A sex-linked enzyme in birds—Z-chromosome conservation but no dosage compensation

Abstract

In birds, the female is the heterogametic sex and the sex-determining system is referred to as ZZ/ZW. In mammals the male is heterogametic, and the sex-determining system is referred to as XX/XY. The mammalian X chromosome appears to have been conserved largely intact during evolution. Thus the structural gene loci for glucose-6-phosphate de-hydrogenase, phosphoglycerate kinase, α-galactosidase and hypoxanthine–guanine phosphoribosyltransferase are situated on the X chromosome of a wide variety of mammals. In addition, mammals show ‘dosage compensation’; that is, although females possess two doses of each X-linked gene while males possess only one, females produce the same level of gene product as males. In birds, cytological studies and data on sex-linked morphological mutants suggest that¹ the Z chromosomes of all birds are homologous, and that buds do not show dosage compensation for sex-linked genes. Ultimate proof of these hypotheses requires the discovery of proteins whose structural gene loci are encoded by the Z chromosome of birds¹, none of which has previously been found. Here we report that the cytoplasmic isozyme of aconitase is Z-linked in the guinea fowl and probably Z-linked in the domestic fowl, house sparrow and two species of cockatoos, thereby providing evidence for Z-chromosome homology in birds. We also show that there is an apparent lack of dosage compensation for cytoplasmic aconitase in the domestic fowl, house sparrow and spotted turtledove.