Proportion (A) and density plot (B) of expression levels of sex-biased genes on each chromosome. (A) Proportion of sex-biased genes (number of sex-biased genes/total expressed genes on each chromosome). Blue and red bars respectively show proportions of male- and female-biased genes on each chromosome. Please see Additional file 1: Table S7 for detailed data. Chromosome 4 is not shown due to the small gene numbers. (B) Density plot of expression levels of sex-biased genes on the X chromosome in antennae. Male-biased genes are shown in blue and female-biased genes in red. Significance was assessed by a chi-squared test.