The marmoset and cotton rat as animal models for the study of sperm chromatin packaging

In the human, canonical histones are largely replaced by protamine 1 (P1) and protamine 2 (P2) during late spermatogenesis. Recent studies have demonstrated that abnormal replacement of the histones is associated with severe male infertility and has profound implications for early embryogenesis. In this review the hispid cotton rat and the common marmoset are evaluated as animal models for the study of chromatin packaging in sperm. The DNA sequences of protamine 1 and protamine 2 in the cotton rat are also reported. We have found that the P1 and P2 genes share a 64% and 56% amino acid identity with human, respectively. The hispid cotton rat expresses both protamines, has a similar P1:P2 ratio, and a gene sequence homologous to human, thus making it a possible model organism for chromatin studies. The common marmoset also expresses both protamine genes and has a very similar spermatogenetic characteristic to man. This review considers both animals as possible models to study the mechanisms of protamine regulation and the effects of altered expression caused by environmental or physiological perturbations.