Abstract

One of the most astonishing examples of chromatin remodelling occurs during the maturation of male germ cells, where changes in protein structure, as well as chromatin compaction, take place. During the post-meiotic formation of sperm (spermiogenesis), chromatin appears super-condensed, and transcriptionally inactive, allowing for a more hydrodynamic sperm head, and preventing physical and chemical damage to DNA. DNA is closely condensed in the mature sperm nucleus through its linkage with sperm-specific nuclear proteins called sperm nuclear basic proteins, clustered in three categories: histones, protamines and protamine-like proteins. Fish represent a unique group of vertebrates, including species with specific proteins from each type, such as Sparus aurata, Mullus surmuletus, Dicentrarchus labrax, and Scyliorhinus canicula, which display histones, protamine-like proteins, protamines, and keratinous protamines, respectively. Phylogenetically, these proteins are evolutionarily related, presenting a sporadic and non-random distribution, as a result of vertical evolution, where only histones would be found in more primitive species, and protamines would be restricted to those species located at the uppermost branches of the phylogenetic tree. The relative frequency of this transition is almost insignificant during the differentiation of genera, and species, and very small amongst different families, but, very noticeable amongst different orders. Thus, the aim of this study is to gather the existing background related to sperm nuclear basic proteins in fish, showing a general perspective of the state of the art about diversity, and nuclear reorganization of sperm chromatin during spermiogenesis, and the evolution and phylogenetic distribution of these proteins in fish sperm.