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Histone H1 in two subspecies of Chironomus thummi with different genome sizes: homologous chromosome sites differ largely in their content of a specific H1 variant.

Chromatin of Chironomus thummi (Diptera) contains seven sequence variants of histone H1. A structurally divergent H1, variant I-1, accounts for about 20% of the total H1 in C. th. piger and for about 30% in C. th. thummi. Monoclonal antibodies against this protein have been induced and have revealed its restriction to the centromeres and to a limited number of other bands in the salivary gland chromosomes. Indirect immunofluorescence of the somatically paired homologous chromosomes of F1 hybrids indicates that the difference between the two subspecies in H1 I-1 content largely depends on differences situated at a number of distinct homologous chromosome bands. These bands were intensely decorated by antibodies against H1 I-1 in C. th. thummi but appeared virtually black in C. th. piger. The same bands, however, were decorated equally in both subspecies by an antibody that reacts with other H1 variants but does not recognize H1 I-1 and by a polyclonal anti-H1 antibody. The results suggest that H1 variant I-1 is characteristic of a specific type of chromatin that is confined to distinct chromosome segments and that is more frequent in the subspecies C. th. thummi, which has a 27% larger genome.