Conditions for the Evolution of Soldier Sperm Classes

Abstract

Abstract. There has been wide disagreement as to whether sperm competition among animals can produce a soldier class of sperm to fight against other males. Utilizing mathematical models, we analyze the appropriate conditions for the evolution and maintenance of a soldier sperm class. We conclude that: (1) soldier sperm evolve even if one soldier sperm can kill or block less than one competing sperm; (2) soldier sperm evolve faster when there is a large variance in the number of competing sperm; (3) soldier ratio increases until reproductive sperm are too scarce to fertilize all ova or a sperm intensely refuses to become a soldier; and (4) soldier sperm are more likely to be smaller than reproductive sperm. Our models suggest that the conditions for the evolution of a soldier sperm class are not stringent.

What Use Is an Infertile Sperm? A Comparative Study of Sperm-Heteromorphic Drosophila

“…The hypothesis that parasperm are produced to enhance males' success in cryptic female choice predicts the production of larger/more parasperm when postcopulatory sexual selection is more frequent (Holman and Snook 2006), which we did not observe. In contrast, a model of the evolution of hypothetical "soldier sperm" (e.g., displacers or killers of rival eusperm) suggested that selection would minimize the size of each soldier (Kura and Nakashima 2000). The idea that parasperm are displacers has not been experimentally tested in the obscura group, but our results suggest it may be worth pursuing.…”

Sperm Sociality: Cooperation, Altruism, and Spite

“…As our simple analysis shows (Figure 1), negative relatedness between sperm abounds within the sperm storage organ of a promiscuous female, in which sperm of multiple males mingle, setting the scene for spite. The idea that sperm harm one another dates back 25 years in the empirical literature [26–29]. But is this really spite?…”

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

“…Males that invest some of their resources in parasperm may consequentially have more eusperm available for sperm competition and/or have an increased likelihood of fertilisation in matings with females that only mate once. The evolution of parasperm for 'sacrificial' functions has been modelled before, but this work models a situation where parasperm attack the sperm of rival males ('kamikaze sperm'; Kura & Nakashima, 2000). Our model also differs from the Kura & Nakashima (2000) model in that rather than simply demonstrating that Yes, as parasperm number/quality is predicted to be an important determinant of male reproductive success Offence Defence parasperm could evolve, we also estimate how much should be invested in parasperm.…”

“…The evolution of parasperm for 'sacrificial' functions has been modelled before, but this work models a situation where parasperm attack the sperm of rival males ('kamikaze sperm'; Kura & Nakashima, 2000). Our model also differs from the Kura & Nakashima (2000) model in that rather than simply demonstrating that Yes, as parasperm number/quality is predicted to be an important determinant of male reproductive success Offence Defence parasperm could evolve, we also estimate how much should be invested in parasperm. Another model on the evolution of sperm number in response to female spermicide (Greeff & Parker, 2000) only considers changes in sperm-monomorphic ejaculates.…”

“…Models 1 and 2 suggest that the SSH is most likely to apply to species with parasperm that are cheaper to produce than eusperm and are relatively good at absorbing spermicide (although under some conditions, parasperm can evolve even if more than one parasperm is needed to save one eusperm). A model examining whether parasperm could evolve as 'soldiers', killing eusperm of rival males, also concluded that parasperm have to be cheap to produce and that they do not necessarily need to kill one or more eusperm each (Kura & Nakashima, 2000). Several sperm-heteromorphic groups possess parasperm that are smaller, and presumably cheaper, than the eusperm, including stalk-eyed flies (Presgraves et al, 1999), the D. obscura species group (Snook, 1997) and the Lepidoptera (whose parasperm are also free of expensive DNA; Silberglied et al, 1984).…”