Variation in hostplant relations and courtship signals of weed-associated populations of the brown planthopper, Nilaparvata lugens (Stål), from Australia and Asia: a test of the recognition species concept

Populations of the morphological species, Nilaparvata lugens (Stål), were found to breed and feed on the grass, Leersia hexandra Schwartz, at six sites in Queensland, Australia. They differ from sympatric rice-feeding populations in characters of pulse repetition frequencies of male and female acoustic courtship signals. The two host-derived populations hybridize freely in the laboratory, but in mate choice experiments show very significant preferences for homogametic matings. No indication of field hybridization has been found, so that the two morphologically inseparable populations represent sympatric biological species in Australia.
Populations from L. hexandra are also reported from four localities in Sri Lanka and one in Orissa, India. These resemble previously studied populations from the Philippines. They differ significantly in courtship call characters, both from sympatric rice-associated populations and from allopatric Leersia -associated populations from Australia.
The geographical variation reported for acoustic signals is not consistent with Paterson's recognition concept of species, but may be interpreted in terms of theories of allopatric speciation involving sexual selection for mate recognition signals.     

Specificity of acoustic signals and mate choice in the brown planthopper Nilaparvata lugens

Mate choice experiments were made between populations of N. lugens from the Philippines, Solomon Is. and northern Australia. Significant barriers to mating were found between the Australian insects and the other two populations. The acoustic signals of successful males in hybridisation experiments were recorded and their pulse repetition frequencies (PRFs) were compared with the mean PRF for a random sample from their own population. In crosses between insects from Australia and the Philippines or Solomon Is. succesful males were characterised by PRFs significantly different from their own populations and closer to those of the female populations in each cross than to their own. It is concluded that PRF of male calls is an important species recognition signal in N. lugens.

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Résumé La communication sexuelle de N. lugens s'effectue par signaux acoustiques transmis par le substrat. Quelques populations géographiquement définissables diffèrent par la fréquence des répétitions des impulsions (PRF) produits par les appels des mâles. Des populations des Philippines, des Iles Salomon et de l'Australie du Nord ont été étudiées.Des expériences de sélection sexuelle ont montré que des obstacles au transfert de spermatozoïdes apparaissaient dans les croisements qui impliquent un parent originaire de la population australienne. Des expériences dhybridation ont été répétées entre individus males et females de chacune des 3 populations étudiées. Le taux de succès étrait faible quand l'un des parents était originaire d'Australie. Le PRF des males de chaque croisement réussi a été déterminé et comparé au PRF moyen d'un échantillon constitué au hasard de males provenant de la même population. Pour les croisements impliquant un parent australien, le PRF des males victorieux différait de la moyenne et se rapprochait de celui des males de la population dont dérivait la female.On en a conclu que le PRF des appels males est important dans la reconnaissance spécifique de N. lugens, mais que chaque population montre un polymorphisme important pour ce caractère.

     

The variability of mating signals and preferences in the brown planthopper,Nilaparvata lugens (Homoptera: Delphacidae)

Male and female brown planthoppers, Nilaparvata lugens(Stål) (Homoptera: Delphacidae), exchange substrate-transmitted signals prior to mating. The pulse repetition frequency of the male song is known to be involved in mate recognition and also to vary among geographical populations. Here the variability of male signals, female signals, and female preferences has been examined within a population. Female preference variation has been partitioned into variation in mean preference and variation in the window of preference of individuals. The genetic component of variation has been examined using isofemale lines. Male signal variation was limited (CV=8%) and was mainly within individuals. Female signal variation was greater (CV=15%). Female mean preference varied little (CV=10%) and was closely matched to the male signal mean, but the preference window was wide (> 4 male signal standard deviations on average) and variable (CV=56%). There was evidence for genetic variation only for preference window. These results are discussed in relation to theories of signal system evolution.     

Sympatry with the devil: reproductive interference could hamper species coexistence

1. As species are often considered discrete natural units, interspecific sexual interactions are often disregarded as potential factors determining community composition. Nevertheless reproductive interference, ranging from signal jamming to hybridization, can have significant costs for species sharing similar signal channels. 2. We combined laboratory and field experiments to test whether the coexistence of two congeneric ground-hopper species with overlapping ranges might be influenced by sexual interactions. 3. In the laboratory experiment the number of conspecific copulations of Tetrix ceperoi decreased substantially in the presence of Tetrix subulata. Males of T. ceperoi performed more mating attempts with heterospecific females, whereas females of T. subulata rejected these heterospecific approaches more often than those of conspecifics. Although no heterospecific matings occurred in the laboratory, the reproductive success of T. ceperoi was reduced substantially in field experiments. Negative effects on T. subulata were found only at high densities. 4. Our results suggest that reproductive interference could have similar consequences as competition, such as demographic displacement of one species ('sexual exclusion'). As reproductive interference should be selected against, it may also drive the evolution of signals (reproductive character displacement) or promote habitat, spatial or temporal segregation.     

Variation in acoustic signals within and among leafhopper species of the genus Alebra (Homoptera, Gicadellidae)

Acoustic behaviour was studied in the four species of Alebra Fieber leafhoppers found in Britain, which are morphologically the closest in the genus: A. wahlbergi (Boheman), A. coryli Le Quesne, A. albostriella (Fallen) and A. viridis Rey. One male call, of a diagnostic structure and produced in all behavioural contexts observed, was recorded for each species. No female acoustic signals were identified. Using male call pattern the three species of Alebra coexisting on Castanea sativa Miller in Kastanitsa, southern Greece, were identified as A. wahlbergi, A. albostriella and A. viridis. Individuals of A. viridis from Kastanitsa had more pronounced external pigmentation than those from Cardiff, South Wales. Discriminant and principal component analysis on variables of the male call of A. viridis distinguished the populations from Cardiff and Kastanitsa, but could not differentiate populations from the two British host plants, Quercus petraea and Q. cerris , in Cardiff. The British and Greek populations differed in the covariation as well as the duration of call components.
Castanea sativa was also recognized as a host plant of A. viridis in Cardiff. Individuals collected from C. saliva had external patterning in the same range of variation as that found for individuals from species of Quercus.
Present evidence strongly suggests that divergence in the acoustic species recognition signals of Alebra is occurring among geographic rather than sympatric host plant-associated populations.     

Genetic control and evolution of acoustic signals in planthoppers (Homoptera: Delphacidae)

Acoustic signals are part of the specific mate recognition system of planthoppers. The genetic control of acoustic signal characters was studied in the planthopperRibautodelphax imitans. Artificial selection for interpulse interval in the female call revealed a large additive genetic component for this polygenic character. Other female call characters showed a correlated response. Some male call characters also appeared to be genetically correlated with the female character selected for, despite the rather different structure of male and female calls. Parent-offspring regression provided significant heritability estimates for those male call characters that also responded to artificial selection in the female call, one of which appeared to be influenced by sex-linked genes. It is argued that the differentiation of this mate recognition system in planthopper populations and species could be the result of founder effects, enabled by the genetic plasticity of the call characters and the existence of a wing length dimorphism in these animals.     

Hybrid origin of a swordtail species (Teleostei: Xiphophorus clemenciae) driven by sexual selection

The swordlike exaggerated caudal fin extensions of male swordtails are conspicuous traits that are selected for through female choice. Swords are one of only few examples where the hypothesis of a pre-existing bias is believed to apply for the evolution of a male trait. Previous laboratory experiments demonstrated that females prefer males with longer swords and even females from some swordless species show an affiliation for males of sworded species. Earlier phylogenetic studies based on maternally inherited mitochondrial DNA placed the sworded southern swordtail Xiphophorus clemenciae with swordless platies, contradicting its morphology-based evolutionary affinities. The analyses of new nuclear DNA markers now recover its traditional phylogenetic placement with other southern swordtails, suggesting that this species was formed by an ancient hybridization event. We propose that sexual selection through female choice was the likely process of hybrid speciation, by mating of platy females with males of an ancestral swordtail lineage. In artificial crosses of descendent species from the two potential ancestral lineages of X. clemenciae the hybrid and backcross males have swords of intermediate lengths. Additionally, mate choice experiments demonstrate that hybrid females prefer sworded males. These experimental lines of evidence make hybridization through xeno-specific sexual selection by female choice the likely mechanism of speciation.     

Male versus female mate choice: sexual selection and the evolution of species recognition via reinforcement

Male mate choice, expressed through courtship preferences, sometime occurs even under the mating system of polygyny, when the operational sex ratio is skewed toward males. The conditions under which male mate choice may be expected during polygyny are not well established. Servedio and Lande (2006, Evolution 60:674-685), assuming strict polygyny where all females have equal mating success, show that when having a preference does not increase the amount of energy that a male can put into courtship, male preferences for "arbitrary" female ornaments should not be expected to evolve; direct selection acts against them because they place males that carry them into situations in which there is high competition for mates. Here I explore in detail two situations under which logic dictates that this effect may be overcome or reversed. First I determine the contributions that direct and indirect selection place on male versus female preferences for traits that increase viability, using notation that allows the exact expression of these measures of selection. I find that direct selection against male preferences still predominates in the male mate choice model, causing less evolution by male than female preferences under these conditions. Second I address whether male mate choice is likely to evolve as a mechanism of premating isolation leading to species recognition, driven by the process of reinforcement. Reinforcement is compared under male and female mate choice, using a variety of models analyzed by both analytical techniques assuming weak selection and numerical techniques under broader selective conditions. I demonstrate that although under many conditions stronger premating isolation evolves under female mate choice, reinforcement may indeed occur via male mate choice alone.     

Patterns of variation in the acoustic calling signals of Chloriona planthoppers (Homoptera: Delphacidae) coexisting on the common reed Phragmites australis

Populations of the Tour species of Chloriona commonly found in the Netherlands C dorsata, C. glaucescens, C. smaragdula and C. vasconica –were cultured in the laboratory on Phragmites australis, their exclusive host plant in the field. The low frequency substrate-transmitted signals produced during the calling phase of mating behaviour were digitally analysed for the males and females of each species. Variables selected to cover most aspects of the call were measured, and variation was quantified within species and statistically tested among species using univariate and multivariate techniques. The calls of the males were more complex in structure than those of the females, with two different phase patterns recognized, and were species-specific. C. glaucescens and C vasconica were also separated on variables of the female call, but C dorsata and C. smaragdula completely overlapped. No significant differences were found between the calls of the long-winged and.short-winged female morphs of C. smaragdula. A possible role for variation in calling signals, and responses to them, in the evolution and maintenance of reproductive isolation in Chlonona is discussed.     

Mate recognition in a freshwater fish: geographical distance, genetic differentiation, and variation in female preference for local over foreign males

It often is assumed that more distant allopatry should reflect reduced rates of contemporary gene flow and/or greater divergence in mate recognition systems. This assumption, however, is rarely tested and may not always be appropriate. Here we investigated female preference for local and foreign males in a morphologically variable Australian freshwater fish, the Pacific blue-eye Pseudomugil signifer. Using a multidisciplinary approach that combined molecular phylogeography with conventional mate choice experiments, we found female blue-eyes spent more time in association with local males only when the alternative was a foreigner from a geographically and genetically more distant population. When offered the choice between two foreign males, females associated more with males from the population that was more closely adjacent to their own. Our results suggest that female preference for local over foreign males in blue-eyes may depend on how genetically and geographically separated populations are from one another.     

Character displacement from the receiver's perspective: species and mate recognition despite convergent signals in suboscine birds

Many social animals use long-distance signals to attract mates and defend territories. They face the twin challenges of discriminating between species to identify conspecific mates, and between individuals to recognize collaborators and competitors. It is therefore often assumed that long-distance signals are under strong selection for species-specificity and individual distinctiveness, and that this will drive character displacement when closely related species meet, particularly in noisy environments. However, the occurrence of signal stereotypy and convergence in rainforest species seems to contradict these ideas, and raises the question of whether receivers in these systems can recognize species or individuals by long-distance signals alone. Here, we test for acoustically mediated recognition in two sympatric antbird species that are known to have convergent songs. We show that male songs are stereotyped yet individually distinctive, and we use playback experiments to demonstrate that females can discriminate not only between conspecific and heterospecific males, but between mates and strangers. These findings provide clear evidence that stereotypy and convergence in male signals can be accommodated by fine tuning of perceptual abilities in female receivers, suggesting that the evolutionary forces driving divergent character displacement in animal signals are weaker than is typically assumed.     

Variation in courtship signals and hybridization between geographically definable populations of the rice Brown planthopper, Nilaparvata lugens (Stål)

Populations of Nilaparvata lugens from 18 geographically defined and widely distributed regions in Asia and Australasia were maintained in the laboratory on growing rice plants. Crosses between some of these showed varying degrees of success in hybridization. Those between insects from Australia and the Solomon Islands had the lowest success rates, but in successful individual crosses there was little evidence of hybrid inviability. Behavioural barriers in the form of substrate transmitted courtship signals appeared to be primarily responsible for low success in hybridization. Pulse repetition frequencies of male calls were distinctive for different populations: those from Australia and the Solomon Islands showed the greatest difference. Divergence in mate recognition signals (pre-mating ethological isolating mechanisms) has apparently evolved in advance of general genetic incompatibilities (post-mating isolating mechanisms) in this species.     

Geographic variation in male ornamentation and female mate preference in the house finch: a comparative test of models of sexual selection

I compared the mate preferences of female house finches (Carpodacusmexicanus) from populations in which males differ in both plumagecoloration and the extent of ventral pigmentation (patch size).Phylogenetic analysis indicated that the small patch of ventralcoloration displayed by males in some populations is derivedfrom a larger-patched ancestral state. Regardless of the appearanceof males in their own populations, however, females from allpopulations showed a preference for the most brightly coloredmales and males with the largest patches. A reduction in patchsize independent of a change in female mate preference is notconsistent with sensory-bias or reproductive isolation modelsof sexual selection or with general predictions of runaway modelsof sexual selection. In contrast, a lack of congruence betweenfemale mate preference and male trait expression is predictedby the honest advertisement model, with house finches respondingto variation in regional and local access to carotenoid plumagepigments.     

Mate choice and the genetic basis for colour variation in a polymorphic dart frog: inferences from a wild pedigree

Understanding how reproductive barriers evolve during speciation remains an important question in evolution. Divergence in mating preferences may be a common first step in this process. The striking colour pattern diversity of strawberry dart frog (Dendrobates pumilio) populations has likely been shaped by sexual selection. Previous laboratory studies have shown that females attend to male coloration and prefer to court with males of their own colour, suggesting that divergent morphs may be reproductively isolated. To test this hypothesis, we used molecular data to estimate pedigree relationships from a polymorphic population. Whereas in the laboratory both red and yellow females preferred to court with males of their own phenotype, our pedigree shows a pattern of assortative mating only for red females. In the wild, yellow females appear to be less choosy about their mates, perhaps because they incur higher costs associated with searching than females of the more common red phenotype. We also used our pedigree to investigate the genetic basis for colour-pattern variation. The phenotype frequencies we observed were consistent with those expected if dorsal background coloration is controlled by a single locus, with complete dominance of red over yellow. Our results not only help clarify the role of sexual selection in reducing gene flow, but also shed light on the mechanisms underlying colour-pattern variation among sympatric colour morphs. The difference we observed between mating preferences measured under laboratory conditions and the pattern of mate choice observed in the wild highlight the importance of field studies for understanding behavioural reproductive isolation.     

Female mate preference explains countergradient variation in the sexual coloration of guppies (Poecilia reticulata)

We tested the hypothesis that mate choice is responsible for countergradient variation in the sexual coloration of Trinidadian guppies (Poecilia reticulata). The nature of the countergradient pattern is that geographical variation in the carotenoid content of the orange spots of males is counterbalanced by genetic variation in drosopterin production, resulting in a relatively uniform pigment ratio. A female hue preference could produce this pattern, because hue is the axis of colour variation most directly affected by the pigment ratio. To test this hypothesis, we crossed two populations differing in drosopterin production and produced an F(2) generation with variable drosopterin levels. When the carotenoid content of the orange spots was held constant, female guppies preferred males with intermediate drosopterin levels. This shows that females do not simply prefer males with greater orange spot pigment content; instead, the ratio of the pigments also affects male attractiveness. To our knowledge, this is the first direct evidence for a hypothesized agent of countergradient sexual selection.     

Effects of breeding site density on competition and sexual selection in the European lobster

The availability of breeding sites has been predicted to affectthe intensity of sexual selection, including mate competition,mate choice and ultimately, variation in mating success. Wetested the hypothesis that reduced density of shelters wouldcause an increase in the intensity of sexual selection in Europeanlobsters, Homarus gammarus. However, we found little supportfor our predictions. For example, within-sex competition bymales and by females was not more intense when shelters werescarce. Indeed, females attempted to evict one another fromshelters significantly more often when shelters were common.When shelters were abundant, shelter-holding males had greatermating success than males without shelters, yet females didnot show more interest towards these males during courtshipencounters. Mate attraction was more strongly related to largemale body size when shelters were scarce. Overall, the resultssuggest that reduced shelter density does not lead to more overtwithin-sex aggression in this species. Instead, we suggest thatimpacts of breeding resource availability on sexual selectionmay depend on the range over which resources are measured, withextreme scarcity of shelters rendering overt competition uneconomical.Furthermore, females may become more selective of male traitssuch as large size, which enhance male control of breeding sitesand hence protection of females.     

The influence of environmental quality on sexual selection in Nauphoeta cinerea (Dictyoptera: Blaberidae)

We examined the impact of environmental conditions on the sexpheromone and mating behavior of the cockroach, Nauphoeta cinerea.Previous research on this species has shown that female behaviorduring courtship reflects female mate choice, male behaviorcorrelates with male social status, and the male sex pheromoneis the character used by females to assess males. In the presentstudy, males and females were allowed to develop from adultemergence to sexual maturity in either a high- or low-qualityenvironment. The environment affected the quantities of sexpheromone components. We found significantly less 3-hydroxy-2-butanoneand 4-ethyl-2-methoxyphenol, but not 2-methylthiazolidine, inthe pheromone glands of males from a poor environment. Pheromonequality was also affected; the ratios involving 2-methylthiazolidinewere altered, while the ratio 3-hydroxy-2-butanone to 4-ethyl-2-methoxyphenoldid not change. Development to sexual maturity under these environmentalconditions also influenced male and female sexual behavior.Male courtship activity reflected environmental influences;males from the low-quality environment took longer to initiatecourtship and spent more time copulating with females from allenvironments. Male quality, as assessed by females, was alsoaffected by their environment. Females were slower to respondto the courtship of males from the poor environment, regardlessof the females' own rearing environments. However, females fromthe low-quality environment also took longer to respond to thecourtship, and required more courtship, regardless of the males'rearing environments. Thus, poor environments also increasefemale choosiness. However, there was only one significant interactionterm, suggesting that the environmental effects are generaland that females do not show adaptive plasticity in mate choice.Studies of sexual selection that consider the effects of variableenvironments on behavior as well as the sexually selected morphologyin other systems are likely to provide new insights into thisevolutionary process