Sperm wars and the evolution of male fertility

Abstract

Females frequently mate with several males, whose sperm then compete to fertilize available ova. Sperm competition represents a potent selective force that is expected to shape male expenditure on the ejaculate. Here, we review empirical data that illustrate the evolutionary consequences of sperm competition. Sperm competition favors the evolution of increased testes size and sperm production. In some species, males appear capable of adjusting the number of sperm ejaculated, depending on the perceived levels of sperm competition. Selection is also expected to act on sperm form and function, although the evidence for this remains equivocal. Comparative studies suggest that sperm length and swimming speed may increase in response to selection from sperm competition. However, the mechanisms driving this pattern remain unclear. Evidence that sperm length influences sperm swimming speed is mixed and fertilization trials performed across a broad range of species demonstrate inconsistent relationships between sperm form and function. This ambiguity may in part reflect the important role that seminal fluid proteins (sfps) play in affecting sperm function. There is good evidence that sfps are subject to selection from sperm competition, and recent work is pointing to an ability of males to adjust their seminal fluid chemistry in response to sperm competition from rival males. We argue that future research must consider sperm and seminal fluid components of the ejaculate as a functional unity. Research at the genomic level will identify the genes that ultimately control male fertility.

Female monopolization mediates the relationship between pre- and postcopulatory sexual traits

“…In all these taxa, sexually selected traits important in both selective episodes are well characterized (Supplementary Note 1). Within each taxon, we examined the relationship between the expression of sexually dimorphic traits under precopulatory sexual selection (for example, sexual size dimorphism, the length and dimorphism of canines, size of horns and antlers, tarsal spur length or the span of eye stalks) with interspecific variation in relative testes size (combined testes mass or mean testis length; testes number in schistosomes), a commonly used proxy for the strength of postcopulatory sexual selection [23][24][25][26] . All sexually selected traits are described in the Methods section; for complete data sets and phylogenies see Supplementary Tables 1-6 We first focused our analysis on the relationship between sexual size dimorphism (SSD), a proxy for the strength of precopulatory sexual selection [28][29][30] , and testes size as a proxy for postcopulatory sexual selection (see above), using phylogenetic multiple regressions controlling for body size (mass or length; Supplementary Note 1).…”

Sperm in hot water: Direct and indirect thermal challenges interact to impact on brown trout sperm quality

“…Evidence that increased sperm velocity increases the competitive ability and therefore fertilizing capability of sperm exists for a number of fish species, including salmonids (Gage et al, 2004;Liljedal et al, 2008; reviewed by Simmons and Fitzpatrick, 2012). Motility, the proportion of live cells in an ejaculate, is another important factor influencing fertilization success (Hoysak and Liley, 2001;Simmons and Fitzpatrick, 2012).…”

“…During paired matings (Esteve, 2005), salmonids release gametes into the surrounding water, where fertilization success is determined by the proximity to eggs, as well as the number and quality of the sperm released (Fitzpatrick and Liley, 2008;Gage et al, 2004;Maekawa and Onozato, 1986;Mjolnerod et al, 1998;Simmons and Fitzpatrick, 2012;Tuset et al, 2008), which is partly determined by the maturation status of the males (Lahnsteiner and Leitner, 2013). The processes of spermatogenesis and spermiation occur as distinct phases during salmonid reproductive periods, causing seasonal fluctuations in sperm quality parameters (Billard, 1986).…”

Female mediation of competitive fertilization success in Drosophila melanogaster

“…In some cases, these modifications are known to involve biochemical ejaculate-female interactions (21), with direct implications for competitive fertilization success [e.g., (30,31)]. A major focus in the study of postcopulatory sexual selection has been to understand the evolution of ejaculate quality traits that are likely to influence competitive fertilization success, such as swimming velocity [reviewed by (32)(33)(34)]. Variation in these phenotypes has almost exclusively been assayed in vitro and interpreted as intrinsic to males.…”