Avoidance of reproductive interference causes resource partitioning in bean beetle females

Heterospecific mating interactions as an interface between ecology and evolution

Reproductive interference (costly interspecific sexual interactions) is well-understood to promote divergence in mating-relevant traits (i.e. reproductive character displacement: RCD), but it can also reduce population growth, eventually leading to local extinction of one of the species. The ecological and evolutionary processes driven by reproductive interference can interact with each other. These interactions are likely to influence whether the outcome is coexistence or extinction, but remain little studied. In this paper, we first develop an eco-evolutionary perspective on reproductive interference by integrating ecological and evolutionary processes in a common framework. We also present a simple model to demonstrate the eco-evolutionary dynamics of reproductive interference. We then identify a number of factors that are likely to influence the relative likelihoods of extinction or RCD. We discuss particularly relevant factors by classifying them into four categories: the nature of the traits responding to selection, the mechanisms determining the expression of these traits, mechanisms of reproductive interference and the ecological background. We highlight previously underappreciated ways in which these factors may influence the relative likelihoods of RCD and local extinction. By doing so, we also identify questions and future directions that will increase our holistic understanding of the outcomes of reproductive interference.

Experimental evolution of competing bean beetle species reveals long-term reversals of short-term evolution, but no consistent character displacement

Interspecific competition for shared resources should select for evolutionary divergence in resource use between competing species, termed character displacement. Many purported examples of character displacement exist, but few completely rule out alternative explanations. We reared genetically diverse populations of two species of bean beetles, Callosobruchus maculatus and Callosobruchus chinensis, in allopatry and sympatry on a mixture of adzuki beans and lentils, and assayed oviposition preference and other phenotypic traits after four, eight, and twelve generations of (co)evolution. C. maculatus specializes on adzuki beans; the generalist C. chinensis uses both beans. C. chinensis growing in allopatry emerged equally from both bean species. In sympatry, the two species competing strongly and coexisted via strong realized resource partitioning, with C. chinensis emerging almost exclusively from lentils and C. maculatus emerging almost exclusively from adzuki beans. However, oviposition preferences, larval survival traits, and larval development rates in both beetle species did not vary consistently between allopatric versus sympatric treatments. Rather, traits evolved in treatment-independent fashion, with several traits exhibiting reversals in their evolutionary trajectories. For example, C. chinensis initially evolved a slower egg-to-adult development rate on adzuki beans in both allopatry and sympatry, then subsequently evolved back toward the faster ancestral development rate. Lack of character displacement is consistent with a previous similar experiment in bean beetles and may reflect lack of evolutionary trade-offs in resource use. However, evolutionary reversals were unexpected and remain unexplained. Together with other empirical and theoretical work, our results illustrate the stringency of the conditions for character displacement.

Male harassment leads to fitness costs for females by disrupting oviposition site preferences

In many species, a difference in the optimal number of copulations for males and females leads to sexual conflict. This is well documented in the bean beetle Callosobruchus maculatus, where both sexes mate multiply and females incur fitness costs from injuries caused by the male genitalia. Here, we demonstrate that sexual conflict also decreases female fitness due to male harassment. We hypothesized that harassment costs would come as 1) decreased clutch size, egg size, or both and by 2) disruption of female preference for higher-quality oviposition substrate. Mated females were housed with two bean types—cowpeas, their preferred natal hosts, and toxic pinto beans. They were then submitted to either no, moderate, or high male harassment in the oviposition site. Females under harassment produced smaller clutch sizes but not smaller eggs, resulting in the absence of an egg-size/clutch-size trade-off. Additionally, females did not exhibit a preference for their natal cowpeas hosts over toxic pinto beans when males were present at the oviposition site, although they do so when harassing males are not present. Harassment disrupted female responses to variation in oviposition substrate quality, resulting in considerable fitness consequences in the form of lower offspring production and survival.

Co-occurrence of related asexual, but not sexual, lineages suggests that reproductive interference limits coexistence

We used randomizations to analyse patterns of co-occurrence of sexual and apomictic (asexual) members of the North American Crepis agamic complex (Asteraceae). We expect strong asymmetry in reproductive interactions in Crepis: apomicts produce clonal seeds with no need for pollination and are not subject to reproductive interference from co-occurring relatives. However, because they still produce some viable pollen, apomicts can reduce reproductive success of nearby sexual relatives, potentially leading to eventual local exclusion of sexuals. Consistent with this, randomizations reveal that sexuals are over-represented in isolated sites, while apomicts freely co-occur. Incorporation of taxonomic and phylogenetic evidence indicates that this pattern is not driven by local origins of asexuals. Our evidence that patterns of local co-occurrence are structured by reproductive interference suggests an underappreciated role for these interactions in community assembly, and highlights the need for explicit tests of the relative contributions of ecological and reproductive interactions in generating patterns of limiting similarity.

Coevolution of competing Callosobruchus species does not stabilize coexistence

Interspecific resource competition is expected to select for divergence in resource use, weakening interspecific relative to intraspecific competition, thus promoting stable coexistence. More broadly, because interspecific competition reduces fitness, any mechanism of interspecific competition should generate selection favoring traits that weaken interspecific competition. However, species also can adapt to competition by increasing their competitive ability, potentially destabilizing coexistence. We reared two species of bean beetles, the specialist Callosobruchus maculatus and the generalist C. chinensis, in allopatry and sympatry on a mixture of adzuki beans and lentils, and assayed mutual invasibility after four, eight, and twelve generations of evolution. Contrary to the expectation that coevolution of competitors will weaken interspecific competition, the rate of mutual invasibility did not differ between sympatry and allopatry. Rather, the invasion rate of C. chinensis, but not C. maculatus, increased with duration of evolution, as C. chinensis adapted to lentils without experiencing reduced adaptation to adzuki beans, and regardless of the presence or absence of C. maculatus. Our results highlight that evolutionary responses to interspecific competition promote stable coexistence only under specific conditions that can be difficult to produce in practice.

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

Reproductive interference through mating between related species can cause fitness reduction and affect population dynamics of the interacting species. In experimental matings between two seed beetles, Callosobruchus chinensis and Callosobruchus maculatus, C. maculatus females, but not C. chinensis females, suffer from significant loss of fecundity when conspecific mating is followed by heterospecific mating. We hypothesized that male traits associated with sexual conflict, which are often harmful to females, pleiotropically affect fitness of heterospecific females through interspecific mating. We examined the effect of ejaculate of C. chinensis males on C. maculatus females as the cause of the fecundity loss in C. maculatus females due to interspecific copulation. We found that frequent interspecific copulation occurred between C. maculatus females and C. chinensis males, but not between C. chinensis females and C. maculatus males, resulting in frequent interspecific ejaculate transfer from C. chinensis males to C. maculatus females. However, injection of the extract from C. chinensis male reproductive organs into C. maculatus females did not significantly affect C. maculatus fecundity compared with saline injection, indicating that the effect of the heterospecific ejaculate transfer on fecundity is negligible. We suggest that other harmful male traits such as genital spines of C. chinensis males are mainly responsible for the fecundity reduction in C. maculatus females that have experienced interspecific mating.