a, Percentage of genes with female-biased (red), non-biased (grey) and male-biased (blue) expression on chromosome arms. Muller's elements and arms are indicated (X chromosomes in bold). Significant under-representation from a random distribution of genes in an expression class (chi-squared test) is noted (*P<10−4). The top 20% of differentially expressed genes (by ranked P values) are assigned a sex-biased expression class. b, Box plots of average hybridization intensities for males by chromosome arm. Twenty-fifth to seventy fifth percentiles (boxes), medians (lines in boxes) and ranges (whiskers) are indicated. c, As in a, but with genes with predicted testis-biased expression removed. D. melanogaster, D. mel; D. simulans, D. sim, D. yakuba, D. yak; D. ananassae, D. ana; D. pseudoobscura, D. pse; D. virilis, D. vir; D. mojavensis, D. moj.