Unusual chromatin state in Rhynchosciara americana (Diptera: Sciaridae).

Euchromatin and heterochromatin are usually defined by the degree of DNA compaction, gene content and combinations of histone and non-histone proteins. More recent studies on protein location have been able to specify a variety of chromatin types thus adding chromatin configurations other than the two basic reference states. Chromatin research exploiting non-model organisms has the potential to provide novel information related to epigenetic modifications and their impact on chromosome structure and function. Polytene chromosomes of Rhynchosciara americana display a particular region within the A9 sub-section characterised by lack of DNA compaction as well as an usual polytene banding pattern. DNA content in the sub-section seems to be low as deduced by DAPI staining. Antibodies to H3K4me, a conserved epigenetic transcription marker,labelled the A9 sub-section strongly. In contrast,transcriptional activity in the region, if any, seems to be low as inferred by detection of RNA polymerase II and RNA. Histone markers related to heterochromatin formation such as H3K9me and H3K27me are underrepresented in the A9 sub-section. However, a chromodomain-containing sciarid protein was detected in the region, displaying levels of fluorescence very close to those observed in pericentric heterochromatin.A plasmid micro-library constructed with microdissected DNA from the A9 sub-section was screened for repetitive DNA. The proportion of inserts containing repeats was found to be similar to that contained in another micro-library made with DNA from a single chromosome end of this species. The data suggest an unusual "chromatin colour" indicating that high levels of histone markers related to transcription coexist with a significant presence of chromodomain-containing proteins and the virtual absence of histone modifications observed in heterochromatin formation.