Analysis of a silkworm F₁ hybrid with yellow cocoon generated by crossing two white-cocoon strains: further evidences for the roles of Cameo2 and CBP in formation of yellow cocoon.

In this report, we examined the gene expression related to carotenoid transport for a silkworm F1 hybrid with yellow cocoon generated by crossing two white-cocoon strains, Qiubai and 12-260. Our results showed that, in Qiubai, Cameo2, a transmembrane protein gene belonging to the CD36 family genes, was expressed normally in the silk gland, but no intact carotenoid-binding protein (CBP) mRNA (only the truncated CBP mRNA) was detected in the midgut. In 12-260, we detected the intact CBP mRNA expression in the midgut, but no Cameo2 expression in the silk gland. Regarding the F1 hybrid from crossing Qiubai and 12-260, both Cameo2 and intact CBP mRNA expressed normally in the silk gland and midgut. HPLC detection confirmed that in the F1 hybrid the carotenoids could be absorbed from dietary mulberry leaves through the midgut and transferred to silk gland via the hemolymph, which eventually colored cocoons into yellow. We also identified four CBP mRNA isoforms expressed in the midgut of the F1 hybrid, subsequently named as variants 5-8. Our results provide further evidences for the roles of Cameo2 and CBP in the formation of yellow cocoon of silkworm.