Positional cloning of silkworm white egg 2 (w-2) locus shows functional conservation and diversification of ABC transporters for pigmentation in insects

The white, scarlet and brown genes of Drosophila melanogaster encode three half-type ATP-binding cassette (ABC) transporters. In Drosophila, precursors of ommochromes and pteridines are transported by White/Scarlet and White/Brown heterodimers, respectively. The white egg 2 (w-2) mutant of the silkworm, Bombyx mori, has white eggs and eyes because of lack of ommochrome granules in the serosa and eyes. Here, we report that the silkworm w-2 locus encodes an ortholog of Drosophila scarlet. Our results indicate that Bombyx Scarlet forms a heterodimer with Bombyx White to transport ommochrome precursors, suggesting that formation of a White/Scarlet heterodimer and its involvement in the transport of ommochrome precursors are evolutionarily ancient and widely conserved traits in insects. Contrary to dipteran insects, white and scarlet were juxtaposed in a head-to-tail orientation in the silkworm genome, suggesting that the origin of white and scarlet was a tandem duplication of their ancestral transporter gene. In Bombyx, White is also essential for the transport of uric acid in larval epidermis. However, our results suggest that a Bombyx White/Scarlet heterodimer is not involved in this process. Our results emphasize the functional conservation and diversification of half-type ABC transporter families in insects, which may contribute to their extremely diverse color patterns.