Underreplication of a polytene chromosome arm in the chironomid Prodiamesa olivacea.

The genome of Prodiamesa olivacea (Diptera, Chironomidae) has a 2 CDNA content of 0.25 pg. Mitotic metaphases reveal 3 pairs of chromosomes: 2 metacentric ones and one submetacentric. The latter comprises 20.8% of total Feulgen DNA. During larval polytenization the complemental portion of the 3rd falls to 6.5%. Concomitantly the polytene 3rd chromosome is much shorter than expected. It has no constriction and is shaped like a ball sector. -- Underreplication is understood as suppression of DNA syntheses mainly in the long arm of the 3rd chromosome at the first to third endoreplicative cycle. Most of the dense heterochromatin seen in the apex of the 3rd polytene element is not itself underreplicated; it conceals the underreplicated long arm of this chromosome. -- In ovarian nurse cells which are closely connected with the germ line the longer heterochromatic arm of the 3rd polyneme chromosome is fully replicated. -- Underreplication is discussed in the context of "dna silencing".