He began his experiments in 1913 by using 40 concrete ponds and many earthenware bowls set in his garden in Kyoto. After 7 years' experiments his first paper was published in 1921. He showed that genotypes in homozygotes responsible for various color effects are represented by the following constitutions: brown (wild type), BB, RR ; blue, BB, rr; orange-red, bb, RR ; white, bb, rr, variegated orange-red, B'B',RR; variegated white, B'B',rr. The triple alleles B, B' and b controlling the formation of melanin in melanophores, are autosomal. The B which is dominant in effect to both B' and b permits full melanin formation. The B' is dominant in effect to b, and caused variegation.

The R controls full xanthic pigmentation in xanthophores and its recessive allele r stands for little or no pigmentation. The fact that blue (B, r) and white (b, r) fish in the F2 of the cross between white females (bb, rr) and brown males (BB, RR) are solely of females led him to the conclusion that R and r are sex-linked and that the sex determining mechanism of the medaka is XX for females and XY for males, viz., male is heterogametic. In the cross of white females (bb, XrXr) with brown males (BB,XRYR), blue (BB, rr or Bb, rr) and white (bb, rr) are usually females (XrXr) because there is no Yr chromosome in this mating type. Aida showed also that crossing over between the Xr and YR chromosomes rarely takes place and XR and Yr spermatozoa are produced. The Xr ovum fertilized by Yr spermaotozoon produces white male (bb, XrYr).

In 1951 Eiji Nakabori (1894-1951), a medaka fancier, collected a pair of the gray medaka in the northern outskirts of Kyoto. Aida studies this pair in his later days and found that gray fish is caused by color interferer (ci) in autosome which is independent of B, b or R, r genes. According to him when this gene is homozygous, body color becomes paler. The F1 of the gray (cici, BB, RR) mated with white (+ci+ci, bb, rr) are brown. The phenotype ci, B, R was called "pale blue" by him. However, it is preferable to call it gray. He performed two matings, viz. gray ´ white and variegated white ´ gray. He found that the new mutant gene behaved as a Mendelian autosomal recessive and independent of color genes known till then. His disease prevented him from publishing his result.

Following him the writer (Takeuchi, 1968, 1969) reported on inheritance of the ci-gene. The crossing gray (cici, BB, RR) with white (+ci+ci, bb, rr) produces the F1 which are all brown (ci+ci, Bb, Rr). In the F2 eight phenotypes, brown (+ci, B, R), gray (ci, B, R), blue (+ci, B, r), orange-red (+ci, b, r), light-blue (ci, B, r), cream (ci, b, R) and milky (ci, b, r) appeared in a 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1 retio. Of these light-blue, cream, and milky are new color races.

In the following paragraphs, details of these new color types and the other five ones are given. In reading these lines the reader is asked to refer to color photos of living fish of color varieties taken by Yamamoto (Color Plates, 1-12) as well as their color photomicrographs of chromatophores taken by the writer (Color Plates, 15-18) at the end of this book.

Light-blue (pale blue) - The body surface of the adult light-blue fish is opaque and appears light-blue, and is easily distinguished from the normal blue by the naked eye. The skin is observed to have evenly distributed melanophores. These melanophores are remarkably well-developed as those of the blue and brown. The only difference between the light-blue and other fish (brown and blue) is that the light-blue has greatly well-developed leucophores but no xanthohores are observable.

Cream - The body color of the cream fish is opaque orange yellow and is easily distinguished from the orange-red and white. In the skin of the cream there are xanthophores and leucophores similar to those in the light-blue. Xanthophores in the cream are light yellow in color and they are much fewer than those in brown and orange-red.

Milky - The milky medaka appears milky white and is easily distinguished from the white just after hatching. No xanthophores are observed in the skin. The difference between the milky and normal white is that the former has more and well-developed leucophores than the normal white. These leucophores are similar in appearance to those of the cream, and there are quite a few of them per one scale.

Fully pigmented xanthophores are observable in the skin of brown, gray, orange-red and cream which have the R gene. No pigmented xanthophores are observable in the blue, light-blue, white and milky which contain the recessive r gene.

The ci gene interfers with the effect of the R gene suppressing the deposition of the xanthic pigments in xanthophores and resulting the reduction in number of xanthophores to one half of their usual number. So that it is quite reasonable that Aida designated the gene as color interferer (ci). However, the most significant effect of the ci gene is concerned with leucophores.

As stated before, the gray, light-blue, cream and milky which contain the ci gene homozygously, have three or four times as many leucophores as the brown, blue, orange-red and white which contain the +ci gene. Furthermore, leucophores of all the ci-fish are larger in size than those of +ci-fish. This fact seems to indicate that the ci gene plays a role in the formation of leucophores. In short, the ci gene is an enhancer of leucophore formation and an inhibitor of xanthic pigmentation.

Goodrich (1927) observed chromatophores in the skin of the medaka. According to him, there are three kinds of chromatophores, viz., melanophores, brown chromatophores and xanthophores. His brown chromatophores seem to be synonymous with leucophores which appear brown through transmitted light.

Aida, T., 1921 On the inheritance of color in a freshwater fish Aplocheilus latipes Temminck and Schlegel, with special reference to sex-linked inheritance. Genetics 6 : 554-573.

Goodrich, H.B., 1927 A study of the development Mendleian characters in Oryzias latipes. J. Exp. Zool. 49:261-280.

Ishiwara, M., 1916 Inheritance of body color in Oryzias latipes. (In Japanese) Zool. Mag. 28: 177-194.

Ishiwara, M., 1917 On the inheritance of body-color in Oryzias latipes . Mitt. Med. Fac. Kais. Univ. Kyushu 4:43-47.

Ishikawa, C., 1913 Lectures on zoology. Kanazashi-Horyu-Do, Tokyo, 1:372-373.

Takeuchi, T., 1963 Relation between leucophore and ci gene in the medaka Oryzias latipes. Japan. J. Genetics 43:442.

Takeuchi, T., 1969 A study of genes in the gray medaka, Oryzias latipes, in reference to body color. Biol. J. Okayama Univ. 15: 1-24.

Takeuchi, T. and Y. Ogasawara, 1969 Electron microscopic study of the chromatophores in the medaka. Zool. Mag. 78: 89.

Toyama, K., 1916 Ichinino Mendel-seisitu ni tuite (On some Mendelian characters). Nippon Ikusyugakkai Hokoku (Rep. Jap. Breed. Soc.). 1: 1-9.