Why is there such a diversity of sperm nuclear basic proteins in animals, plants, protozoans and algae? In somatic cells, evolutionarily conservative histones combine with DNA to form nucleosomes, fundamental particles of chromatin organization. However, in animal sperm cells, basic proteins that bind to DNA include: type 1 arginine-rich protamines of low molecular weight in salmon; type 2 arginine- and cystine-rich keratinous protamines in man and other mammals; type 3 intermediate basic proteins containing histidine and lysine as well as arginine in molluscs; type 4 somatic-like histones in echinoderms and type 5 sperm lacking basic proteins altogether in crabs. Our investigations, extending over three decades, indicate that there is an evolutionary trend in the vertebrates from variability of sperm nuclear basic proteins in bony fish and frogs with external fertilization, to relative constancy amongst reptiles, birds and mammals where fertilization is internal. In invertebrates, sperm nuclear basic proteins in animals with internal fertilization are usually arginine-rich protamines, whereas the proteins in external fertilizers are either sperm histones, protamine-like proteins or arginine-rich protamines. To probe this question of diversity in greater depth we are focussing on those organisms where related species show differences in their mode of reproduction. In collaboration with research groups in Victoria, B.C., and Barcelona, Spain, we are also comparing sperm nuclear basic proteins in animals with those in plants, algae and protozoans in order to determine the origins of these sperm nuclear basic proteins and their roles in the dynamic remodeling of chromatin during spermiogenesis.