Does heterospecific seminal fluid reduce fecundity in interspecific copulation between seed beetles?

Reproductive Interference Alters Species Coexistence in Nematodes due to Asymmetric Sperm-Induced Harm

Species coexistence is shaped by a range of biotic and abiotic factors. Beyond predation, parasitism and competition, one species may interfere with another's reproduction to induce sexual exclusion from a habitat. Here, we test for reproductive interference from inter-species mating between sympatric nematodes Caenorhabditis macrosperma and C. nouraguensis. Higher intrinsic population growth of C. nouraguensis arises from greater reproductive output by both sexes, predicting it to be superior in resource competition. Mate discrimination between species is incomplete, however, with inter-species mating errors reducing lifespan and reproductive fitness of female C. nouraguensis only. These asymmetric costs arise within hours, due to ectopic migration of C. macrosperma's giant sperm cells. We modelled the population dynamic impacts of reproductive interference, then confirmed rapid sexual exclusion in mixed-species communities with multi-generation experiments. These findings demonstrate the profound ecological implications of reproductive interference for demographic parameters and species coexistence through a cell-mediated mechanism of inter-species harm.

The evolution of between-species reproductive interference capability under different within-species mating regimes

Sexual selection sometimes favors male traits that benefit their bearers, but harm their mates. The harmful effects of male traits may also extend to females of other species via heterospecific mating interactions. This could affect the coexistence of closely related species during secondary contact. We examined the evolution of the interspecific interfering capability of a beetle (Callosobruchus chinensis) with a congener (C. maculatus) using C. chinensis males reared under conditions of monogamy and polygamy for 17 generations. After experimental evolution, C. chinensis males reared under polygamous conditions imposed greater impacts on offspring production by C. maculatus females than did C. chinensis males reared under monogamous conditions. However, the mechanism by which differential mating regimes altered the effect of C. chinensis males on C. maculatus females was unclear, because we did not find evidence for the expected genital evolution in C. chinensis, despite their body size divergence. Our findings suggest that traits that originally evolved through sexual selection in two allopatric species could influence the coexistence of these species or the likelihood of reinforcement during secondary contact.

A generalized population dynamics model for reproductive interference with absolute density dependence

Interspecific mating interactions, or reproductive interference, can affect population dynamics, species distribution and abundance. Previous population dynamics models have assumed that the impact of frequency-dependent reproductive interference depends on the relative abundances of species. However, this assumption could be an oversimplification inappropriate for making quantitative predictions. Therefore, a more general model to forecast population dynamics in the presence of reproductive interference is required. Here we developed a population dynamics model to describe the absolute density dependence of reproductive interference, which appears likely when encounter rate between individuals is important. Our model (i) can produce diverse shapes of isoclines depending on parameter values and (ii) predicts weaker reproductive interference when absolute density is low. These novel characteristics can create conditions where coexistence is stable and independent from the initial conditions. We assessed the utility of our model in an empirical study using an experimental pair of seed beetle species, Callosobruchus maculatus and Callosobruchus chinensis. Reproductive interference became stronger with increasing total beetle density even when the frequencies of the two species were kept constant. Our model described the effects of absolute density and showed a better fit to the empirical data than the existing model overall.

Exaggerated male genitalia intensify interspecific reproductive interference by damaging heterospecific female genitalia

Male-male competition over fertilization can select for harmful male genital structures that reduce the fitness of their mates, if the structures increase the male's fertilization success. During secondary contact between two allopatrically formed, closely related species, harmful male genitalia may also reduce the fitness of heterospecific females given interspecific copulation. We performed a laboratory experiment to determine whether the extent of genital spine exaggeration in Callosobruchus chinensis males affects the fitness of C. maculatus females by injuring their reproductive organs. We found that males with more exaggerated genital spines were more likely to injure the females via interspecific copulation and that the genital injury translated into fecundity loss. Thus, as predicted, reproductive interference by C. chinensis males on C. maculatus females is mediated by exaggeration of the genital spine, which is the evolutionary consequence of intraspecific male-male competition. Harmful male traits, such as genital spines, might generally affect the extent of interaction between closely related species.