Asymmetric Reproductive Interference Research Articles

Spatiotemporal niche overlap, asymmetric reproductive interference, and population genetics between the sympatric species, Rhododendron diversipilosum and Rhododendron subarcticum, in alpine fellfield habitat

ABSTRACT Reproductive interference between sympatric-related species often causes adverse impacts on rare species, which increases the risk of local extinction, particularly in small and isolated populations. To evaluate the congeneric interactions in alpine plants, we compared the ecological and genetic properties and assessed the reproductive interference between tetraploid Rhododendron diversipilosum (widespread species) and diploid Rhododendron subarcticum (rare species in alpine fellfields) in northern Japan. In alpine fellfields, R. diversipilosum is commonly distributed close to shrubby patches, whereas R. subarcticum tends to grow in more exposed places, although they are sometimes mixed. R. subarcticum initiated flowering one week earlier; however, the flowering periods overlapped between species, indicating incomplete phenological isolation. The pollination experiment showed that both species were self-incompatible. Furthermore, heterospecific pollination occurred only for R. subarcticum; however, hybrid seeds seldom germinated. These results indicate that reproductive interference is asymmetric between species, where only the rare R. subarcticum may suffer from heterospecific pollination. Genetic analysis showed that R. subarcticum populations had lower genetic diversity but higher divergence among populations than R. diversipilosum. Although spatiotemporal niche separations may mitigate reproductive interference, the risk of local extinction could be higher in R. subarcticum populations because of isolated distributions with low genetic diversity.

Heterospecific pollination by an invasive congener threatens the native American bittersweet, Celastrus scandens

Invasive plants have the potential to interfere with native species’ reproductive success through a number of mechanisms, including heterospecific pollination and hybridization. This study investigated reproductive interactions between a native North American woody vine (American bittersweet, Celastrus scandens) and an introduced congener (oriental bittersweet, C. orbiculatus). The decline of C. scandens in the eastern portion of its range is coincident with the introduction and spread of C. orbiculatus, and the two species are known to hybridize. The relationship between proximity and floral production of conspecific and heterospecific males on fertilization and hybridization rates was measured at a field site in northwestern Indiana, USA where both species occur and reproduce. We found that the invasive vine had an extreme advantage in both male and female floral production, producing nearly 200 times more flowers per staminate plant and 65 times more flowers per pistillate plant than the native. Using nuclear microsatellite DNA markers we found that hybridization rates were asymmetric; 39% of the C. scandens seeds tested were hybrids, compared to only 1.6% of C. orbiculatus seeds. The asymmetric hybridization rates were likely not solely due to greater abundance of C. orbiculatus pollen because experimental hand crosses revealed that C. scandens had a higher rate (41%) of heterospecific fertilization than C. orbiculatus (2.4%). We previously reported that few hybrids were observed in the wild, and hybrids had greatly reduced fecundity. Thus, in our system, the threat posed by heterospecific pollen is not replacement by hybrids or introgression, but rather asymmetric reproductive interference. Reproductive interference extended to distances as great as 100 meters, thus, efforts to conserve the native species must reduce its exposure to C. orbiculatus over a relatively large spatial scale.

Where Vectors Collide: The Importance of Mechanisms Shaping the Realized Niche for Modeling Ranges of Invasive Aedes Mosquitoes.

The vector mosquitoes Aedes aegypti (L.), native to Africa, and Aedes albopictus (Skuse), native to Asia, are widespread invasives whose spatial distributions frequently overlap. Predictive models of their distributions are typically correlative rather than mechanistic, and based on only abiotic variables describing putative environmental requirements despite extensive evidence of competitive interactions leading to displacements. Here we review putative roles of competition contributing to distribution changes where the two species meet. The strongest evidence for competitive displacements comes from multiple examples of habitat segregation where the two species co-occur and massive reductions in the range and abundance of A. aegypti attributable to A. albopictus invasions in the southeastern U.S.A. and Bermuda (U.K). We summarize evidence to support the primacy of asymmetric reproductive interference, or satyrization, and larval resource competition, both favoring A. albopictus, as displacement mechanisms. Where evidence of satyrization or interspecific resource competition is weak, differences in local environments or alternative ecologies or behaviors of these Aedes spp. may explain local variation in the outcomes of invasions. Predictive distribution modeling for both these major disease vectors needs to incorporate species interactions between them as an important process that is likely to limit their realized niches and future distributions. Experimental tests of satyrization and resource competition are needed across the broad ranges of these species, as are models that incorporate both reproductive interference and resource competition to evaluate interaction strengths and mechanisms. These vectors exemplify how fundamental principles of community ecology may influence distributions of invasive species.

Asymmetric reproductive interference: The consequences of cross-pollination on reproductive success in sexual-apomictic populations of Potentilla puberula (Rosaceae).

Apomixis evolves from a sexual background and usually is linked to polyploidization. Pseudogamous gametophytic apomicts, which require a fertilization to initiate seed development, of various ploidy levels frequently co‐occur with their lower‐ploid sexual ancestors, but the stability of such mixed populations is affected by reproductive interferences mediated by cross‐pollination. Thereby, reproductive success of crosses depends on the difference in ploidy levels of mating partners, that is, on tolerance of deviation from the balanced ratio of maternal versus paternal genomes. Quality of pollen can further affect reproductive success in intercytotype pollinations. Cross‐fertilization, however, can be avoided by selfing which may be induced upon pollination with mixtures of self‐ and cross‐pollen (i.e., mentor effects). We tested for reproductive compatibility of naturally co‐occurring tetraploid sexuals and penta‐ to octoploid apomicts in the rosaceous species Potentilla puberula by means of controlled crosses. We estimated the role of selfing as a crossing barrier and effects of self‐ and cross‐pollen quality as well as maternal: paternal genomic ratios in the endosperm on reproductive success. Cross‐fertilization of sexuals by apomicts was not blocked by selfing, and seed set was reduced in hetero‐ compared to homoploid crosses. Thereby, seed set was negatively related to deviations from balanced parental genomic ratios in the endosperm. In contrast, seed set in the apomictic cytotypes was not reduced in hetero‐ compared to homoploid crosses. Thus, apomictic cytotypes either avoided intercytotype cross‐fertilization through selfing, tolerated intercytotype cross‐fertilizations without negative effects on reproductive success, or even benefitted from higher pollen quality in intercytotype pollinations. Our experiment provides evidence for asymmetric reproductive interference, in favor of the apomicts, with significantly reduced seed set of sexuals in cytologically mixed populations, whereas seed set in apomicts was not affected. Incompleteness of crossing barriers further indicated at least partial losses of a parental genomic endosperm balance requirement.

Male origin determines satyrization potential of Aedes aegypti by invasive Aedes albopictus

Rapid displacements of resident Aedes aegypti by invasions of Aedes albopictus have been documented in the southeastern United States and Bermuda. Interspecific mating has been detected in nature between these species and proposed as a likely mechanism for these displacements by means of asymmetric reproductive interference, or satyrization. However, rapid displacements of A. aegypti have not been detected in most localities where these two invasive species are known to co-occur. Aedes albopictus invaded the United States and Brazil at approximately the same time, in the mid-1980s, but the origins of the invading strains are known to be different. Here we tested the hypothesis, in standardized cage environments, that A. albopictus males from Brazil are less capable of satyrizing A. aegypti females than A. albopictus males from the United States. Using strains of A. aegypti and A. albopictus from the United States of known susceptibility to and capacity for interspecific mating, we demonstrate that A. albopictus colonized from collections in the Brazilian cities of Rio de Janeiro and Manaus were relatively unsuccessful in inseminating virgin female A. aegypti from Key West Florida compared to A. albopictus from peninsular Florida. We suggest that the low satyrization potential of Brazilian A. albopictus males may contribute to the lack of documented competitive displacements of A. aegypti in that country.

Interspecific interference in the closely related predatory mites Neoseiulus womersleyi and Neoseiulus longispinosus (Acari: Phytoseiidae)

ABSTRACTReproductive interference is known to occur in the morphologically similar species Neoseiulus womersleyi and Neoseiulus longispinosus: crosses between N. longispinosus females and N. womersleyi males produce (a few) female offspring, reciprocal crosses produce only male offspring. Details of the interspecific reproduction process remain largely unknown. In this article, we investigated intraspecific and interspecific crosses, spermathecal vesicles and spermatophores in females, and molecular identification of F1 hybrids based on the internal transcribed spacer (ITS) region of nuclear ribosomal RNA (rRNA). The copulation times of interspecific crosses were similar to those of intraspecific crosses, yet interspecific crosses between N. womersleyi females and N. longispinosus males exhibited extremely low fecundity with no female offspring. On the other hand, 1 out of 35 reciprocal crosses produced viable, but sterile female offspring (F1). Intraspecific crosses of N. womersleyi and N. longispinosus showed female-biased offspring sex ratios. Most N. womersleyi females (12 out of 15 females tested) that mated with N. longispinosus males had inflated vesicle(s) and were inseminated. Of the crosses between N. longispinosus females and N. womersleyi males, 14 pairs had neither inflated spermathecal vesicles nor were inseminated, and in only one pair, vesicles were inflated and insemination occurred that enabled the production of female offspring. Polymerase chain reaction products of the ITS region of rRNA gene digested with ClaI exhibited two fragments (361 and 278 bp) for the N. womersleyi and only a single fragment (642 bp) for the N. longispinosus, whereas three fragments for the F1 hybrids (642, 361, and 278 bp), confirming the identification. Our results confirmed the asymmetric reproductive interference between N. womersleyi and N. longispinosus and demonstrated that the F1 female offspring produced were (sterile) hybrids.

Attack of the clones: reproductive interference between sexuals and asexuals in the Crepis agamic complex.

Negative reproductive interactions are likely to be strongest between close relatives and may be important in limiting local coexistence. In plants, interspecific pollen flow is common between co‐occurring close relatives and may serve as the key mechanism of reproductive interference. Agamic complexes, systems in which some populations reproduce through asexual seeds (apomixis), while others reproduce sexually, provide an opportunity to examine effects of reproductive interference in limiting coexistence. Apomictic populations experience little or no reproductive interference, because apomictic ovules cannot receive pollen from nearby sexuals. Oppositely, apomicts produce some viable pollen and can exert reproductive interference on sexuals by siring hybrids. In the Crepis agamic complex, sexuals co‐occur less often with other members of the complex, but apomicts appear to freely co‐occur with one another. We identified a mixed population and conducted a crossing experiment between sexual diploid C. atribarba and apomictic polyploid C. barbigera using pollen from sexual diploids and apomictic polyploids. Seed set was high for all treatments, and as predicted, diploid–diploid crosses produced all diploid offspring. Diploid–polyploid crosses, however, produced mainly polyploidy offspring, suggesting that non‐diploid hybrids can be formed when the two taxa meet. Furthermore, a small proportion of seeds produced in open‐pollinated flowers was also polyploid, indicating that polyploid hybrids are produced under natural conditions. Our results provide evidence for asymmetric reproductive interference, with pollen from polyploid apomicts contributing to reduce the recruitment of sexual diploids in subsequent generations. Existing models suggest that these mixed sexual–asexual populations are likely to be transient, eventually leading to eradication of sexual individuals from the population.

Asymmetrical reproductive interference between two sibling species of tea looper: Ectropis grisescens and Ectropis obliqua.

Ectropis grisescens Warren and Ectropis obliqua (Prout) are two morphologically similar sibling species with overlapping ranges. In this study, manipulative laboratory experiments were conducted to examine the possibility of reproductive interference in sympatric populations of E. grisescens and E. obliqua and the potential consequences of the mating interaction. Our results showed that the presence of males or females of different species could incur mating interference and significant reduction of F 1 offspring. The reduction was not significant relevant to the initial relative abundance of E. grisescens and E. obliqua. Detailed observations of mating opportunity showed that female mating frequencies of both species were not significantly affected by the absolute species density, but the mating success of E. obliqua females with conspecific males depended on species ratio. In addition, adding males to the other species resulted in lower number of offspring suggesting that the males' behaviour might be linked with mating interference. Males of both E. grisescens and E. obliqua could interfere the intraspecific mating of the other species, but the impact of the mating interference differed. These combined data indicated that asymmetric reproductive interference existed in E. grisescens and E. obliqua under laboratory conditions, and the offspring of the mixed species were significantly reduced. The long term outcome of this effect is yet to be determined since additional reproductive factors such as oviposition rate and progeny survival to adulthood may reduce the probability of demographic displacement of one species by the other in overlapping niches.

Intraspecific and interspecific competition for mates: Rana temporaria males are effective satyrs of Rana dalmatina females

Reproductive interference can seriously affect the reproductive success of involved individuals and may lead to local exclusion of the competitively inferior species. The components of male competitiveness in direct bodily fights for females have been studied rarely in a heterospecific context. In explosively breeding anuran amphibians, males are often coercive and indiscriminate, which frequently leads to interactions among heterospecific males. We experimentally assessed the competitiveness of male Rana dalmatina (RD) and R. temporaria (RT), two species with overlapping breeding both in time and space. The mating speed of the RD males decreased over the course of the breeding season, whereas RT males retained their swiftness. In addition, the RD males were rarely able to replace the RT males from amplexus with RD females, while takeovers frequently occurred among the RD males. Further, larger RD males were more successful in maintaining amplexus and in achieving takeovers when facing smaller conspecifics. Our results suggest that male body size is important for intraspecific competition among the RD males, but the RT males outcompete the RD males of all sizes, perhaps due to their larger body size. Hence, RT males are likely to be effective satyrs of RD females, because they may lower the reproductive success of the latter. Such interspecific differences in the competitiveness of males may represent an important mechanism that contributes to the arousal of asymmetric reproductive interference between species. Reproductive interference—interactions between species during mating with negative consequences on the fitness of involved individuals—is widespread in nature. Even though differences in the competitiveness of males of interfering species may have severe consequences, competitiveness of males has rarely been studied in an interspecific scenario in vertebrates. Males of anuran amphibians are often indiscriminate and coercive and engage in bodily fights for the possession of both conspecific and heterospecific females, which may often compromise the reproductive success of involved males and females. Here, we demonstrate that interspecific differences among ranid males can be large in the swiftness of pair formation, in the ability to defend the mating position and to perform takeovers. These differences are likely to lower the reproductive performance of the competitively inferior species, potentially leading to its local extinction.

Within-species reproductive costs affect the asymmetry of satyrization in Drosophila.

Understanding how species interactions influence their distribution and evolution is a fundamental question in evolutionary biology. Theory suggests that asymmetric reproductive interference, in which one species induces higher reproductive costs on another species, may be more important in delimiting species boundaries than interspecific competition over resources. However, the underlying mechanisms of such asymmetry remain unclear. Here, we test whether differences in within-species reproductive costs determine the between-species asymmetry of costs using three allopatric Drosophila species belonging to the melanogaster subgroup. Our results support this hypothesis, especially in a pair of insular species. Males of one species that induce costs to their conspecific females led to a 5-fold increase of heterospecific females mortality with dead flies bearing spectacular large melanized wounds on their genitalia. Males of the other species were harmful neither to their conspecific nor heterospecific females. Comparative studies of within-species reproductive costs may therefore be a valuable tool for predicting between-species interactions and community structures.

Hybridization and pre-zygotic reproductive barriers in Plasmodium.

Sexual reproduction is an obligate step in the life cycle of many parasites, including the causative agents of malaria (Plasmodium). Mixed-species infections are common in nature and consequently, interactions between heterospecific gametes occur. Given the importance of managing gene flow across parasite populations, remarkably little is understood about how reproductive isolation between species is maintained. We use the rodent malaria parasites P. berghei and P. yoelii to investigate the ecology of mixed-species mating groups, identify proteins involved in pre-zygotic barriers, and examine their evolution. Specifically, we show that (i) hybridization occurs, but at low frequency; (ii) hybridization reaches high levels when female gametes lack the surface proteins P230 or P48/45, demonstrating that these proteins are key for pre-zygotic reproductive isolation; (iii) asymmetric reproductive interference occurs, where the fertility of P. berghei gametes is reduced in the presence of P. yoelii and (iv) as expected for gamete recognition proteins, strong positive selection acts on a region of P230 and P47 (P48/45 paralogue). P230 and P48/45 are leading candidates for interventions to block malaria transmission. Our results suggest that depending on the viability of hybrids, applying such interventions to populations where mixed-species infections occur could either facilitate or hinder malaria control.

The Effects of Interspecific Courtship on the Mating Success of Aedes aegypti and Aedes albopictus (Diptera: Culicidae) Males.

Satyrization, a form of asymmetric reproductive interference, has recently been shown to play a role in competitive displacements of Aedes aegypti (L.) by Aedes albopictus (Skuse). Furthermore, female Ae. aegypti from populations in sympatry with Ae. albopictus have evolved reproductive character displacement and changes in mating behavior to reduce interspecific mating. In this article, we examine evolutionary responses of males to interspecific mating and show that satyrization has also evoked reproductive character displacement in males. We demonstrate that the presence of heterospecific females negatively influences conspecific mating success in male Ae. aegypti, most likely due to misdirected courting or mating efforts, and that males of this species from populations in sympatry with Ae. albopictus have evolved to be less influenced by the presence of heterospecific females than their allopatric counterparts. Conversely, we suggest that the presence of conspecifics may, in some circumstances, increase interspecific mating. This study demonstrates that co-occurrences of these two invasive species may lead to evolution and adaptation of reproductive behaviors to changing circumstances. Understanding the processes driving development of mate choice preferences or avoidance mechanisms may help predict future changes in the distribution and abundance of insect vectors or pests.

Effects of reproductive interference on the competitive displacement between two invasive whiteflies

Reproductive interference is one of the major factors mediating species exclusion among insects. The cryptic species Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) of the whitefly Bemisia tabaci complex have invaded many parts of the world and often exhibit niche overlap and reproductive interference. However, contrasting patterns of competitive displacement between the two invaders have been observed between regions such as those in USA and China. Understanding the roles of reproductive interference in competitive interactions between populations of the two species in different regions will help unravel other factors related to their invasion. We integrated laboratory population experiments, behavioural observations and simulation modelling to investigate the role of reproductive interference on species exclusion between MEAM1 and MED in China. In mixed cohorts of the two species MEAM1 always excluded MED in a few generations when the initial proportion of MEAM1 was ⩾0.25. Even when the initial proportion of MEAM1 was only 0.10, however, MEAM1 still had a higher probability of excluding MED than that for MED to exclude MEAM1. Importantly, we show that as MEAM1 increased in relative abundance, MED populations became increasingly male-biased. Detailed behavioural observations confirmed that MEAM1 showed a stronger reproductive interference than MED, leading to reduced frequency of copulation and female progeny production in MED. Using simulation modelling, we linked our behavioural observations with exclusion experiments to show that interspecific asymmetric reproductive interference predicts the rate of species exclusion of MED by MEAM1. These findings not only reveal the importance of reproductive interference in the competitive interactions between the two invasive whiteflies as well as the detailed behavioural mechanisms, but also provide a valuable framework against which the effects of other factors mediating species exclusion can be explored.

Rapid evolution of reduced receptivity to interspecific mating in the dengue vector Aedes aegypti in response to satyrization by invasive Aedes albopictus

In this paper we examine the effect of reproductive interference on the dynamics of two mosquito vectors of public health concern and add to the growing literature on the strength and speed with which interspecific reproductive interference may drive evolution. Recent evidence supports a role for asymmetric reproductive interference, or satyrization, in competitive displacements of Aedes aegypti by Aedes albopictus. However, populations of A. aegypti sympatric with A. albopictus in nature evolve resistance to satyrization. Here we report that A. aegypti from Tucson, Arizona (USA), where A. albopictus are not known to occur, are satyrization-susceptible. Furthermore, in cage experiments we demonstrate rapid evolution in satyrization-susceptible lines. Exposing allopatric strains of A. aegypti to A. albopictus in cages led to significant reductions, within 1-3 generations, in the frequency of reproductive interference. We also demonstrate that satyrization-resistant A. aegypti females derived from selection experiments are significantly slower to mate with conspecific males, suggesting a cost for the evolution of satyrization-resistance. Results show how interspecific interactions between these vector species are rapidly evolving, with implications for the arboviral diseases, especially dengue and chikungunya, which they transmit.

Asymmetric reproductive isolation and interference in neriid flies: the roles of genital morphology and behaviour

The processes underlying reproductive isolation, and the traits involved, are the subject of considerable debate in evolutionary biology. Studying the costly sexual interaction of species in secondary sympatry, a phenomenon known as reproductive interference, can help to shed light on past and present isolating mechanisms, as well as the implications of sympatry for individual fitness. We investigated the roles of two sets of traits, genitalic and behavioural, in reproductive isolation and interference in two species of Australian neriid flies, Telostylinus lineolatus and T. angusticollis. Surprisingly, although these species differ markedly in male but not in female genitalia, we found evidence that genital morphology resulted in asymmetric reproductive isolation: T. lineolatus males could transfer sperm to T. angusticollis females, but T. angusticollis males were unable to transfer sperm to T. lineolatus females. However, neither type of cross produced any viable hybrids. Behavioural responses also contributed asymmetrically to both reproductive isolation and reproductive interference. Males pursued both conspecific and heterospecific females. Females of both species discriminated against heterospecific males, but T. lineolatus females exhibited stronger discrimination than T. angusticollis females. Curiously, T. angusticollis males both fought and attempted to copulate with T. lineolatus males, resulting in reduced mating success for T. angusticollis males with conspecific females. Our findings show that both genitalic and behavioural traits can play important roles in reproductive isolation, but the consequences of interspecific divergence in these traits can be surprisingly complex, resulting in asymmetric effects on many aspects of interspecific interactions.

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

AbstractMale 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.

Asymmetric reproductive interference between specialist and generalist predatory ladybirds

1. Closely related species often differ greatly in the quality and breadth of resources exploited, but the actual mechanisms causing these differences are poorly understood. Because in the laboratory specialized species often survive and perform as well or better on host species that are never utilized in nature, negative ecological interactions restricting host range must exist. Here, we focused on reproductive interference, which has been theoretically predicted to drive niche separation between closely related species with overlapping mating signals. 2. We examined the interspecific sexual interactions in relation to ecological specialization and generalization in two sibling ladybird species, Harmonia yedoensis and Harmonia axyridis. Harmonia yedoensis is a specialist predator that preys only on pine aphids, which are highly elusive prey for ladybird hatchlings, whereas H. axyridis is a generalist predator with a broad prey and habitat range. 3. We experimentally showed that conspecific sperm fertilized the vast majority of eggs regardless of mating order (i.e. conspecific sperm precedence) when a female of H. yedoensis or H. axyridis mated with both a conspecific and a heterospecific male. Moreover, we demonstrated that mating opportunities of H. yedoensis females strongly decreased as heterospecific density increased relative to conspecific density. In contrast, in H. axyridis, female mating success was high regardless of conspecific or heterospecific density. 4. Our results suggest that the generalist H. axyridis should be dominant to the specialist H. yedoensis in terms of reproductive interference. Our results support the hypothesis that asymmetric reproductive interference from the dominant species may force the non-dominant species to become a specialist predator that exclusively utilizes less preferred prey in nature.

Evidence for pre-zygotic reproductive barrier between the B and Q biotypes ofBemisia tabaci(Hemiptera: Aleyrodidae)

The degree of reproductive isolation between the B and Q biotypes of the whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is currently not clear. Laboratory experiments have shown that the two biotypes are capable of producing viable F1 hybrids but that these females are sterile as their F2 generation failed to develop, indicating, most likely, a post-zygotic reproductive barrier. Here, we confirm, by molecular and ecological tools, that the B and Q biotypes of Israel are genetically isolated and provide two independent lines of evidence that support the existence of a pre-zygotic reproductive barrier between them. Firstly, monitoring of mating behaviors in homogeneous and heterogeneous couples indicated no copulation events in heterogeneous couples compared to approximately 50% in homogeneous B and Q couples. Secondly, we could not detect the presence of sperm in the spermathecae of females from heterogeneous couples, compared to 50% detection in intra-B biotype crosses and 15% detection in intra-Q biotype crosses. The existence of pre-zygotic reproductive barriers in Israeli B and Q colonies may indicate a reinforcement process in which mating discrimination is strengthened between sympatric taxa that were formerly allopatric, to avoid maladaptive hybridization. As the two biotypes continued to perform all courtship stages prior to copulation, we also conducted mixed cultures experiments in order to test the reproductive consequences of inter-biotype courtship attempts. In mixed cultures, a significant reduction in female fecundity was observed for the Q biotype but not for the B biotype, suggesting an asymmetric reproductive interference effect in favour of the B biotype. The long-term outcome of this effect is yet to be determined since additional environmental forces may reduce the probability of demographic displacement of one biotype by the other in overlapping niches.

Plasticity in mating behaviour drives asymmetric reproductive interference in whiteflies

Reproductive interference between species with incomplete mate recognition can disrupt the process of mate acquisition. Accordingly, reproductive interference can reduce female and male fitness and lead to sexual exclusion. Here, we tested the hypothesis that flexible mating behaviours could mitigate the fitness-reducing effects of reproductive interference between reproductively incompatible biotypes of the haplodiploid whitefly Bemisia tabaci. We show that females of the globally distributed and invasive B biotype respond to reproductive interference from the Q biotype by increasing their acceptance of copulation attempts from B males. This behavioural plasticity increases the ability of B females to mate successfully and maintain a constant sex ratio in their offspring despite reproductive interference. In contrast, females of competing biotypes have invariant behaviour and produce fewer female offspring because of reproductive interference from the B biotype. Heuristic simulation models incorporating data on behavioural and life history traits of the B and Q biotypes obtained here, and published data on other biotypes, show that this plasticity in mating behaviour of B females could contribute to sexual exclusion of closely related biotypes. Our results demonstrate a powerful link between mating behaviour, reproductive interference and sexual exclusion, suggesting that variation in mating behaviour could determine the effects of reproductive interference and drive sexual exclusion among closely related species or among biotypes of a species.

Reproductive interference determines persistence and exclusion in species interactions

1. Reproductive interference is a negative interspecific sexual interaction that adversely affects the fitness of males and females during reproductive process. Theoretical studies suggest that because reproductive interference is characterized by positive frequency dependence it is far more likely to cause species exclusion than the density dependence of resource competition. However, the respective contributions of resource competition and reproductive interference to species exclusion, which have been frequently observed in many competition studies, remain unclear. 2. We show that reproductive interference is a far more critical cause of species exclusion than resource competition in the competition between Callosobruchus bean weevil species. In competition experiments over several generations, we manipulated the initial relative abundance of the adzuki bean beetle, Callosobruchus chinensis, and the southern cowpea beetle, Callosobruchus maculatus. When the initial adult ratio of C. chinensis : C. maculatus were 6 : 2 and 4 : 4, C. chinensis excluded C. maculatus. However, when C. maculatus was four times more abundant than C. chinensis at the start, we observed the opposite outcome. 3. A behavioural experiment using adults of the two species revealed asymmetric reproductive interference. The fecundity and longevity of C. maculatus females, but not those of C. chinensis females, decreased when the females were kept with heterospecific males. Fecundities of females of both species decreased as the number of heterospecific males increased. In contrast, resource competition at the larval stage resulted in higher survival of C. maculatus than of C. chinensis. 4. These results suggest that the positive frequency-dependent effect of reproductive interference resulted in species exclusion, depending on the initial population ratio of the two species, and the asymmetry of the interference resulted in C. chinensis being dominant in this study, as in previous studies. Classical competition studies should be reviewed in light of this evidence for reproductive interference.