BACKGROUND: Protamines are sperm nuclear proteins with a crucial role in 
chromatin condensation. Their function is strongly linked to sperm head 
morphology and male fertility. Protamines appear to be affected by a complex 
pattern of selective constraints. Previous studies showed that sexual selection 
affects protamine coding sequence and expression in rodents. Here we analyze 
selective constraints and post-copulatory sexual selection acting on protamine 2 
(Prm2) gene sequences of 53 species of primates and rodents. We focused on 
possible differences in selective constraints between these two clades and on 
the two functional domains of PRM2 (cleaved- and mature-PRM2). We also assessed 
if and how changes in Prm2 coding sequence may affect sperm head dimensions.
RESULTS: The domain of Prm2 that is cleaved off during binding to DNA 
(cleaved-Prm2) was found to be under purifying selection in both clades, whereas 
the domain that remains bound to DNA (mature-Prm2) was found to be positively 
selected in primates and under relaxed constraint in rodents. Changes in 
cleaved-Prm2 coding sequence are significantly correlated to sperm head width 
and elongation in rodents. Contrary to expectations, a significant effect of 
sexual selection was not found on either domain or clade.
CONCLUSIONS: Mature-PRM2 may be free to evolve under less constraint due to the 
existence of PRM1 as a more conserved and functionally redundant copy. The 
cleaved-PRM2 domain seems to play an important role in sperm head shaping. 
However, sexual selection on its sequence may be difficult to detect until it is 
identified which sperm head phenotype (shape and size) confers advantages for 
sperm performance in different mammalian clades.