Cardinal Black - January Came Close (2022) [24-48]

In this study, we found that a missense mutation in the Bm-cardinal gene is responsible for the silkworm pe locus. The original Drosophila cardinal gene product has been regarded as a phenoxazinone synthase for converting 3-hydroxykynurenine into xanthommatin (Howells et al., 1977; Harris et al., 2011). Although both the Bombyx pe mutant and the Drosophila cardinal mutant seem to accumulate the ommochrome precursor 3-hydroxykynurenine (Supplementary Figure S6a, Inagami, 1954; Howells et al., 1977), it should be noted that they have different eye color phenotypes. Drosophila cardinal has reduced ommochrome levels at eclosion and becomes close to the wild-type compound eye color during aging, but the pink eyes of the Bombyx pe mutant do not become black like those of the wild type. Notably, eye pigments of most insects, including Bombyx, have been reported to be a mixture of purple ommin pigments and reddish xanthommatin, which gives eyes black coloration in many insects, whereas ommin is absent in eyes of higher Dipteran species including Drosophila (Linzen, 1974). In the present study, we found that decarboxylated xanthommatin is also a major pigment of wild-type B. mori eyes (Supplementary Figure S10). Because decarboxylated xanthommatin can be removed by methanol wash, it has been missed in the previous study. By TLC and HPLC-MS results, we confirmed obvious decrease of not only red-purple pigment that is a good candidate for ommin, but also xanthommatin and decarboxylated xanthommatin in Bm-cardinal-deficient insect eyes. Instead, we found that substances with different molecular weights are the majority of the mass peak of the same retention time with xanthommatin and decarboxylated xanthommatin, suggesting that other substances, possibly pigments, are produced in these insects. We could not identify these substances as well as ommins by HPLC-MS, and this deserves future studies.

Cardinal Black - January Came Close (2022) [24-48]

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