Evolution of vertebrate chromosomal sperm proteins: implications for fertility and sperm competition

The three major types of sperm nuclear basic proteins (SNBPs), histone (H type), protamine-like (PL type) and protamine (P type), are well represented in vertebrates. The three groups are evolutionarily related through a vertical evolutionary process (H --> PL --> P) that involves a transition from lysine to arginine-rich proteins and results in a sporadic but non-random distribution that can be phylogenetically traced. The arginine-rich P type has been selected in the course of evolution of the vertebrates, probably due to constraints imposed by internal fertilisation. Protamines are subject to a positive Darwinian selection process that results in the characteristic fast evolutionary rate shown by these proteins. This makes their use very suitable for the reconstruction of phylogenies of the different vertebrate groups. In mammals, two different types of protamines (P1 and P2) are present which, in the course of the evolution of this vertebrate group, have undergone a further transition to cysteine-rich proteins which further enhanced their DNA packing efficiency. From a functional perspective, protamines provide the most efficient packaging of sperm chromatin and can probably influence the shape of the sperm nucleus and chromatin stability, both of which have direct implications for fertility. In mammals, alterations of the ratio between P1 and P2 protamines as well as the ratio between histones and protamines are important determinants of sperm fertility. All of this suggests a potential involvement of protamines in sperm competition which is discussed in this paper.