PROPOSED ROLE OF W CHROMOSOME INACTIVATION AND THE ABSENCE OF DOSAGE COMPENSATION IN AVIAN SEX DETERMINATION

Three features of avian sex chromosomes - female heterogamety (ZZ male , ZW female), the apparently inactive state of the W chromosome, and d ose-dependent expression of Z-linked genes - are examined in regard to their possible relation to sex determination. It is proposed that the W chromosome is facultatively heterochromatic and that the Z and W ch romosomes carry one or more homologous sex-determination genes. The ab sence of dosage compensation in ZZ embryos, and W inactivation in ZW e mbryos, would then bring about a 2n(ZZ)-n(ZW) inequality in the effect ive copy number of such genes. The absence of dosage compensation of Z -linked genes in ZZ embryos is viewed as a means by which two copies o f Z-W homologous sex determination genes are kept active to meet the r equirements of testis determination. W inactivation may promote ovaria n development by reducing the effective copy number of these genes fro m 2n to n. If there is a W-specific gene for femaleness, spread of het erochromatization to this gene in cells forming the right gonadal prim ordium may explain the latter's normally undifferentiated state; rever sal of heterochromatization may similarly explain the development of t he right gonad into a testis following left ovariectomy.