Sperm competition and sperm cooperation: the potential role of diploid and haploid expression.

Sperm competition is a powerful selective force driving the evolution of sperm shape and function. Recent findings suggest that sperm cooperation is a potential evolutionary response to sperm competition. Sperm cooperation may enhance the performance of the ejaculate increasing a male's chance to outcompete rival males in competition for fertilisation. Whether and how sperm cooperation may evolve is the focal point of this review. The relative importance of haploid and diploid gene expression for the evolution of sperm cooperation and the potential conflict of interest between (i) haploid sperm and diploid male and (ii) among sibling sperm, since sibling sperm only share an average of 50% of their genes in a diploid organism, are discussed. Furthermore, sperm cooperation is defined and the literature for empirical evidence of sperm cooperation is reviewed in light of the author's definitions.