Chromatin Diminution

Abstract

Chromatin diminution is defined as chromosomal fragmentation, followed by the elimination of part of the chromosome during mitosis. The process was first observed in early cleavage divisions of the parasitic nematode Parascaris equorum embryo by Boveri in 1887. It was later found to also occur in some other parasitic nematodes, as well as a number of unicellular and metazoan species in diverse taxonomic groups. While chromatin diminution occurs in diverse higher order taxa, within individual taxa, it occurs only in a rather small number of species. The process appears to play different biological roles in different organisms, may use different mechanisms and is likely to have arisen multiple times during evolution.

Key Concepts

  • The term ‘chromatin diminution’ describes the programmed elimination of parts of chromosomes during mitotic divisions and is distinguished from chromosome elimination, during which entire chromosomes are eliminated.
  • Chromatin diminution, like chromosome elimination, can affect both copies of a chromosome leading to the complete removal of the corresponding genetic information.
  • Alternatively, chromatin diminution or chromosome elimination can affect only one of the two homologous chromosomes leading to a reduction in gene dose. In other systems such a state where in one cell different regions of the genome are present at different copy numbers can be achieved by differential amplification of various regions of the genome.
  • If chromatin diminution occurs only in some of the cells of an individual, it leads to a deviation from the dogma that all cells in an organism contain equal genetic information.
  • In most known cases, chromatin diminution creates a difference between somatic and germ line cells or between the sexes.
  • Chromatin diminution is a mechanism to silence genes (throwing them away approach to gene regulation) or reduce gene dosage.
  • Chromatin diminution may also serve to reduce the burden of ‘selfish DNA’ in somatic cells while maintaining it in the gem line.
  • Chromatin diminution has arisen independently multiple times during evolution (convergent evolution) and its mechanisms likely differ.

Keywords: chromatin diminution; DNA elimination; programmed genome rearrangement; germ line‐soma differentiation; sex determination

Figure 1. Chromatin diminution in Parascaris univalens. (a) Anaphase of the first cleavage division. (b) Anaphase of the second cleavage division. Chromatin diminution occurs in the upper S1 cell but not in the lower P1 cell. (c) Four‐cell stage after completion of the second cleavage division. The cells S1a, S1b and S2 give rise to the somatic cells, while the P2 cell represents the germ line. P0, zygote; C, centromere; E, eliminated chromatin; N, nucleus; P0–P2, germ line and S1, S1a, S1b, S2, presomatic cells.Reproduced with permission from Müller et al. 1996 © John Wiley & Sons Ltd.
Figure 2. Cell lineage of the early embryo of P. univalens. Chromatin diminution is indicated by a broken bar. The germ line lineage is drawn in thick red lines. Presomatic cell lineages before elimination are represented in thin red lines and somatic cell lineages in broken red lines.Reproduced with permission from Müller et al. 1996 © John Wiley & Sons Ltd.
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Further Reading

Bracht JR (2014) Beyond transcriptional silencing: is methylcytosine a widely conserved eukaryotic DNA elimination mechanism? Bioessays 36: 346–352.

Escriba MC and Goday C (2013) Histone H3 phosphorylation and elimination of paternal X chromosomes at early cleavages in sciarid flies. Journal of Cell Science 126: 3214–3222.

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Streit A, Wang J, Kang Y and Davis RE (2016) Gene silencing and sex determination by programmed DNA elimination in parasitic nematodes. Current Opinion in Microbiology 32: 120–127.

Zufall RA, Robinson T and Katz LA (2005) Evolution of developmentally regulated genome rearrangements in eukaryotes. Journal of Experimental Zoology. Part B, Molecular and Developmental Evolution 304: 448–455.

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Streit, Adrian, and Davis, Richard E(Nov 2016) Chromatin Diminution. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001181.pub2]