A comparative study of interspecies mating of Phratora vulgatissima and P. vitellinae using behavioural tests and molecular markers

The leaf beetle genus Phratora L. (Coleoptera: Chrysomelidae) has been used to study the ecology of host plant chemicals in herbivore preference, and the evolution of host use in chemical defence. Phratora vulgatissima and P. vitellinae are sympatric species distributed widely across Europe. Their trophic niches are largely separate due to strong differences in their host feeding preference, but they have occasionally been recorded together, feeding on Salix burjatica‘Germany’ and, only in early spring, on Populus trichocarpa (Torr & A. Gray) ‘Trichobel’. Using behavioural tests and recently developed species‐specific microsatellite markers, the intra‐ and interspecific mating of both beetle species were investigated. The microsatellite markers provided evidence that interspecific mating occurred under field conditions. Interspecific mating also took place under laboratory conditions, but less frequently than mating within species. Females of both species laid fewer eggs, and fewer eggs per clutch, when isolated with an interspecific male than with a conspecific male. Female P. vulgatissima were polyandrous, as microsatellite markers showed that their larvae were the progeny of both P. vulgatissima males that had been isolated with a single female. While only 0.55% of eggs laid in interspecific pair combinations hatched, microsatellite markers provided evidence of hybridisation between beetle species; however, these larvae died within a week when reared in a Petri dish containing ‘Germany’ and P. trichocarpa leaves. It can therefore be inferred that reproductive isolation is complete. The results are discussed in relation to species integrity and the implications for diverse mixtures of short‐rotation coppice willow plantations.     

Frequency of interspecific matings and of hybrid offspring in sympatric populations of Vtviparus ater and V. contectus (Mollusca: Prosobranchia)

In species pairs able to produce fertile hybrids, the amount of introgression can be limited by the frequency of interspecific copulations. We investigated the frequency of interspecific copulations between Kviparus ater and V. contectus and the proportion of hybrid offspring of V. ater females in natural populations in northern Italy. Although there was significant assortative mating, the frequency of interspecific copulations was substantial. At sites where V. contectus was rare, 5.9% of all matings, and up to 73% of matings in which V. contectus was involved, were interspecific. At sites where both species were abundant 14.8% of all matings were interspecific. Male-male matings occurred in approximately 3% of copulations. In 1993, 12 out of 325 V. ater females collected from 10 sympatric populations and held in isolation for two to five months gave birth to 25 hybrid offspring, out of a total of 2548 offspring produced by all females. The proportion of hybrid offspring (ranging from 0 to 2.3% depending on the site) was positively related to the proportion of V. contectus in the population. Since interspecific copulations are frequent and hybrid offspring are produced in nature, we conclude that assortative mating is not an effective isolating mechanism between V. ater and V. contectus.     

Introgression in natural populations of bioindicators: a case study of Carabus splendens and Carabus punctatoauratus

The evolutionary importance of hybridization in wild plants and animals has become increasingly widely recognized in the last decade. In practical terms, hybridization provides an exceptionally tough set of problems for conservation biologists. We illustrate this in a case study of two Carabidae species widely used to evaluate the impact of human activities on biodiversity. These two species live in a complex mosaic of sympatry/allopatry and are known to hybridize in controlled conditions. Hybridization has not been quantified in natural populations to date due to the lack of a simple set of phenotypic traits for identifying hybrids. We thus screened for hybrids in natural populations, by multilocus genotyping at nine microsatellite loci. A high level of genetic differentiation between these two taxa was observed, as shown by allelic frequency distributions. Two Bayesian assignment procedures without obligatory pure taxon references were used to infer different classes of hybrids (F(1), F(2) and backcrosses) and mixture proportions between the two species. A low level of hybridization (F(1) genotypes) was observed in natural populations, contrasting with results obtained in controlled conditions. A high level of introgression was, however, detected at three of 12 sites, as revealed by the detection of backcrossed genotypes. This interspecific gene flow was detected in a limited zone of the common geographical range of the two species and was not related to the pattern of sympatry/allopatry. We then considered the origin and repercussions of this introgression, based on intraspecific genetic diversity and geographical structure.     

Inter- and Intraspecific Effects of Volatile and Nonvolatile Sex Pheromones on Males, Mating Behavior, and Hybridization in Eretmocerus mundus and E. eremicus (Hymenoptera: Aphelinidae)

Eretmocerus species (Hym. Aphelinidae) are solitary parasitoids of Bemisia tabaci (Gennadius). Mate finding and mating behavior of two species, E. mundus and E. eremicus, were studied under laboratory conditions. We used three populations of Eretmocerus: typical arrhenotokous populations of E. eremicus (from USA) and E. mundus (from Spain), and an atypical thelytokous population of E. mundus (from Australia). We studied the intra- and interspecific responses of males to volatile and nonvolatile components of the female sex pheromones, mating behavior, and hybridization between populations and species. In both arrhenotokous populations, males reacted to volatile pheromones by walking toward conspecific virgin females. Males also reacted to nonvolatile pheromones by spending more time on and around patches on leaves of poinsettia plants that had been exposed to virgin females. Males of E. eremicus showed the same reaction to the nonvolatile sex pheromone of E. mundus females, but E. mundus males did not show any reaction to the nonvolatile sex pheromone of E. eremicus. There was no response of males of both species to thelytokous females of E. mundus. In both species three phases were distinguished in the mating behavior: premating, mating, and postmating. The duration of the phases differed between the three populations. Successful copulation between the two Eretmocerus species did not occur. In contrast, we recorded some successful copulations between Australian males and Spanish females of E. mundus, but they did not produce any hybrid females.     

A General Model of Distant Hybridization Reveals the Conditions for Extinction in Atlantic Salmon and Brown Trout

Interspecific hybridization is common in nature but can be increased in frequency or even originated by human actions, such as species introduction or habitat modification, which may threaten species persistence. When hybridization occurs between distantly related species, referred to as “distant hybridization,” the resulting hybrids are generally infertile or fertile but do not undergo chromosomal recombination during gametogenesis. Here, we present a model describing this frequent but poorly studied interspecific hybridization to assess its consequences on parental species and to anticipate the conditions under which they can reach extinction. Our general model fully incorporates three important processes: density-dependent competition, dominance/recessivity inheritance of traits and assortative mating. We demonstrate its use and flexibility by assessing population extinction risk between Atlantic salmon and brown trout in Norway, whose interbreeding has recently increased due to farmed fish releases into the wild. We identified the set of conditions under which hybridization may threaten salmonid species. Thanks to the flexibility of our model, we evaluated the effect of an additional risk factor, a parasitic disease, and showed that the cumulative effects dramatically increase the extinction risk. The consequences of distant hybridization are not genetically, but demographically mediated. Our general model is useful to better comprehend the evolution of such hybrid systems and we demonstrated its importance in the field of conservation biology to set up management recommendations when this increasingly frequent type of hybridization is in action.     

A comparison of the developmental and reproductive biology of two soybean pod borers, Etiella spp. in Indonesia

Experiments conducted in West Java, Indonesia investigated the developmental biology and reproductive behavior of two sympatric soybean pod borers, Etiella zinckenella Treitschke and E. hobsoni Butler (Lepidoptera: Pyralidae). It was determined that: (1) significant interspecific differences occurred between the egg, larval, pupal and total egg-to-adult developmental periods of laboratory raised E. zinckenella and E. hobsoni; (2) the pre-pupal and total egg-to-adult development periods of female E. zinckenella were significantly shorter than for males; (3) the longevity of virgin female E. zinckenella was significantly longer than that of virgin males, or virgin male and female E. hobsoni; (4) interspecific differences occurred in the female: male sex-ratios of laboratory raised adults; (5) peak mating for both species occurred on the second night after eclosion; (6) interspecific differences occurred in the temporal distribution of calling and mating behaviors; (7) repeated mating was observed for both species at a very low frequency; (8) interspecific mating did not occur; (9) female E. zinckenella were significantly more fecund than E. hobsoni; (10) the duration in copulo of E. zinckenella was significantly longer than that of E. hobsoni; and (11) wingtraps baited with virgin females caught only conspecific males, and reduced numbers of males were captured in traps simultaneously baited with virgin females of both species. This study demonstrates distinct biological differences and reproductive isolation between the two Etiella spp.     

BODY SIZE,NATURAL SELECTION,AND SPECIATION IN STICKLEBACKS

There is little evidence from nature that divergent natural selection is crucial to speciation. However, divergent selection is implicated if traits conferring adaptation to alternative environments also form the basis of reproductive isolation. We tested the importance of body size differences to premating isolation between two sympatric sticklebacks. The species differ greatly in size, and several lines of evidence indicate that this difference is an adaptation to alternative foraging habitats. Strong assortative mating was evident in laboratory trials, but a few hybridization events occurred. Probability of interspecific mating was strongly correlated with body size: interspecific spawning occurred only between the largest individuals of the smaller species and the smallest individuals of the larger species. Probability of spawning between similar-sized individuals from different species was comparable to spawning rates within species. Disruption of mating between individuals from different species can be traced to increased levels of male aggression and decreased levels of male courtship as size differences increased between paired individuals. Interspecific mate preferences in sympatric sticklebacks appears to be dominated by body size, implicating natural selection in the origin of species.     

Sexual conflict and ecology: Species composition and male density interact to reduce male mating harassment and increase female survival

Sexual conflict is a pervasive evolutionary force that can reduce female fitness. Experimental evolution studies in the laboratory might overestimate the importance of sexual conflict because the ecological conditions in such settings typically include only a single species. Here, we experimentally manipulated conspecific male density (high or low) and species composition (sympatric or allopatric) to investigate how ecological conditions affect female survival in a sexually dimorphic insect, the banded demoiselle (Calopteryx splendens). Female survival was strongly influenced by an interaction between male density and species composition. Specifically, at low conspecific male density, female survival increased in the presence of heterospecific males (C. virgo). Behavioral mating experiments showed that interspecific interference competition reduced conspecific male mating success with large females. These findings suggest that reproductive interference competition between con‐ and heterospecific males might indirectly facilitate female survival by reducing mating harassment from conspecific males. Hence, interspecific competitors can show contrasting effects on the two sexes thereby influencing sexual conflict dynamics. Our results call for incorporation of more ecological realism in sexual conflict research, particularly how local community context and reproductive interference competition between heterospecific males can affect female fitness.     

Levels of mate recognition within and between two Drosophila species and their hybrids

If sexual selection is to result in speciation, traits involved in mate choice within species need to be capable of producing sexual isolation between species. We investigated the association between mate choice and sexual isolation using interspecific hybrids between two sibling species, Drosophila serrata and Drosophila birchii. A perfuming experiment demonstrated that olfaction was involved in the sexual isolation between the two species. A quantitative genetic analysis using 30 populations of hybrids between the two species indicated that mating success in hybrid individuals was predominately determined by cuticular hydrocarbons; the average genetic correlation between mating success and cuticular hydrocarbon profile was 0.84, and in some instances exceeded 0.95. Multivariate analysis of the cuticular hydrocarbons of the two species revealed that there were three independent blends of cuticular hydrocarbons that separated three levels of organization: species, sex, and sex within species. The hydrocarbons used by hybrids in mate choice included those that separated the two species, demonstrating that species-specific characters may be used in mate choice within populations. The interspecific reciprocal cross had major effect on which cuticular hydrocarbons were associated with mating success, indicating that the expression of the cuticular hydrocarbons was strongly sex linked.     

A genetic analysis of some components of reproductive isolation between two closely related species,Spodoptera latifascia (Walker) and S. descoinsi (Lalanne-Cassou and Silvain) (Lepidoptera: Noctuidae)

The two closely related noctuid moths Spodoptera latifascia and S. descoinsi have been found in sympatry in some parts of French Guiana. They differ by i) ecological preferences, ii) some anatomical features of the genital tract, iii) the relative amounts of the two main components of the female pheromonal blend, and iv) the temporal pattern of sexual behaviour. Viable and fertile interspecific hybrids can be obtained in the laboratory, thus permitting a genetic study of some of these traits. The anatomical differences cause perturbations in mating efficiency. Experimental results show that these differences are hereditarily transmitted, but their exact mode of inheritance remains unknown. The timing of initiation of female calling activity is very likely to be under polygenic control. Conclusive evidence showed that a pair of major autosomal alleles determined the pheromone composition. Moreover, some of the data suggest that the descoinsi allele would become lethal in certain mosaic latifascia/descoinsi genomic environments. The ecological factor is likely to be the major component of reproductive isolation between the two species in natural conditions. However in contact zones, the differences in pheromone composition and in timing of sexual activity are probably involved in preventing interspecific hybridization. Two speciation scenarios are proposed, both sympatric. In one of them, the primary event would be a fragmentation of the tropical forest followed by disruptive selection associated with habitat diversification. In the other, the speciation process might have been initiated by the occurrence of mutations acting on the pheromone mediated mate recognition system.     

Are 'preferences' and 'tolerances' in cichlid mate choice important for speciation?

Although early experience influences the development of preferences in cichlid mate choice, the same species can be very 'tolerant' and spawn with members of other species appearing very different from conspecifics. The conditions under which such interspecific spawnings occur in captivity are described, and are considered to exist in the field.
Many cichlid species are genetically similar, and interspecific spawning often produces fertile hybrids. Pre-mating behavioural barriers to hybridization are important. The ability of cichlids to develop preferences based on subtle cues, but also to produce fertile hybrids, is discussed in the context of speciation: genetic isolation of morphs (and possibly of hybrids) from their parental species, through social learning.     

Alternative Mating Tactics as Potential Prezygotic Barriers to Gene Flow Between Two Sister Species of Frugivorous Fruit Flies

Rhagoletis completa Cresson and Rhagoletis zoqui Bush, are two sister species in the suavis group that were thought to occupy non-overlapping geographical ranges. Recent discovery of a contact zone in North Eastern México where natural interspecific hybrids with intermediate morphotypes can be found has led to laboratory studies in small enclosures that failed to identify the existence of reproductive barriers capable of maintaining the genetic integrity of both species. Because the experimental approach used in such studies could have interfered with natural behavior of flies, a series of additional mating compatibility tests under much less restrictive conditions were performed. Our observations, confirmed the fact that males and females of both species engage in interspecific matings. However, R. completa females were more reluctant to engage in heterospecific matings than R. zoqui females, revealing asymmetries in sexual isolation. Also, careful scrutiny of male guarding behavior and species specific partitioning of mating location unveiled subtle differences that could result in reproductive isolation if reinforced during secondary contact.     

Frequency of interspecific mating in salamanders of the plethodontid genus Desmognathus: different experimental designs may yield different results

Heterosexual courtship trials were staged between dusky salamanders of two species ( Desmognathus fuscus and D. santeetlah ) from two allopatric populations in the southern Appalachian Mountains of eastern North America. Two experiments were conducted in order to test for effects of experimental design on estimates of the strength of sexual isolation between these species. In Experiment 1, frequencies of interspecific mating were determined when: (1) a male was presented with a conspecific and a heterotypic female successively; and (2) both types of females were presented simultaneously to a male. The frequency of interspecific mating by male D. fuscus was greatest when females were presented successively. The frequency of interspecific mating by male D. santeetlah was not influenced by method of female presentation. An index of sexual isolation of +0.29 was obtained between D. fuscus and D. santeetlah when females were presented successively, and of +0.63 when females were presented simultaneously. In Experiment 2, interspecific mating frequencies were determined in courtship arenas of two different sizes. No significant effect of arena size on frequency of interspecific mating was found for D. fuscus. In D. santeetlah , a trend towards greater mating frequency in small arenas was almost significant. These results suggest that interspecific mating frequencies (and thus estimates of the strength of sexual isolation between populations or species) may be sensitive to the experimental design employed. The validity and generality of this conclusion for other animals is an urgent, empirical issue.     

Intraspecific sexual selection on a speciation trait, male coloration, in the Lake Victoria cichlid Pundamilia nyererei

The haplochromine cichlids of Lake Victoria constitute a classical example of explosive speciation. Extensive intra- and interspecific variation in male nuptial coloration and female mating preferences, in the absence of postzygotic isolation between species, has inspired the hypothesis that sexual selection has been a driving force in the origin of this species flock. This hypothesis rests on the premise that the phenotypic traits that underlie behavioural reproductive isolation between sister species diverged under sexual selection within a species. We test this premise in a Lake Victoria cichlid, by using laboratory experiments and field observations. We report that a male colour trait, which has previously been shown to be important for behavioural reproductive isolation between this species and a close relative, is under directional sexual selection by female mate choice within this species. This is consistent with the hypothesis that female choice has driven the divergence in male coloration between the two species. We also find that male territoriality is vital for male reproductive success and that multiple mating by females is common.     

Asymmetric Mating Interference between Two Related Mosquito Species: Aedes (Stegomyia) albopictus and Aedes (Stegomyia) cretinus

Aedes (Stegomyia) albopictus (Skuse) and Aedes (Stegomyia) cretinus Edwards are closely related mosquito species with common morphological features and bio-ecological similarities. Recent mosquito surveillance in Athens, Greece, showed that they are sympatric mosquito species, with Ae. Albopictus developing quite higher population densities than Ae. Cretinus. The potential of mating interference between these species was investigated by reciprocal and homologous mating experiments in cages under laboratory conditions. In non-choice interspecific crosses (groups of males and females) females of both species produced sterile eggs. Insemination rate was 58% for Ae. Cretinus females and only 1% for Ae. Albopictus females. Aedes albopictus males were sexually aggressive and inseminated Ae. Cretinus females (31%) in choice experiments, where males of one species had access to mate with females of both species. Whereas, interspecific mating of Ae. Albopictus females with Ae. Cretinus males in the co-occurrence of Ae. Cretinus females was weaker (4%). Aedes cretinus females from non-choice crossing with Ae. Albopictus or Ae. Cretinus males were paired individually with conspecific males. The percentage of fertile Ae. Cretinus females was 17.5% when had encaged before with Ae. Albopictus males, compared to 100% when Ae. Cretinus females were encaged with conspecific males only. Probable ecological consequences of asymmetric mating between these ecologically homologous species in nature are discussed.     

Evolutionary genetics of the Drosophila montium subgroup. I. Reproductive isolations and the phylogeny

Seventeen species of the Drosophila montium subgroup, originated from the Southeast Asia, were genetically examined to clarify the phylogenetic relationships. Among 272 interspecific crosses, 61 combinations were successful in mating and 39 combinations produced hybrid flies. These results enabled us to classify the subgroup into three species complexes: the kikkawai complex (6 species), the jambulina complex (4 species) and the auraria complex (7 species), which were very similar to that obtained by the electrophoretic classification (Ohnishi and Watanabe, 1984). Asymmetrical mating preference between species was found in the present experiment. They were applied to estimate the relative age of species according to the hypothesis proposed by Watanabe and Kawanishi (1979). The evolving order was as follows: kikkawai, leontia, pennae, lini-like, lini, bocki in the kikkawai complex, punjabiensis, punjabiensis-like, jambulina, barbarae in the jambulina complex, and quadraria, yuwanensis, rufa, subauraria, biauraria, triauraria, auraria in the auraria complex. Hybrid flies, if produced, were mostly fertile or partially fertile in both sexes (32/39) within species complex crosses. Therefore premating isolation played a more important role than postmating isolation in speciation of the D. montium subgroup.     

Sexually antagonistic coevolution in a mating system: combining experimental and comparative approaches to address evolutionary processes

We combined experimental and comparative techniques to study the evolution of mating behaviors within in a clade of 15 water striders (Gerris spp.). Superfluous multiple mating is costly to females in this group, and consequently there is overt conflict between the sexes over mating. Two alternative hypotheses that could generate interspecific variation in mating behaviors are tested: interspecific variation in optimal female mating rate versus sexually antagonistic coevolution of persistence and resistance traits. These potentially coevolving traits include male grasping and female antigrasping structures that further the interests of one sex over the other during premating struggles. Both processes are known to play a role in observed behavioral variation within species. We used two large sets of experiments to quantify behavioral differences among species, as well as their response to an environmentally (sex-ratio) induced change in optimal female mating rate. Our analysis revealed a large degree of continuous interspecific variation in all 20 quantified behavioral variables. Nevertheless, species shared the same set of behaviors, and each responded in a qualitatively similar fashion to sex-ratio alterations. A remarkably large proportion (> 50%) of all interspecific variation in the magnitude of behaviors, including their response to sex ratio, could be captured by a single multivariate axis. These data suggest tight coevolution of behaviors within a shared mating system. The pattern of correlated evolution was best accounted for by antagonistic coevolution in the relative abilities of each sex to control the outcome of premating struggles. In species where males have a relative advantage, mating activity is high, and the opposite is found in species where females have gained a relative advantage. Our analyses also suggested that evolution has been unconstrained by history, with no consistent evolutionary tendency toward or away from male or female relative advantage.     

MATING SYSTEM AND ASYMMETRIC HYBRIDIZATION IN A MIXED STAND OF EUROPEAN OAKS

The sessile (Quercus petraea [Matt.] Liebl.) and pedunculate (Quercus robur L.) oaks are two closely related species having a wide sympatric distribution over Europe. Under natural conditions, they frequently form mixed forests, where hybridization is suspected to occur. In this paper, two different approaches have been applied to the study of the mating system and the interspecific gene flow in a mixed stand formed by the two species. The mating systems of both species have been studied separately by means of the mixed-mating model. The relative contribution of the parental species to the progenies have been estimated with two different methods. The first uses the admixture model. The second is an extension of the mixed-mating model and subdivides the outcrossing rate into intra- and interspecific components. The two species were almost completely outcrossing. This high level of outcrossing and interspecific gene flow could play an important role in the maintenance of the genetic diversity in these long-lived forest tree species. The contribution of the sessile oak to the pedunculate oak progenies varied from 17% to 48%. In contrast, ovules of sessile oak trees appear to be preferentially fertilized by other extreme sessile genotypes. We suggest that interspecific and directional gene flow was responsible for such patterns. Pedunculate oak is considered as a pioneer species and is progressively replaced by sessile oak. Our present findings add a further genetic component to this succession scheme, suggesting that unidirectional gene flow reinforces succession between the two species.     

Conflicting intersexual mate choices maintain interspecific sexual interactions

Reproductive interference, interspecific sexual interactions that affect reproductive success, is found in various taxa and has been considered as a fundamental source of reproductive character displacement (RCD). Once RCD has occurred, persistent interspecific sexual interactions between species pairs are expected to diminish. However, reproductive interference has been reported from some species pairs that sympatrically coexist. Thus, the question arises, can reproductive interference persist even after RCD? We modeled the evolutionary dynamics of signal traits and mate recognition that determine whether interspecific sexual interactions occur. Our models incorporate male decision making based on the recognition of signal traits, whereas most previous models incorporate only female decision making in mate selection. Our models predict the following: (1) even when male decision making is incorporated, males remain promiscuous; (2) nevertheless, the frequency of interspecific mating is maintained at a low level after trait divergence; (3) the rarity of interspecific mating is due to strict female mate recognition and the consequent refusal of interspecific courtship by females; and (4) the frequency of interspecific mating becomes higher as the cost to females of refusing interspecific courtship increases. These predictions are consistent with empirical observations that males of some species engage in infrequent heterospecific mating. Thus, our models predict that reproductive interference can persist even after RCD occurred.     

Spatial swarm segregation and reproductive isolation between the molecular forms of Anopheles gambiae

Anopheles gambiae, the major malaria vector in Africa, can be divided into two subgroups based on genetic and ecological criteria. These two subgroups, termed the M and S molecular forms, are believed to be incipient species. Although they display differences in the ecological niches they occupy in the field, they are often sympatric and readily hybridize in the laboratory to produce viable and fertile offspring. Evidence for assortative mating in the field was recently reported, but the underlying mechanisms awaited discovery. We studied swarming behaviour of the molecular forms and investigated the role of swarm segregation in mediating assortative mating. Molecular identification of 1145 males collected from 68 swarms in Donéguébougou, Mali, over 2 years revealed a strict pattern of spatial segregation, resulting in almost exclusively monotypic swarms with respect to molecular form. We found evidence of clustering of swarms composed of individuals of a single molecular form within the village. Tethered M and S females were introduced into natural swarms of the M form to verify the existence of possible mate recognition operating within-swarm. Both M and S females were inseminated regardless of their form under these conditions, suggesting no within-mate recognition. We argue that our results provide evidence that swarm spatial segregation strongly contributes to reproductive isolation between the molecular forms in Mali. However this does not exclude the possibility of additional mate recognition operating across the range distribution of the forms. We discuss the importance of spatial segregation in the context of possible geographic variation in mechanisms of reproductive isolation.