Figure 1.
Genetic Relatedness among Sperm and Males as a Function of Female Re-Mating Rate (Risk of Sperm Competition)
Social evolution theory predicts that relatedness is central to social behaviour. When two individuals share more genes in common than the population average, they are genetically related, and natural selection can favour altruistic behaviours that invest in another's reproduction, as with social insect workers. Formally, relatedness is calculated as (pR- p)/(pA - p) where pR, pA, and p denote focal gene frequency in recipients, actors, and the population (Box 1, [42]). Calculations of relatedness require one to assign the relevant population scale at which individuals interact and compete (see Box 1, [16]). And, importantly, we are taking a different scale for the male and the sperm here: we assume that all evolutionary competition for sperm occurs within the female: she is the population for each sperm (Box 1). If the actions of sperm were to harm the female, there would also be competition among sperm in different females, which would change the relatedness values and, perhaps, evolutionary predictions [44].
(A) Sperm's perspective (population is at the scale of the female). If a female mates once, all sperm have the same probability of sharing genes, and relatedness at the scale of the female is zero. Adaptations that result from natural selection on sperm, therefore, are expected to favour the individual sperm's personal fitness interests. This may mean temporary alliances with other sperm, but may also mean strong competition among the sperm of the same ejaculate. If a female mates again, things change. The second male's sperm are less likely than average to share genes with the first (negative relatedness, Box 1), which can favour sperm that harm themselves just to reduce the chance that the other male's sperm fertilise eggs (spite). However, the mixing of sperm from competing males also means that a sperm cell is now more likely to share genes with sperm from the same male than with the average sperm present in the female (positive relatedness). This situation can favour altruism, and indeed, as the sperm of our focal male become rarer, altruism becomes a better option than spite (it is more difficult to knock-down a majority than support a minority).
(B) Male perspective (population is at the scale of the real population). The only conflict for the male is with other males, and this conflict strengthens as the number of sperm inseminated by other males into the same female increases.
Table 1.
The Four Types of Social Action Based on Their Effect on the Direct Fitness (Lifetime Personal Reproduction) of the Actor and Recipient
Figure 2.
(A) Wood mouse A. sylvaticus sperm train where sperm are attached hook-to-hook or hook-to-flagellum (credit: Harry Moore).
(B) Motile grouping of wood mouse sperm (credit: Harry Moore).
(C) Apical hook morphology across different species of rodents (1, Bunomys fratrorum; 2, M. musculus; 3, R. norvegicus; 4, Dasymys incomtus; 5, Pseudomys oralis; 6, Maxomys surifer; 7, Melomys burtoni; 8, A. sylvaticus; 9, A. speciosus). From [10].
(D) The shape (left graph) and curvature (right graph) of the apical hook across different species of murid rodents in relation to the level of sperm competition (relative testes mass). From [10].
Figure 3.
Conjugate Sperm Pairs in American Opossums
(A) Paired and single sperm of the short-tailed opossum Monodelphis domestica.
(B) Pairs of conjugate sperm attached by the heads, the top pair starting to separate after capacitation.
(C) Pair of conjugate sperm separating.
(D) Electron microscopy of exquisite sperm head alignment in conjugate sperm pair (credit: Harry Moore).
Figure 4.
(A) Immature Oregon triton (Fusitriton oregonensis) lancet parasperm seen with scanning electron microscopy, showing the tail brush still present, which later develops into part of the body of the parasperm.
(B) Montage of side-by-side transmission electron microscopy sections of the carrier (i) and lancet (ii) parasperm.
(C) Montage of two transmission electron microscopy sections of a carrier parasperm transporting eusperm (long dark nuclei) with some cross-sections of eusperm and carrier and lancet parasperm (credit: John Buckland-Nicks).