In order to examine if Z-chromosome inactivation, which is analogous to X-c
hromosome inactivation in mammals, takes place in male birds having ZZ sex
chromosomes, five Z-linked genes of chickens which are expressed in both se
xes in certain tissues were selected: i.e. genes for growth hormone recepto
r, nicotinic acetylcholine receptor beta3, aldolase B, beta1,4-galactosyltr
ansferase I, and iron-responsive element-binding protein (also known as cyt
osolic aconitase). Antisense or sense riboprobe was prepared from an intron
ic sequence of each gene and subjected to fluorescence in situ hybridizatio
n to nascent transcripts of each gene in a nucleus. Each antisense riboprob
e hyridized to two spots of nascent RNA which corresponded to its gene loci
on the two Z chromosomes in a majority of nuclei in a tissue of the male.
The efficiency of detection of two spots per nucleus was comparable to that
for the glyceraldehyde-3-phosphate dehydrogenase gene, an autosomal housek
eeping gene. These results suggest strongly that Z-chromosome inactivation,
i.e. virtual silence of transcription at one of the alleles, does not take
place for these five Z-linked genes in male chickens.