A dual chromatin organization in the sperm of the bivalve mollusc Spisula solidissima

Most of the DNA in the sperm of the bivalve mollusc. Spisula solidissima, is found to be associated with a specific high-molecular-mass, protamine-like component, sharing features common both to protamines and to histones. We have found that this component coexists, in the mature sperm nucleus, with a complete set of histones, including an H1-like histone. Such histones account for approximately 20% of the whole protein content in the sperm chromatin, the overall protein/DNA ratio (w/w) being 0.87. These data, together with micrococcal nuclease digestions in combination with salt fractionation, have allowed us to propose a structural model for this chromatin in which short nucleosomal domains are interspersed in a highly saturated protamine-DNA complex.