Spermicide, cryptic female choice and the evolution of sperm form and function

Abstract

Sperm competition and cryptic female choice profoundly affect sperm morphology, producing diversity within both species and individuals. One type of within‐individual sperm variation is sperm heteromorphism, in which each male produces two or more distinct types of sperm simultaneously, only one of which is typically fertile (the ‘eusperm’). The adaptive significance of nonfertile ‘parasperm’ types is poorly understood, although numerous sperm‐heteromorphic species are known from many disparate taxa. This paper examines in detail two female‐centred hypotheses for the evolution and maintenance of this unconventional sperm production strategy. First, we use game theoretical models to establish that parasperm may function to protect eusperm from female‐generated spermicide, and to elucidate the predictions of this idea. Second, we expand on the relatively undeveloped idea that parasperm are used by females as a criterion for cryptic female choice, and discuss the predictions generated by this idea compared to other hypotheses proposed to explain sperm heteromorphism. We critically evaluate both hypotheses, suggest ways in which they could be tested, and propose taxa in which they could be important.

Sperm competition promotes diversity of sperm bundles in Ohomopterus ground beetles

“…Their function may be in providing nutrients to the female, moving rapidly to the fertilization site, or enhancing the longevity of the sperm by providing nutrients, by guarding their acrosomes or by avoiding toxins that are secreted by rival males to incapacitate sperm within the female (Sivinski 1984;Siva-Jothy 1997;Holman and Snook 2006). In opossum (Moore 1996), fishflies (Hayashi 1998) and wood mice (Moore et al 2002), paired or aggregated sperm improve swimming speed within viscous environments probably because of synchronized flagella movements, giving support to the motility hypothesis (but see Ishijima et al 1999).…”

Evolution of genitalia: theories, evidence, and new directions

“…Experimental manipulation of the intensity of malefemale conflicts (e.g., by imposing strict monogamy on males and females) is also a powerful tool (Arnqvist and Rowe 2005). Nevertheless, I do not share the optimism of some of the leading workers on CFC and SAC (e.g., Hosken and Stockley 2004;Holman and Snook 2006;Moore et al 2003;Orteiza et al 2005;Pizarri and Snook 2003;Rice and Chippendale 2001) that another use of studies of captive populations, to study the overall reproductive costs and benefits to females in the laboratory, is likely to resolve the relative importance of SAC and CFC in the evolution of genitalia (or other traits). I say this despite the fact that the crucial difference between CFC and SAC models hinge on the balance between a female's overall gains from traits that result in rejecting some males in terms of direct fitness.…”

Postcopulatory sexual selection: Darwin's omission and its consequences

“…Parasperm are widespread in Lepidoptera, have evolved repeatedly in other groups (66), and constitute more than half of the ejaculate in some species. Hypotheses for parasperm functions (66,70) include provisioning the female with nutrients, displacing or killing rival sperm, blocking access for rival sperm, promoting movement of fertile sperm within the female, influencing CFC (for instance, by packing female storage organs to induce the female not to remate), influencing long-term vs. short-term survival in the female, and defense against female spermicides (71). The data needed to test these hypotheses are largely lacking (see critical discussions in ref.…”