Mating behavior and the evolution of sperm design

Abstract

Sperm are the most diverse of all animal cell types, and much of the diversity in sperm design is thought to reflect adaptations to the highly variable conditions under which sperm function and compete to achieve fertilization. Recent work has shown that these conditions often evolve rapidly as a consequence of multiple mating, suggesting a role for sexual selection and sexual conflict in the evolution of sperm design. However, very little of the striking diversity in sperm design is understood functionally, particularly in internally fertilizing organisms. We use phylogenetic comparative analyses covering 16 species of the hermaphroditic flatworm genus Macrostomum to show that a complex sperm design is associated with reciprocal mating and that this complexity is lost secondarily when hypodermic insemination--sperm injection through the epidermis--evolves. Specifically, the complex sperm design, which includes stiff lateral bristles, is likely a male persistence trait associated with sexual conflicts over the fate of received ejaculates and linked to female resistance traits, namely an intriguing postcopulatory sucking behavior and a thickened epithelium of the sperm-receiving organ. Our results suggest that the interactions between sperm donor, sperm, and sperm recipient can change drastically when hypodermic insemination evolves, involving convergent evolution of a needle-like copulatory organ, a simpler sperm design, and a simpler female genital morphology. Our study documents that a shift in the mating behavior may alter fundamentally the conditions under which sperm compete and thereby lead to a drastic change in sperm design.

Fig. 1.

Morphology of the sperm and stylet of two Macrostomum species. (A) M. lignano, a species that represents the reciprocal mating syndrome. (B) M. hystrix, a species that represents the hypodermic mating syndrome.

Fig. 2.

Phylogenetic mapping of reproductive character states. Variation in sperm and stylet morphology, mating behavior, and female antrum morphology among 16 species in the genus Macrostomum. (Details on character states are given in Table S1.) The character states are mapped on the maximum-likelihood tree of the genus (complete ssrDNA and partial lsrDNA), and nodes marked with a circle have Bayesian posterior probability >0.95 and maximum-likelihood bootstrap support of >70%. (A detailed tree with outgroups and exact nodal support values is given in Fig. S1.) Note that the stylet of Macrostomum tuba is depicted at one-third of the original size.

Fig. 3.

Correlated evolution of reproductive character states. Schematic diagram showing the most probable evolutionary routes for transitions between the four possible combinations of character states of two binary variables. For analysis A, the first variable represents the sperm morphology (0, bristles absent; 1, bristles present) or the stylet morphology (0, needle-like; 1, not needle-like) (SI Materials and Methods). For analysis B, the first variable represents female antrum morphology (0, simple; 1, thickened). For both analyses A and B, the second variable represents the copulation behavior (0, hypodermic; 1, reciprocal). Because the results of the two analyses are qualitatively similar, we show only one summary graph. The different arrows indicate three classes of transition rate parameters: transition rates with a high probablitiy (>55%, thin arrows), intermediate probability (30–40%, medium arrows), and low probability (<15%, thick arrows) of being zero. (The actual distributions of the transition rate parameters are given in Fig. S4.) Thus, thicker arrows represent more likely transitions. Under the null hypothesis of independent evolution, certain pairs of transition parameters must be equal. (The rationale is discussed in SI Materials and Methods.) The results clearly reject this null hypothesis, suggesting strong evidence for correlated evolution between the analyzed character states. Specifically, the estimated transition rate parameters tend to favor correlated evolution toward either the hypodermic mating or reciprocal mating syndrome.