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.     

The mechanism of the fecundity reduction in Callosobruchus maculatus caused by Callosobruchus chinensis males

Male promiscuity sometimes results in interspecific reproductive interaction, also known as reproductive interference. Reproductive interference entails costs for the individuals involved and affects the community structure by reducing the population growth rate. However, our understanding of the mechanisms generating reproductive interference is still insufficient. Two congeneric bean weevils, Callosobruchus chinensis and C. maculatus, show asymmetric reproductive interference; only C. chinensis males reduce the fecundity of the other species. Here we investigated the mechanism of reproductive interference by C. chinensis males on C. maculatus females in terms of lifetime fecundity. Callosobruchus chinensis males with ablated genitals, which could harass C. maculatus females but not copulate with them, did not reduce the C. maculatus fecundity, suggesting that interspecific copulation was necessary. However, a single interspecific copulation did not affect C. maculatus fecundity as long as the females also copulated with a conspecific male. Exposure to C. chinensis males for 24 h prior to oviposition significantly reduced C. maculatus fecundity, and fecundity was negatively correlated with the number of C. chinensis males the females were exposed to. Additionally, C. maculatus females experienced more interspecific copulations when they were housed with more C. chinensis males. Together these findings suggest that multiple interspecific copulations by C. chinensis males reduce the fecundity of C. maculatus females. Thus in general, even if a single interspecific copulation is apparently harmless, repeated interspecific copulations can be costly for the individuals involved. Furthermore, only by quantifying reproductive success were we able to identify the precise mechanism of reproductive interference.     

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.     

Variation in mate recognition specificities among four Callosobruchus seed beetles

Differentiation of mate recognition systems is one of the important steps for speciation in animals. For some insects, a contact sex pheromone present on the cuticular surface is indispensable in discriminating reproductive partners. In Callosobruchus species (Coleoptera: Chrysomelidae: Bruchinae), contact sex pheromones have been found in two species, Callosobruchus chinensis (L.) and Callosobruchus maculatus (Fabricius). It was suggested, however, that these two species lacked the ability to discriminate their conspecific and/or heterosexual partners. To elucidate this inconsistency, we verified the existence of contact sex pheromones from two other species, Callosobruchus rhodesianus (Pic) and Callosobruchus analis (Fabricius). As a result, unlike C. chinensis and C. maculatus, the males of C. rhodesianus and C. analis were able to discriminate their heterosexual partners. Comparing cross‐copulation behavior, i.e., copulation behavior between two species, against these four species indicated that the mate recognition specificities were quite different. Males of C. rhodesianus and C. analis had highly species‐specific mating behavior, whereas males of C. chinensis and C. maculatus were much less specific. These results indicate that variation in mate recognition can arise even among congeneric species living in a sympatric environment, and this variation might have arisen during species differentiation. Based on our results in combination with previous reports on interspecific competition, we suggest that the observed asymmetric cross‐copulation behavior might be, at least partially, an adaptation for surviving interspecific competition.     

Avoidance of reproductive interference causes resource partitioning in bean beetle females

Reproductive interference is any interspecific sexual interaction that adversely affects female fitness through indiscriminate reproductive activities. It can be a driving force of resource partitioning in conjunction with resource competition. We previously showed that the bean beetle Callosobruchus maculatus is superior in larval resource competition, but vulnerable in reproductive interference, compared with its congener C. chinensis. We hypothesized that these two species might use two resources differently if one of the resources modified the intensity of reproductive interference or resource competition. We observed that C. maculatus females often enter gaps between beans to avoid mating attempts of heterospecific males, and hypothesized that removing bean gaps would strengthen reproductive interference. Therefore, we provided normal beans with gaps and split beans without gaps to females of the two species housed with conspecific or heterospecific males or no males and compared the number of eggs on each bean type among treatments. Callosobruchus maculatus females housed with heterospecific males were more likely to oviposit on normal beans than C. chinensis females. As a result, more C. chinensis adults hatched from split beans and more C. maculatus hatched from normal beans when females and males of both species were housed together. Thus, oviposition resource partitioning resulted from the combination of female avoidance of reproductive interference and resource competition.     

Sexual differences in head form and diet in a population of Mexican lance‐headed rattlesnakes,Crotalus polystictus

Sexual dimorphism of phenotypic traits associated with resource use is common in animals, and may result from niche divergence between sexes. Snakes have become widely used in studies of the ecological basis of sexual dimorphism because they are gape‐limited predators and their head morphology is likely to be a direct indicator of the size and shape of prey consumed. We examined sexual dimorphism of body size and head morphology, as well as sexual differences in diet, in a population of Mexican lance‐headed rattlesnakes, Crotalus polystictus, from the State of México, Mexico. The maximum snout–vent length of males was greater than that of females by 21%. Males had relatively larger heads, and differed from females in head shape after removing the effects of head size. In addition, male rattlesnakes showed positive allometry in head shape: head width was amplified, whereas snout length was truncated with increased head size. By contrast, our data did not provide clear evidence of allometry in head shape of females. Adults of both males and females ate predominately mice and voles; however, males also consumed a greater proportion of larger mammalian species, and fewer small prey species. The differences in diet correspond with dimorphism in head morphology, and provide evidence of intersexual niche divergence in the study population. However, because the sexes overlapped greatly in diet, we hypothesize that diet and head dimorphisms in C. polystictus are likely related to different selection pressures in each sex arising from pre‐existing body size differences rather than from character displacement for reducing intersexual competition. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 633–640.     

Extreme Eye Projection in the Male Weevil <Emphasis Type="Italic">Exechesops leucopis</Emphasis> (Coleoptera: Anthribidae): Its Effect on Intrasexual Behavioral Interferences

Fungus weevils, Exechesops leucopis (Anthribidae), are sexually dimorphic in the degree of eye protrusion and antenna elongation. I examined the allometric relationships of eye span, eyestalk length, antenna length, elytra width, and wing length against body size (pronotum width), and their effects on the outcome of male-male combat in the laboratory. Male eye span, eyestalk length, and antenna length indicated positive allometry, while elytra width showed isometry, and wing length showed negative allometry. In male-male combat, males with a larger eye span, antenna length, and body size defeated those with smaller attributes. However, when males fought experimentally males of similar body size, only eye span affected the outcome of combat, independent of body size and antenna length. In the female-female contests, the prior residency was an important determinant of victory the other than any morphological traits.     

The presence of heterospecific males causes an Allee effect

The Allee effect is a positive causal relationship between any component of fitness and population density or size. Allee effects strongly affect the persistence of small or sparse populations. Predicting Allee effects remains a challenge, possibly because not all causal mechanisms are known. We hypothesized that reproductive interference (an interspecific reproductive interaction that reduces the fitness of the species involved) can generate an Allee effect. If the density of the interfering species is constant, an increase in the population of the species receiving interference may dilute the per capita effect of reproductive interference and may generate an Allee effect. To test this hypothesis, we examined the effect of heterospecific males on the relationship between per capita fecundity and conspecific density in Callosobruchus chinensis and C. maculatus. Of the two species, only C. maculatus females suffer reproductive interference from heterospecific males. Only C. maculatus, the species susceptible to reproductive interference, demonstrated an Allee effect, and only when heterospecific males were present. In contrast, C. chinensis, the species not susceptible to reproductive interference, demonstrated no Allee effect regardless of the presence of heterospecific males. Our results show that reproductive interference in fact generated an Allee effect, suggesting the potential importance of interspecific sexual interactions especially in small or sparse populations, even in the absence of a shared resource. It may be possible to predict Allee effects produced by this mechanism a priori by testing reproductive interference between closely related species.     

Environmental effects on sexual size dimorphism of a seed-feeding beetle

Sexual size dimorphism is widespread in animals but varies considerably among species and among populations within species. Much of this variation is assumed to be due to variance in selection on males versus females. However, environmental variables could affect the development of females and males differently, generating variation in dimorphism. Here we use a factorial experimental design to simultaneously examine the effects of rearing host and temperature on sexual dimorphism of the seed beetle, Callosobruchus maculatus. We found that the sexes differed in phenotypic plasticity of body size in response to rearing temperature but not rearing host, creating substantial temperature-induced variation in sexual dimorphism; females were larger than males at all temperatures, but the degree of this dimorphism was smallest at the lowest temperature. This change in dimorphism was due to a gender difference in the effect of temperature on growth rate and not due to sexual differences in plasticity of development time. Furthermore, the sex ratio (proportion males) decreased with decreasing temperature and became female-biased at the lowest temperature. This suggests that the temperature-induced change in dimorphism is potentially due to a change in non-random larval mortality of males versus females. This most important implication of this study is that rearing temperature can generate considerable intraspecific variation in the degree of sexual size dimorphism, though most studies assume that dimorphism varies little within species. Future studies should focus on whether sexual differences in phenotypic plasticity of body size are a consequence of adaptive canalization of one sex against environmental variation in temperature or whether they simply reflect a consequence of non-adaptive developmental differences between males and females.     

Effect of host age on life cycle and morphological characteristics of Glyptapanteles liparidis (Hymenoptera: Braconidae), a parasitoid of Acronicta rumicis (Lepidoptera: Noctuidae)

The life cycle of Glyptapanteles liparidis was 23.75 ± 1.26, 21.95 ± 2.44 and 20.83 ± 0.78 days when fed on the first, second and third instar larvae of Acronicta rumicis, respectively. Although insufficient numbers hindered statistical analysis, the life cycle of G. liparidis appeared to be shortest, 19 days, when fed on fourth instar larvae. The life cycle of G. liparidis tends to shorten as the larvae of A. rumicis fed upon are more advanced. The body length, forewing length and head capsule width of female G. liparidis fed on first instar larvae of A. rumicis were greater than those of males, while the antennae of males were longer than those of females. When fed on second instar larvae, there was no difference in body length and head capsule width between males and females, but the male antennae were longer than the female, and the female forewings were longer than the male. When fed on third instar larvae, there was no significant difference in head capsule width between the sexes, but female body length and forewing length were greater than the male, and the male antennae were longer than the female. On the whole, females were bigger than males in terms of body length and forewing length, while antennae of the males were longer than those of the females. There was no difference in head capsule width between males and females. Body length, antenna length, forewing length and head capsule width of male and female G. liparidis were relatively larger when fed on first instar larvae of A. rumicis than when fed on second and third instar larvae.