Hs. Chandra, PROPOSED ROLE OF W CHROMOSOME INACTIVATION AND THE ABSENCE OF DOSAGE COMPENSATION IN AVIAN SEX DETERMINATION, Proceedings - Royal Society. Biological Sciences, 258(1351), 1994, pp. 79-82
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.