Conjugate Sperm Pairs in American Opossums

<div><p>(A) Paired and single sperm of the short-tailed opossum Monodelphis domestica.</p> <p>(B) Pairs of conjugate sperm attached by the heads, the top pair starting to separate after capacitation.</p> <p>(C) Pair of conjugate sperm separating.</p> <p>(D) Electron microscopy of exquisite sperm head alignment in conjugate sperm pair (credit: Harry Moore).</p></div

Genetic Relatedness among Sperm and Males as a Function of Female Re-Mating Rate (Risk of Sperm Competition)

<div><p>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 (<i>p</i>_R- <i>p</i>)/(<i>p</i>_A - <i>p</i>) where <i>p</i>_R, <i>p</i>_A, and p denote focal gene frequency in recipients, actors, and the population (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060130#pbio-0060130-box001" target="_blank">Box 1</a>, [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060130#pbio-0060130-b042" target="_blank">42</a>]). Calculations of relatedness require one to assign the relevant population scale at which individuals interact and compete (see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060130#pbio-0060130-box001" target="_blank">Box 1</a>, [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060130#pbio-0060130-b016" target="_blank">16</a>]). 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 (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060130#pbio-0060130-box001" target="_blank">Box 1</a>). 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 [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060130#pbio-0060130-b044" target="_blank">44</a>].</p> <p>(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, <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060130#pbio-0060130-box001" target="_blank">Box 1</a>), 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).</p> <p>(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.</p></div

Mollusc Parasperm

<div><p>(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.</p> <p>(B) Montage of side-by-side transmission electron microscopy sections of the carrier (i) and lancet (ii) parasperm.</p> <p>(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).</p></div

Sperm Trains in Rodents

<div><p>(A) Wood mouse A. sylvaticus sperm train where sperm are attached hook-to-hook or hook-to-flagellum (credit: Harry Moore).</p> <p>(B) Motile grouping of wood mouse sperm (credit: Harry Moore).</p> <p>(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 [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060130#pbio-0060130-b010" target="_blank">10</a>].</p> <p>(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 [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0060130#pbio-0060130-b010" target="_blank">10</a>].</p></div

Female developmental environment and adult reproduction.

<p>(a–c) Female mating frequency: (a) Female Experiment; (b) Male Experiment and (c) Female-Male experiments. (d–f) Female offspring production: (d) Female Experiment; (e) Male Experiment and (f) Female-Male experiments. (g-i) <i>per mating</i> female offspring production: (g) Female Experiment; (h) Male Experiment and (i) Female-Male experiments. Solid dark grey–Homogeneous Large, Solid white–Homogeneous Small, Dark grey striped from bottom left to upper right–Heterogeneous Large, White striped from bottom left to upper right–Heterogeneous Small, Light grey striped from bottom right to upper left–Heterogeneous (combined Large and Small).</p

Female and focal male Bateman gradients.

<p>Estimate ± SE (p-value) are shown. Female estimates of the Bateman gradient are extracted from the univariate model. Male Bateman gradients, paternity share gradients and mate productivity gradients are extracted from the multivariate models. L–Large body size; S–Small body size. Bold–p-value ≤ 0.05. HomL–Homogeneous Large; HomS–Homogeneous Small; HetL–Heterogeneous Large, HetS–Heterogeneous Small, Het–Heterogeneous (combined large and small). The Female Experiment–varying female body size; The Male Experiment–varying male body size and the Female -Male Experiment–varying both female and male body size.</p

Diagram of the experimental design.

<p>We investigated the effects of the developmental environment on the strength of sexual selection. Because larval density strongly influences adult body size we refer to low-larval density flies as “large” and high larval density flies as “small”. <i>The Female Experiment</i>–experiment varying female larval density, keeping males constant (low larval density). Homogeneous social environments consisted of 4 large or 4 small females in addition to 4 large males. Heterogeneous social environments consisted of 2 large <i>and</i> 2 small females in addition to 4 large males. <i>The Male Experiment</i>–experiment varying male larval density, keeping females constant. Homogeneous social environments consisted of 4 large or 4 small males in addition to 4 large females. Heterogeneous social environments consisted of 2 large <i>and</i> 2 small males. <i>The Female -Male Experiment</i>–experiment varying both male and female larval density. Homogeneous social environments consisted of 4 large males and females or 4 small males and females. Heterogeneous social environments consisted of 2 large males and females <i>and</i> 2 small males and females. In all experiments one male, out of each group of four, was the focal individual for which we obtained paternity data (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0154468#sec005" target="_blank">methods</a>).</p

Data for analysis in Table S3

Data used for analysis presented in Table S3 and associated figure

Female Bateman gradients.

<p>Estimate (± SE) extracted from the univariate models. Female Bateman gradients in (a) the Female Experiment; (b) the Male Experiment and (c) the Female -Male Experiment. Solid dark grey–Homogeneous Large, Solid white–Homogeneous Small, Dark grey striped from bottom left to upper right–Heterogeneous Large, White striped from bottom left to upper right–Heterogeneous Small, Light grey striped from bottom right to upper left–Heterogeneous (combined Large and Small).L–Large; S–Small.</p

Focal male Bateman gradients, paternity share gradients and mate productivity gradients.

<p>Estimate (±SE) extracted from the multivariate models. (a-c) the focal male Bateman gradients in (a) the Female Experiment; (b) the Male Experiment and (c) the Female-Male Experiment. (d-f) paternity share gradients in (d) the Female Experiment; (e) the Male Experiment and (f) the Female-Male Experiment. (g-i) mate productivity gradients in (g) the Female Experiment; (h) the Male Experiment and (i) the Female-Male Experiment. Solid dark grey–Homogeneous Large, Solid white–Homogeneous Small, Dark grey striped from bottom left to upper right–Heterogeneous Large, White striped from bottom left to upper right–Heterogeneous Small, Light grey striped from bottom right to upper left–Heterogeneous (combined Large and Small). L–Large; S–Small.</p