Differences in Pigment Content and Expression of Cocoon Color Formation-Related Genes in Multiple Silkworm Strains

Deciphering the coloration mechanism of natural-colored cocoons in the domestic silkworm (Bombyx mori) is of great importance for the green and sustainable development of the sericulture industry. In this study, 14 silkworm strains were selected for studying differences in the coloration mechanism of diverse cocoon colors, and the present research carried out integrated investigations from three aspects: pigment content, differences in gene expression levels, and gene structural variation. The results demonstrated that pigment accumulation presented distinct tissue-specific and strain-specific characteristics. The middle silk gland (MSG) acts as the primary locus for pigment deposition: silkworm strains forming yellow or red cocoons accumulate carotenoids at high levels in this tissue, whereas those producing green cocoons show abundant flavonoid enrichment here. Analysis of gene expression profiles indicated that the expression patterns of core transporter genes are highly correlated with the spatial distribution of pigments. The expression level of CBP gene in the MSG is over 10-fold higher than that in the midgut (MG) among yellow/red cocoon strains. The pivotal glycosylation gene UGT86 displayed remarkably elevated expression in the MSG relative to other tissues across all green cocoon silkworm strains. The CBP gene acts as a core regulatory factor governing the transport of carotenoid pigments, and notable disparities existed in the coding region of the gene among silkworm strains with different cocoon colors. In contrast to yellow and red cocoon strains, the transcription start site of CBP gene is displaced in silkworm varieties that form green or white cocoons. In summary, this study clarified the expression patterns and variations in key pigment deposition-related genes at the population level for the first time and provided data references for the study of the biological basis and coloration mechanism of diverse cocoon colors.