The critical electrolyte concentrations (CEC) of sperm chromatin from anima
l species known or suspected to contain histone H1 variants were compared b
y examining the affinity or their DNA-protein complexes for toluidine blue
in the presence of Mg2+. Bullfrog, sea urchin, bee and bumblebee spermatozo
a were studied. The CEC for Rana catesbeiana and two sea urchin species wer
e similar to that of histone MS-containing chromatin from chicken erythrocy
tes, thus confirming the biochemical and structural similarities of these D
NA-protein complexes. The CEC for bees and the bumblebee, Bombus atratus, s
howed no particular phylogenetic relationship. We concluded that the CEC of
histone HI-containing sperm cell chromatin is a useful indicator of variab
ility in DNA-protein complexes but is of little phylogenetic value.