Diversity of Sperm Basic Chromosomal Proteins in the Vertebrates: A Phylogenetic Point of View

Abstract

In this paper we try to explain the variability of sperm basic proteins in nature by taking the subphylum Vertebrata as a starting point. The data presently available indicate that the appearance of unique sperm basic proteins has not been a sporadic phenomenon during vertebrate evolution. Rather there is a general macroevolutionary trend; namely, extreme variability of sperm basic proteins in bony fish and frogs gives way to a relative constancy of sperm protein types within urodeles, snakes, lizards, turtles, birds, metatherian and eutherian mammals. Cartilaginous fish also have similar sperm basic proteins. Furthermore, within particular orders of frogs and bony fish, certain families of sperm basic proteins are characteristic for particular genera and even individual species can be distinguished by their typical set of sperm proteins. This burst of sperm protein variability in the bony fish and frogs during vertebrate phylogeny coincides with the absence of internal fertilization in these orders, the appearance of sperm motility in the testis rather than the excurrent duct, the existence of polyploidy and the general absence of heteromorphic sex chromosomes. This seems to relieve selection pressure to maintain some relative constancy of sperm protein type in these orders. We speculate that perhaps the set of basic chromosomal proteins required to produce a functional sperm in a particular species of frog or bony fish is due to the time of onset of sexual maturity in that species. Thus, from a phylogenetic point of view, although sperm basic protein evolution in the vertebrates has been much less conservative than that of the nucleosomal histones, it has not been entirely a random affair.