Behavioral sterility of hybrid males in acoustically communicating grasshoppers (Acrididae,Gomphocerinae)

The effectiveness of hybridization barriers determines whether two species remain reproductively isolated when their populations come into contact. We investigated acoustic mating signals and associated leg movements responsible for song creation of hybrids between the grasshopper species Chorthippus biguttulus and C. brunneus to study whether and how songs of male hybrids contribute to reproductive isolation between these sympatrically occurring species. Songs of F1, F2, and backcross hybrids were intermediate between those of both parental species in terms phrase number and duration. In contrast, species-specific syllable structure within phrases was largely lost in hybrids and was produced, if at all, in an irregular and imperfect manner. These divergences in inheritance of different song parameters are likely the result of incompatibility of neuronal networks that control stridulatory leg movements in hybrids. It is highly probable that songs of hybrid males are unattractive to females of either parental species because they are intermediate in terms of phrase duration and lack a clear syllable structure. Males of various hybrid types (F1, F2, and backcrosses) are behaviorally sterile because their songs fail to attract mates.     

Behavioral responses of hybrid lacewings (Neuroptera: Chrysopidae) to courtship songs

The study of hybrid courtship songs and the behavioral responses of hybrids and parental individuals to hybrid songs can be useful in understanding the origin of reproductive isolation among species that differ mainly in their courtship songs. Here we test the hypotheses (a) that hybrid lacewings prefer hybrid songs to either of the parental songs from a cross betweenChrysoperla plorabunda andC. johnsoni, and (b) that parental individuals prefer their own song over those of hybrids. Analysis of songs showed that most features of hybrid songs are intermediate between the two parents. Hybrids organize their songs with a series of simple volleys like those ofC. plorabunda. Female hybrids from two reciprocal crosses and females and males of the parental species were presented with choices of hybrid and parental songs. Hybrids responded more to recordings of hybrid songs than to recordings ofC. plorabunda but did not differ in the responses given toC. johnsoni and hybrid songs. In contrast, males and females of both parental lines preferred to duet with recordings of their own song types and did not respond to hybrid songs. Our results demonstrate that hybrids would be at a disadvantage in nature, because neitherC. plorabunda norC. johnsoni will respond to their songs.     

Behavioural mechanisms of sexual isolation involving multiple modalities and their inheritance

Speciation research dissects the genetics and evolution of reproductive barriers between parental species. Hybrids are the “gatekeepers” of gene flow, so it is also important to understand the behavioural mechanisms and genetics of any potential isolation from their parental species. We tested the role of multiple behavioural barriers in reproductive isolation among closely related field crickets and their hybrids (Teleogryllus oceanicus and Teleogryllus commodus). These species hybridize in the laboratory, but the behaviour of hybrids is unusual and there is little evidence for gene flow in the wild. We found that heterospecific pairs exhibited reduced rates of courtship behaviour due to discrimination by both sexes, and that this behavioural isolation was symmetrical. However, hybrids were not sexually selected against and exhibited high rates of courtship behaviour even though hybrid females are sterile. Using reciprocal hybrid crosses, we characterized patterns of interspecific divergence and inheritance in key sexual traits that might underlie the mating patterns we found: calling song, courtship song and cuticular hydrocarbons (CHCs). Song traits exhibited both sex linkage and transgressive segregation, whereas CHCs exhibited only the latter. Calculations of the strength of isolation exerted by these sexual traits suggest that close‐range signals are as important as long‐distance signals in contributing to interspecific sexual isolation. The surprisingly weak mating barriers observed between hybrids and parental species highlight the need to examine reproductive isolating mechanisms and their genetic bases across different potential stages of introgressive hybridization.     

Hybrid Identification in Clivia(Amaryllidaceae) using Chromosome Banding and Genomic In Situ Hybridization

Giemsa C-banding and genomic in situ hybridization (GISH) wereused to identify parental genomes in hybrids of Clivia(Amaryllidaceae).Of the three groups reputed to be hybrids, onlyC. cyrtanthiflorawas shown to be of hybrid origin. The ‘German hybrids’and ‘Belgian hybrids’ were both shown to be karyotypicallyand genomically similar to C. miniata, and are either selectionsor intraspecific hybrids of that species. Successful genomedifferentiation in F₁hybrids by GISH required high stringencyand high ratios of blocking DNA to probe. The spatial dispositionof different genomes with C-band or GISH markers in the hybridswas investigated in two dimensions on the spread. In five artificiallyproduced hybrids, either C-banding or GISH was used to locatethe position of parental genomes in mitotic metaphase cells.In all cases there was a significant tendency for centromeresof the different parental genomes to occupy two distinct concentricdomains on the metaphase plate. The presence or absence of centromericheterochromatin was not correlated with genome disposition.Results show that chromosome analyses can be a useful way ofidentifying Clivia hybrids in their vegetative phase. Copyright2001 Annals of Botany Company Clivia, genomic in situ hybridization, cultivar origin, parental genome separation     

Distribution of Phoxinus eos,Phoxinus neogaeus,and Their Asexually-Reproducing Hybrids (Pisces: Cyprinidae) in Algonquin Provincial Park,Ontario

Hybrid Phoxinus are one of the few asexually reproducing vertebrates species. The distribution of hybrid Phoxinus among lakes in Algonquin Park, Ontario, was evaluated relative to the distribution of parental species and relative to physiochemical lake characteristics. No association between the distribution of hybrids and the distribution of parental species was found, suggesting that the hybrids can successfully coexist with either parental species. In addition, we found no association between hybrid distribution and the physiochemical characteristics of lakes, suggesting that the hybrids are generalists with respect to the ecological niches available in Algonquin Park. There was a difference between the physiochemical characteristics of lakes with and without the parental species P. neogaeus. The lakes containing P. neogaeus were lower elevation than the lakes containing the other parental species, P. eos. The difference in distribution between the parental species may therefore be due to different dispersal abilities, to later arrival following post-glacial dispersal, or to differences in ecology. These results suggest that asexual reproduction is a successful strategy for hybrid Phoxinus in Algonquin Park because these sperm-dependent asexual hybrids are able to survive and persist regardless of which parental species is present, and regardless of the physiochemical characteristics of their habitat.     

Hybrid vocalizations are effective within, but not outside, an avian hybrid zone

Secondary contact between closely related species can lead tohybridization. The fitness of hybrid individuals within andoutside the hybrid zone determines whether the hybrid zone expandsinto the ranges of the 2 parental species or remains a stable,geographically narrow area in between the allopatric rangesof the parental species. In birds, vocalizations play an importantrole in male–male competition and female mate choice andare often affected by hybridization. One of the factors thatwill influence male hybrid fitness is the ability to defenda territory against competitors by vocalizing. We tested theefficacy of territorial signals of hybrids of 2 dove species,Streptopelia vinacea and Streptopelia capicola, compared withthe vocalizations of the parental species. With playback experiments,we assessed the response to hybrid and the 2 parental speciesvocalizations in the hybrid zone and adjacent allopatric populationsof each species. In the hybrid zone, males did not respond differentlyto the 3 vocalization types. In both allopatric populations,however, males responded more to conspecific than to heterospecificsignals and the response strength to hybrid signals was intermediate.Therefore, in the allopatric populations, hybrid males may havea reduced success in defending territories. In male–maleinteractions in the hybrid zone, hybrids may not have a disadvantagecompared with males of the parental species. The ability todefend a territory against competitors may thus help maintaina stable hybrid zone in the area of overlap.     

Phenotypic variation and covariation indicate high evolvability of acoustic communication in crickets

Studying the genetic architecture of sexual traits provides insight into the rate and direction at which traits can respond to selection. Traits associated with few loci and limited genetic and phenotypic constraints tend to evolve at high rates typically observed for secondary sexual characters. Here, we examined the genetic architecture of song traits and female song preferences in the field crickets Gryllus rubens and Gryllus texensis. Song and preference data were collected from both species and interspecific F1 and F2 hybrids. We first analysed phenotypic variation to examine interspecific differentiation and trait distributions in parental and hybrid generations. Then, the relative contribution of additive and additive‐dominance variation was estimated. Finally, phenotypic variance–covariance ( P ) matrices were estimated to evaluate the multivariate phenotype available for selection. Song traits and preferences had unimodal trait distributions, and hybrid offspring were intermediate with respect to the parents. We uncovered additive and dominance variation in song traits and preferences. For two song traits, we found evidence for X‐linked inheritance. On the one hand, the observed genetic architecture does not suggest rapid divergence, although sex linkage may have allowed for somewhat higher evolutionary rates. On the other hand, P matrices revealed that multivariate variation in song traits aligned with major dimensions in song preferences, suggesting a strong selection response. We also found strong covariance between the main traits that are sexually selected and traits that are not directly selected by females, providing an explanation for the striking multivariate divergence in male calling songs despite limited divergence in female preferences.     

A narrow hybrid zone between the grasshoppers Stenobothrus clavatus and Stenobothrus rubicundus (Orthoptera: Gomphocerinae): female preferences for courtship songs

Two grasshopper species, Stenobothrus rubicundus and Stenobothrus clavatus, were previously shown to hybridize in a narrow contact zone on Mount Tomaros in northern Greece. The species are characterized by complex and completely different courtship songs. In the present study, we investigated female preferences for the courtship songs of S. rubicundus, S. clavatus and hybrids in playback experiments. Playback of the courtship songs revealed assortative preferences in females of the parental species: they significantly more often preferred the songs of conspecific males. Hybrid females showed a lower selectivity than parental females, responding somewhat equally eager to playback of the songs of S. clavatus, S. rubicundus, and natural hybrid song, although less actively to the F₁ hybrid song. The results suggest that hybrid males may lose to males of parental species, whereas hybrid females would even have an advantage over parental females. Comparison of responses of females from allopatric populations and Mount Tomaros to different song types shows no evidence for reinforcement. Asymmetry found in female preferences may have implications for the structure of the hybrid zone. © 2013 The Linnean Society of London     

Songs of hybrid gibbons (Hylobates lar × H. muelleri)

Two hybrid offspring, one male and one female, were produced by a Hylobates muelleri female mated to a Hylobates lar male at Micke Grove Zoo, Lodi, California. Songs of the hybrids were studied at adulthood and compared to the parental-type songs. The hybrid female song is uniquely different from either parental type. The hybrid male song resembles the male song of H. lar, but contains an element that may be unique to H. muelleri. The study demonstrates the utility of sound-spectographic analysis of hybrid vocalizations to further understanding of primate behavioral inheritance.     

A narrow hybrid zone between the grasshoppers Stenobothrus clavatus and Stenobothrus rubicundus: courtship song analysis

The closely related grasshopper species Stenobothrus rubicundus and Stenobothrus clavatus are known to hybridize in a very narrow contact zone on Mt. Tomaros in northern Greece. These species produce very different and complex courtship songs accompanied with visual display. We analyzed the courtship songs and underlying stridulatory movements of the hind legs in natural hybrids from Mount Tomaros. The two species were also hybridized in the laboratory and their songs were compared with the songs of the natural hybrids. Some hybrid songs were shown to have intermediate features between parental songs, whereas other hybrid songs comprised completely new elements. The clavatus‐like song elements were found to dominate in hybrid songs. These song features may influence the mating success of hybrid males in the contact zone. A comparison of hybrid songs with the song pattern of the north European S. rubicundus populations allowed us to suggest a scenario of S. rubicundus and S. clavatus origin. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Processes underlying complex patterns of song trait evolution in a Setophaga hybrid zone

During secondary contact between two species when hybrids are less fit than parents, mating signals are expected to diverge, while aggressive signals are expected to converge. If a single signal trait is used in both mating and aggression, then the dynamics between these two forces could influence the evolutionary trajectory of that trait. We studied such a situation in an avian hybrid zone between two Setophaga species, where birdsong is used in both mate attraction and territory defense. We hypothesized that song modules of the two species will show separate and distinct geographic patterns due to the influence of selective pressures for effective territorial aggression and for effective mate attraction. We conducted geographic cline analyses and playback experiments across this hybrid zone. We found an unexpected geographic pattern of asymmetric introgression of song rhythm, which may be explained by results of the playback experiments that suggest that differences in song rhythm serve a greater role in mate attraction than in territory defense. In contrast, differences in syllable morphology show little evidence of importance in mate attraction or territorial defense. Song features converge in the hybrid zone, yet patterns of trait change suggest that the song production modules may vary in their modes of development and inheritance. Syringeal motor gesturing, which gives rise to syllable morphology, shows a nonclinal mosaic pattern, suggesting that this trait may be predominantly learned. In contrast, respiratory patterning, which forms song rhythm, shows a clinal geographic transition, suggesting that this trait could be more innate. The results indicate that opposing forces act independently on song via distinct modules of the song production mechanism, driving complex patterns of song trait evolution.     

Behavioral reproductive isolation inDrosophila silvestris,D. heteroneura,and their F1 hybrids (Diptera: Drosophilidae)

We investigated the role that courtship and aggressive interactions may have for the maintenance of reproductive isolation betweenDrosophila silvestris andD. heteroneura. We examined the behavioral bases of reproductive isolation between the parental species and we examined the courtship success of each sex of both reciprocal F₁ hybrids when paired with the parental species. We found reduced copulation success among heterotypic parental pairs compared to homotypic pairs, which was primarily due to the lack of courtship initiation betweenD. silvestris males andD. heteroneura females. When hybrid males from both reciprocal crosses were paired with parental females their copulation successes were not significantly different from that of parental males. In contrast, hybrid females from both crosses had reduced copulation success withD. silvestris males, which in turn was primarily due to a reduced success of reaching later stages of courtship. The time spent in copulation by hybrid males was intermediate between the two parental males. We studied aggression by observing the interactions of males of heterotypic pairs, both between the parental species and between the hybrids and parental males. A lack of aggressive interactions betweenD. silvestris males andD. heteroneura males in addition to the lack of courtship suggests thatD. silvestris males do not respond toD. heteroneura individuals of either sex. Hybrid males were equally successful in winning fights with bothD. silvestris andD. heteroneura males. These results indicate that the behavioral isolation betweenD. silvestris andD. heteroneura may be largely a consequence of the earliest stages of interactions. The two species may differ either in activity levels or in morphological or chemical traits that are important for species and mate recognition. The relatively high copulation and aggressive success of hybrids indicates that sexual selection against hybrids alone is unlikely to be a sufficient force to reduce gene flow and maintain species distinctions.     

Mating signal variation and bimodality in a mosaic hybrid zone between Chorthippus grasshopper species

The grasshoppers Chorthippus brunneus and Chorthippus jacobsi are easily distinguished by male calling song and the number of stridulatory pegs on the hind femur, and form a mosaic hybrid zone in northern Spain. In this paper, we fit a two-dimensional cline to variation in male calling song characters, which are of particular interest as they are likely to be involved in mate choice by females. As with variation in peg number, local habitat makes only a small contribution in explaining deviations in mean song score from clinal expectations. However, the fitted width of the cline for song characters is significantly narrower than for peg number, suggesting that mating signals may be associated with reduced hybrid fitness in the field and that recombination rates are sufficient to allow clines for different characters to diverge in width. Despite this, estimates for the overall elevation in linkage disequilibrium at the zone center, based on covariance between peg and song characters, reveal a substantial overrepresentation of parental genotypes at the cline center relative to the expectations of a tension zone of similar width. Examination of covariance at individual sites reveals that this inflated estimate of linkage disequilibrium is caused by several sites where the distribution of phenotypes is effectively bimodal. This substantial variation in linkage disequilibrium at the cline center could result from local variation in the strength of assortative mating or selection against hybrids, or may reflect the long-distance colonization of empty habitat from outside the hybrid zone, which would continually create new contacts between parental genotypes at the cline center. Hybrid zones like this, in which strong linkage disequilibrium occurs in some situations but not in others, are of particular relevance to speciation research and allow investigation of the spread of combinations of alleles through different genetic and ecological backgrounds.     

Age-specific fitness components in hybrid females of Drosophila pseudoobscura and D. persimilis

Most models of hybridization assume that hybrids are less fit than their parental taxa. In contrast, some researchers have explored the possibility that hybrid individuals may actually have higher fitness and so play an important role in the generation of new species or adaptations. By estimating age-specific fitness components, we can determine not only how hybrid fitness differs from parental taxa, but also whether the fitness of hybrids relative to parental taxa changes with age. Here we describe an analysis of age-specific fitness traits in two species of Drosophila, D. pseudoobscura and D. persimilis, and their F1 hybrids. At early ages, hybrid females lay as many eggs as parental individuals, on average, but produce far fewer offspring. By late ages, in contrast, parental taxa show a steep decline in production not seen in hybrids, such that hybrids produce more offspring, on average, than parental taxa. Furthermore, egg-adult survival in hybrids is negatively correlated with egg density, whereas these traits are only weakly correlated in parental taxa. The results are limited somewhat by the fact that we analyze only two strains, and that these may be partially inbred. Nonetheless, the results are certainly illustrative, pointing out not only that at least some hybrid individuals may be as fit or fitter than parental taxa, but also that the difference between hybrids and parental taxa varies with age.     

In Situ Localization of Parental Genomes in a Wide Hybrid

In situ hybridization enabled DNA originating from the two parentalgenomes to be distinguished in plant hybrids. A probe of biotinylatedtotal genomic DNA from Secale africanum labelled the chromosomesof S. africanum origin but not those from Hordeum chilense inroot-tip chromosome spreads of the sexual hybrid between thetwo species. Hybridization of total genomic DNA from S. africanumto DNA on filters (dot blots) confirmed the distinction betweenDNA from Hordeum and Secale. The total genomic probe hybridizedto the whole length of the chromosomes from S. africanum remarkablyuniformly, labelling both euchromatin and heterochromatin, exceptat the centromeric region. The probe binding was visualizedas a yellow colour by the fluorescein-coupled detection systemwhich contrasted with the red fluorescing counterstain of theunlabelled chromatin. The chromosomes originating from bothparents could be seen and distinguished as red and yellow fluorescenceat all stages of the cell cycle. At interphase and prophase,the chromatin originating from the two parental genomes didnot mix. Chromosomes or groups of chromosomes occupied distinctdomains and also tended to be arranged in a Rabl configurationwith the centromeres clustered at one end of the nucleus. Wepropose calling the technique using total genomic DNA as a probe‘genomic in situ hybridization.’ Hordeum chilense, Secale africanum, hybrids, genomic in situ hybridization, DNA, repetitive sequences, chromosomes, chromosome disposition, nuclear order     

Nitrate Reductase Activity: A Biochemical Criterion of Hybrid Vigour in Sorghum bicolor (L.) Moench

F₁ hybrids of Sorghum bicolor (L.) Moench and their inbred parentswere analysed for NADH-nitrate reductase activity during theearly stages of seedling growth. In all the hybrids both mid-parentaland better parental heterosis were discernible in shoots whereasin roots two hybrids out of the three tested, showed heteroticlevels. It is suggested that in sorghum nitrate reductase activityduring seedling stages can be used as a biochemical criterionfor evaluating hybrid vigour. Sorghum bicolor (L.) Moench, sorghum, hybrid vigour, nitrate reductase     

Mating patterns of Darwin's Finch hybrids determined by song and morphology

Three species of Darwin's Finches hybridize on the Galapagos island of Daphne Major. We examined mating patterns to determine if hybrids exhibit mate preferences. Geospiza fortis x G. scandens F₁ hybrids backcrossed to both of the parental species, whereas all backcrosses bred with the parental species to which they were most related, or with hybrids. Paternal song was shown to be the crucial factor determining the mating pattern of G. fortis x G. scandens F₁ hybrids and their offspring. Song is culturally inherited, transmitted faithfully from father to son (with few exceptions) as a result of an imprinting-like process. Size also contributes to the choice of mates. G. fortis x G. scandens F₁ hybrid females paired with large G. scandens -like G. fortis males. G. fortis x G. fuliginosa F₁ hybrids paired negatively assortatively with respect to the size of their G. fortis mates. Non-random mating of hybrids based on song, a non-generic trait, has interesting evolutionary consequences. Song characteristics and nuclear and mitochondrial genes flow from G. fuliginosa into the G. fortis population, whereas the direction of transfer of genetic and song information between G. fortis and G. scandens depends on which song was sung by the father of the hybrids.     

Isozyme Evidence for Natural Hybridization in Phlomis (Lamiaceae): Hybrid Origin of the RareP.xmargaritae

Phlomisxmargaritae is a rare and sterile hybrid taxon foundin a single endangered population in southern Spain. It waspreviously described as a morphologically intermediate hybrid,putatively between Phlomis purpurea and P. composita. The presentstudy used allozymes as molecular markers to assess the hybrididentity ofP.xmargaritae . Ten putative loci from seven enzymesystems were resolved: five were monomorphic and fixed acrossall taxa studied and the rest (Aat-1, Aat-2, 6-Pgdh-2, Pgi andPgm) were polymorphic in at least one taxon. The two parentaltaxa are fixed for different alleles at 6-Pgdh-2 and show distinctallelic frequency differences for four other loci. Phlomisxmargaritaedisplays fixed heterozygous phenotypes for four of the fivepolymorphic loci, these being composed of combinations of thealleles found in the parental taxa. No unique alleles were detectedin P.xmargaritae. We conclude that this taxon is of hybrid originand confirm the identity of the parental taxa involved. It isfurther suggested that this population is composed of individualsthat are recent F₁hybrids that have not undergone backcrossingor introgression. Global conservation measures are necessaryfor the whole hybrid system in this location since further continuousassessment could reveal the evolutionary input of hybridizationin Phlomis.Copyright 2000 Annals of Botany Company Endangered species, genetic diversity, isozyme variation, hybrid species, Phlomisxmargaritae, plant conservation, population genetics, Lamiaceae, Spain.