An unusual histone H4 gene from Tilapia nilotica exhibiting characteristics of both a replication‐dependent histone and a basal‐expression histone: evolutionary considerations

The nucleotide sequence of a histone H4 gene from the fish Tilapia nilotica was determined. The deduced amino acid sequence is identical to that of H4 from the trout, Salmo gairdnerii. The 3′ untranslated region of the Tilapia gene exhibits a unique combination of structural features each of which is usually associated with either a replication‐dependent histone or a basal‐expression histone, but not with both. The direction of nucleotide substitutions in the Tilapia and Salmo lineages was inferred. The Tilapia gene was found to evolve faster and to exhibit a more biased pattern of substitution and codon usage than its Salmo homologue. This combination of features cannot be explained by either mutation or purifying selection. The rapid embryonic development of Tilapia prompts us to suggest that the molecular evolution of the histone H4 gene is driven by fixation of advantageous synonymous mutations.