Chromatin diminution and its possible biological meaning

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Chromatin diminution (СD) is a phenomenon of germline and somatic cell differentiation during early embryonic development, associated with the loss of a portion of chromatin that represents the chromosomal constitutive heterochromatin (CH). In СD, 96-98% of chromatin is deleted in somatic cells (macronucleus), whereas in germline cells (micronucleus) they are completely preserved. Despite the fact that the phenomenon of СD was discovered in the in the late 18th century, its biological role remains to be elucidated. Numerous hypotheses have been proposed, which are the subject of discussions. Based on data obtained during the study of variability of human CH, the hypothesis of their participation in cell thermoregulation (CT) was substantiated. The essence of СT is that it equalizes temperature differences between the nucleus and cytoplasm, namely, excess metabolic heat from the cell nucleus is removed into the cytoplasm by means of a dense layer of condensed chromatin (CC) around the nucleus, consisting of CH. We discuss the view that the phenomenon of CD in parasitic animals may be related to CT in somatic cells, in the sense that in them the problem of thermoregulation is solved by the host organism and for this reason they do not need to retain full CH as the material basis of CT. Chromatin preservation in germ cells is necessary for conjugation, synaptonemal complex formation and recombination of homologous chromosomes during meiosis.