Competition drives cooperation among closely related sperm of deer mice

Abstract

Among the extraordinary adaptations driven by sperm competition is the cooperative behaviour of spermatozoa. By forming cooperative groups, sperm can increase their swimming velocity and thereby gain an advantage in intermale sperm competition. Accordingly, selection should favour cooperation of the most closely related sperm to maximize fitness. Here we show that sperm of deer mice (genus Peromyscus) form motile aggregations, then we use this system to test predictions of sperm cooperation. We find that sperm aggregate more often with conspecific than heterospecific sperm, suggesting that individual sperm can discriminate on the basis of genetic relatedness. Next, we provide evidence that the cooperative behaviour of closely related sperm is driven by sperm competition. In a monogamous species lacking sperm competition, Peromyscus polionotus, sperm indiscriminately group with unrelated conspecific sperm. In contrast, in the highly promiscuous deer mouse, Peromyscus maniculatus, sperm are significantly more likely to aggregate with those obtained from the same male than with sperm from an unrelated conspecific donor. Even when we test sperm from sibling males, we continue to see preferential aggregations of related sperm in P. maniculatus. These results suggest that sperm from promiscuous deer mice discriminate among relatives and thereby cooperate with the most closely related sperm, an adaptation likely to have been driven by sperm competition.

Figure 1

Images of Peromyscus sperm in BWW medium. (a) 1000X phase contrast image of P. maniculatus sperm aggregate attached at sperm heads and (b)head hook to midpiece. (c) 400X image of motile P. polionotus sperm aggregate stained with 400nM Tubulin Tracker (Invitrogen Corp.). (d) 1000X image of aggregated sperm observed in a mixture containing sperm from one P. maniculatus male and one P. polionotus male (midpiece of P. maniculatus sperm is stained with MitoTracker Red 580 and midpiece of P. polionotus sperm with MitoTracker Green FM).

Figure 2

Preferential sperm aggregations. Mean ± s.e.m. proportion of cells in a sperm aggregate labelled with a single probe. Black bars indicate treatments in which sperm of one male is labelled green and sperm of another male is labelled red; white bars indicate controls in which sperm from a single male is labelled with both red and green probes. Pairwise comparisons between treatment and control groups are by paired two-tailed t-test with Bonferroni correction, asterisks indicate P < 0.01. Treatments include: (a) heterospecific mixtures containing live sperm from one P. maniculatus male and one P. polionotus male (n = 15), (b) conspecific mixtures of sperm from two unrelated males of the promiscuous P. maniculatus species (n = 8), (c) conspecific mixture of sperm from two unrelated males from the monogamous P. polionotus (n = 8), and (d) conspecific mixture of sperm from two full-sibling P. maniculatus males (n = 8). Horizontal lines above bars indicate comparisons between aggregations of sperm by unpaired two-tailed t-tests with Bonferroni correction (NS = non-significant).