Dosage analysis of Z chromosome genes using microarray in silkworm, Bombyx mori

In many organisms, dosage compensation is needed to equalize sex-chromosome gene expression in males and females. Several genes on silkworm Z chromosome were previously detected to show a higher expression level in males and lacked dosage compensation. Whether silkworm lacks global dosage compensation still remains poorly known. Here, we analyzed male:female (M:F) ratios of expression of chromosome-wide Z-linked genes in the silkworm using microarray data. The expression levels of genes on Z chromosome in each tissue were significantly higher in males compared to females, which indicates no global dosage compensation in silkworm. Interestingly, we also found some genes with no bias (M:F ratio: 0.8-1.2) on the Z chromosome. Comparison of male-biased (M:F ratio more than 1.5) and unbiased genes indicated that the two sets of the genes have functional differences. Analysis of gene expression by sex showed that M:F ratios were, to some extent, associated with their expression levels. These results provide useful clues to further understanding roles of dosage of Z chromosome and some Z-linked sexual differences in silkworms.