Re: What happens if a 'killer sperm' fertilizes an ovum

Hi Nigel,

Thanks for submitting your question to the MadSci network. The idea that there might be competition between sperm was first proposed by Parker in 1970, and has been popularized in books like Sperm Wars. However, popularization aside, there does not seem to be much evidence for so-called "killer sperm", or "kamikaze sperm" in humans or other species.

Here is an abstract from a paper by Moore et al. in which they tested the "kamikaze sperm hypothesis":
"This study examines one of the possible mechanisms of sperm competition, i.e. the kamikaze sperm hypothesis. This hypothesis states that sperm from different males interact to incapacitate each other in a variety of ways. We used ejaculates from human donors to compare mixes of semen in vitro from the same or different males. We measured the following parameters: (i) the degree of sperm aggregation, velocity and proportion of morphologically normal sperm after 1 and 3 h incubation in undiluted semen samples, (ii) the proportion of viable sperm plus the same parameters as in (i) in 'swim-up' sperm suspensions after 1 and 3 h incubation, (iii) the degree of self and non-self sperm aggregation using fluorescent dyes to distinguish the sperm of different males, and (iv) the extent of sperm capacitation and acrosome-reacted sperm in mixtures of sperm from the same and different males. We observed very few significant changes in sperm aggregation or performance in mixtures of sperm from different males compared with mixtures from the same male and none that were consistent with previously reported findings. The incapacitation of rival sperm therefore seems an unlikely mechanism of sperm competition in humans."

Swallow and Wilkinson review what is known about sperm heteromorphism in insects, and find no support for "killer sperm" in insects either. Here's the abstract from their review:
"Production of more than one morphological type of sperm in a common testis has been documented for a variety of invertebrates, including gastropods, spiders, centipedes, and insects. This unusual phenomenon is difficult to explain by current theory, particularly since available evidence indicates that one sperm type is often incapable of effecting fertilization. In this review we critically examine evidence on the distribution and development of sperm heteromorphisms among insects in light of competing hypotheses for the evolutionary origin, maintenance, and function of a non- fertilizing class of sperm. To date, no single hypothesis, including alternatives which assume non- fertilizing sperm are non-adaptive, or that they provision, facilitate, or compete with fertilizing sperm, has received strong empirical support by any group of insects. The diversity of sperm heteromorphisms suggests that non-fertilizing sperm may have different functions in different clades or even serve multiple functions within a clade. We suggest that insight could be gained from (1) new models for the evolution of sperm polymorphism, (2) comparative studies that focus on multiple traits simultaneously (e.g. sperm number, proportion, length, and remating rate) and utilize clades in which more than one gain or loss of sperm heteromorphism has been documented (e.g. Pentatomidae, Carabidae, or Diopsidae), and (3) experimental studies that exploit individual variation or directly manipulate the composition of the male ejaculate."

However, that does not mean that there is not competition between sperm; Till- Bottraud et al. review what is known about sperm heteromorphism in both animals and plants, and conclude that sperm competition has resulted in the level of heteromorphism seen across many plant an animal species. Pizzari recently showed how competition between sperm results in the evolution of larger, faster sperm, and Martin-Coello et al. recently showed how the evolution of the expression of protamine, which packages DNA and influences the shape of the sperm head, can enhance sperm swimming speed.

In addition, there are instances where sperm of different "castes" are produced, with each caste having a different responsibility (as far as fertilization goes). For example, Holman and Snook have demonstrated how male Drosophila pseudoobscura fruit-flies produce parasperm and eusperm in order to defeat spermacides produced by female Drosophila pseudoobscura fruit-flies. The parasperm are infertile and protect the fertile eusperm from death by female spermicide.

So, there's a quick overview of sperm heteromorphism for you. Some species do produce sperm that are specialized for specific tasks, and there is competition between sperm, but there do not seem to be sperm that are specialized for killing other sperm.

Keep asking questions!

References

Parker, Geoffrey A. (1970) Sperm competition and its evolutionary consequences in the insects Biological Reviews 45: 525-567.

Moore, H.D.M., Martin, M. and Birkhead, T.R. (1999) No evidence for killer sperm or other selective interactions between human spermatozoa in ejaculates of different males in vitro. Proceedings of the Royal Society Series B: Biological Sciences, 266 (1436). p. 2343

Swallow JG and Wilkinson GS (2002) The long and short of sperm polymorphisms in insects. Biological Reviews of the Cambridge Philosophical Society 77:2:153-182.

Till-Bottraud I, Joly D, Lachaise D, Snook, RR (2005) Pollen and sperm heteromorphism: convergence across kingdoms? Journal of Evolutionary Biology. 18(1):1-18.

Pizzari T (2009)Sexual selection: sperm in the fast lane Curr Biol 19(7):R292-4.

Martin-Coello J, Dopazo H, Arbiza L, Ausió J, Roldan ER, Gomendio M (2009). Sexual selection drives weak positive selection in protamine genes and high promoter divergence, enhancing sperm competitiveness. Proc Biol Sci.Apr 8. [Epub ahead of print]

Holman L, Snook RR. (2008) A sterile sperm caste protects brother fertile sperm from female-mediated death in Drosophila pseudoobscura. Curr Biol. 18(4):292-6.