The critical electrolyte concentrations (CEC) of sperm chromatin from animal species known or suspected to contain histone H1 variants were compared by examining the affinity of their DNA-protein complexes for toluidine blue in the presence of Mg2+. Bullfrog, sea urchin, bee and bumblebee spermatozoa were studied. The CEC for Rana catesbeiana and two sea urchin species were similar to that of histone H5-containing chromatin from chicken erythrocytes, thus confirming the biochemical and structural similarities of these DNA-protein complexes. The CEC for bees and the bumblebee, Bombus atratus, showed no particular phylogenetic relationship. We concluded that the CEC of histone H1-containing sperm cell chromatin is a useful indicator of variability in DNA-protein complexes but is of little phylogenetic value.
