Structure, replication and transcriptional activity of the sex-specific heterochromatin in a moth

The female-specific heterochromatin (W chromatin) is derived from the W chromosome. It forms a single sphere in highly polyploid somatic tissues of the wild-type strain of the flour moth Ephestia kuehniella. This heterochromatin sphere consists of chromatin of a high but not uniform electron density. It contains multiple cavities and has a rather smooth outer surface. The W chromatin is well delimited from the remaining chromatin of the nucleus. Thus, probably the whole W chromosome is heterochromatic in these tissues. This assumption is confirmed by the study of strains with mutant W chromosomes. In these mutants a whole autosome is translocated to the W chromosome. The W chromatin in highly polyploid cells of these strains is split up into several bodies with a rugged surface. Cytophotometric measurements of polyploid nuclei up to values of 1000C show the DNA of the W chromatin to increase proportionally to the total DNA of the cell and to contain 3% of the total nuclear DNA. The DNA concentration is 3-8 times higher in the W chromatin than in the rest of the nucleus. As can be seen in autoradiography experiments with [³H]thymidine, replication of the W chromatin DNA is out of phase with the remaining chromatin. Using autoradiography of short-term [³H]uridine incorporation as a measure for transcriptional activity, no such activity could be detected in the female specific heterochromatin. This means that the whole W chromosome, or at least most of it, is heterochromatic and transcriptionally inactive in somatic tissues. However, there are indications that this inactivation is specific for somatic tissues while in the previtellogenetic oocyte of the adult ovary the W chromosome is transcriptionally active.