Dosage compensation is less effective in birds than in mammals

TLDR

Dosage compensation of sex‑chromosome genes is essential for survival and has evolved diverse mechanisms in mammals, but its status in birds with ZW females and ZZ males remains unclear. The study used microarray analysis to compare male‑to‑female expression ratios of Z‑linked and autosomal genes in zebra finch, chicken, mouse, and human. Birds exhibit higher male‑to‑female expression ratios for Z‑linked genes than for autosomal genes, indicating ineffective dosage compensation, whereas mammals show comparable ratios and effective compensation.

Abstract

Abstract Background In animals with heteromorphic sex chromosomes, dosage compensation of sex-chromosome genes is thought to be critical for species survival. Diverse molecular mechanisms have evolved to effectively balance the expressed dose of X-linked genes between XX and XY animals, and to balance expression of X and autosomal genes. Dosage compensation is not understood in birds, in which females (ZW) and males (ZZ) differ in the number of Z chromosomes. Results Using microarray analysis, we compared the male:female ratio of expression of sets of Z-linked and autosomal genes in two bird species, zebra finch and chicken, and in two mammalian species, mouse and human. Male:female ratios of expression were significantly higher for Z genes than for autosomal genes in several finch and chicken tissues. In contrast, in mouse and human the male:female ratio of expression of X-linked genes is quite similar to that of autosomal genes, indicating effective dosage compensation even in humans, in which a significant percentage of genes escape X-inactivation. Conclusion Birds represent an unprecedented case in which genes on one sex chromosome are expressed on average at constitutively higher levels in one sex compared with the other. Sex-chromosome dosage compensation is surprisingly ineffective in birds, suggesting that some genomes can do without effective sex-specific sex-chromosome dosage compensation mechanisms.