L Chromosome Behaviour and Chromosomal Imprinting in Sciara Coprophila

The retention of supernumerary chromosomes in the germ-line of Sciara coprophila is part of a highly-intricate pattern of chromosome behaviours that have fascinated cytogeneticists for over 80 years. Germ-line limited (termed L or “limited”) chromosomes are cytologically heterochromatic and late-replicating, with more recent studies confirming they possess epigenetic hallmarks characteristic of constitutive heterochromatin. Little is known about their genetic constitution although they have been found to undergo cycles of condensation and de-condensation at different stages of development. Unlike most supernumeraries, the L chromosomes in S. coprophila are thought to be indispensable, although in two closely related species Sciara ocellaris and Sciara reynoldsi the L chromosomes, have been lost during evolution. Here, we review what we know about L chromosomes in Sciara coprophila. We end by discussing how study of the L chromosome condensation cycle has provided insight into the site and timing of both the erasure of parental “imprints” and also the placement of a putative “imprint” that might be carried by the sperm into the egg.

Mouse oocyte meiosis is disturbed by knockdown of Suv4-20h

Suv4-20h was initially characterised as a histone methyltransferase (HMTase) that catalyses lysine 20 of histone H4 dimethylation (H4K20me2) and trimethylation (H4K20me3). In the present study, using RNA interference (RNAi), we found that Suv4-20h activity is required for the fidelity of chromosome distribution during meiosis in the mammalian oocyte. Knockdown of Suv4-20h resulted in attenuation of H4K20me3 and the accumulation of H4K20me1. After Suv4-20h knockdown, oocytes exhibited an increasing percentage of aberrant chromosome alignment in MI, together with a decreasing percentage of polar body I extrusion. We conclude that Suv4-20h may be required for normal chromosome behaviour and that it is crucial for proper meiotic progression in mammalian oocytes.