Chromosomal distribution of tissue-specific genes present only in one single tissue. Chromosomal distribution of our own dataset of tissue-specific genes. We required those to be present in a single tissue but completely absent in all other tissues. Testis- and ovary- specific genes are respectively under- and over- represented in the X chromosome (Chi-square test with Yates correction: P = 0.0082, n = 468; P = 0.0001, n = 17, respectively). Other tissue-specific genes were polled together and are not under-represented in the X chromosome (Chi-square test with Yates correction: P = 0.97, n = 118). The sample sizes for individual sets of tissue-specific genes are: accessory glands = 10; head = 37; brain = 20; larval hindgut = 26; larval midgut = 8. Hindgut, midgut, adult carcass, crop, salivary gland, thoracicoabdominal ganglion and larval and adult tubules < 5 (each). Except for brain-specific genes that are over-represented in the X chromosome (Chi-square test with Yates correction: P = 0.01), no other tissue show non-random chromosome distribution (for cases where n ≥4). We analyzed only old genes since the chromosome X is known as enriched with new genes with testis-biased expression. Figure S3 in Additional file 1 show more details for differences between gene age and chromosomal distribution of testis-specific genes.