The unusual inheritance pattern of the courtship songs in closely related grasshopper species of the Chorthippus albomarginatus-group (Orthoptera: Gomphocerinae)

Sibling species of the Chorthippus albomarginatus-group are exceptional among all European grasshopper species because they have an extremely elaborate courtship behaviour. Here, we present a genetic analysis of the courtship song differences between two closely related grasshopper species Ch. albomarginatus and Ch. oschei. Measurements of seven courtship song characters and one parameter of courtship visual display were compared among parentals, F(1) and F(2) hybrids, and backcrosses. Variation in one morphological character, the number of stridulatory pegs, was studied as well. The distributions of song phenotypes were more consistent with a type-III genetic architecture, which involves complementary or duplicate pairs of loci. We suggest that this type of architecture may provide the novel elements of the hybrid songs, even more complex than the parental ones, which may offer a new starting point for sexual selection. One parameter of the visual display appeared to be controlled by a single locus, but differences in other characters of visual display could be explained by a more complex inheritance pattern. Inheritance of the peg number differences was consistent with a simple polygenic additive model. Sex linkage and/or maternal effects were not detected for any trait. We found a disproportionate influence of the Ch. albomarginatus parent on most hybrid song characteristics and the visual display, and this may support a previously suggested idea that Ch. albomarginatus is an ancestral form and Ch. oschei is a derived species.     

Recognition of sex in the acoustic communication of the grasshopper Chorthippus biguttulus (Orthoptera, Acrididae)

Many gomphocerine grasshoppers communicate acoustically: a male's calling song is answered by a female which is approached phonotactically by the male. Signals and recognition mechanisms were investigated in Chorthippus biguttulus with regard to the cues which allow sex discrimination. (1) The stridulatory files on the hindfemur of both sexes are homologous in that they are derived from the same row of bristles, but convergent with respect to the “pegs”. In males the pegs are derived from the bristles, and in females from the wall of the bristle's cup. (2) Male and female songs are generated by similar, probably homologous motor programs, but differ in the duration, intensity, “gappyness” of syllables, risetime of pulses, and the frequency spectra. The hindleg co-ordination during stridulation and the resulting temporal song patterns are less variable in males than in females. (3) For both sexes, recognition of a mate's signal depends on species-specific syllable structure. For males it is essential that the female syllables consist of distinct short pulses, whereas females reject “gappy” syllables. Males strongly prefer “ramped” pulses, females respond to syllables irrespective of steeply or slowly rising ramps. Males react only to the low-frequency component, whereas females prefer spectra containing both, low and high frequency components. Accepted: 20 November 1996     

Selective phonotaxis in <Emphasis Type="Italic">Neoconocephalus nebrascensis</Emphasis> (Orthoptera: Tettigoniidae): call recognition at two temporal scales

The calls of many Orthopteran species are comprised of a simple trill of pulses, the temporal pattern of which is important for call recognition. Male Neoconocephalus nebrascensis produce pulses with a temporal structure typical for the genus. However, they modify this pattern by grouping their pulses into verses, thereby creating a higher order temporal structure. The importance of the pulse pattern and verse structure for call recognition in N. nebrascensis was determined using a walking compensator. Females required the conspecific pulse pattern for call recognition, responding only when the intervals between pulses were short or absent. Females also required the verse structure for call recognition, and recognized the verse structure only when the amplitude modulation depth between verses and pauses exceeded 18 dB. We discuss that the verse recognition mechanism is a derived trait adapted for pre-mating isolation. We hypothesize that the unusually large amplitude modulation required for verse recognition forces males to synchronize their calls in order to preserve an attractive pattern. Call synchrony appears to be the outcome of cooperation, rather than competition, in this species.     

Pharmacological Activation of Stridulation in the Grasshopper Chorthippus albomarginatus (Orthoptera: Gomphocerinae)

Calling and courtship stridulatory behavior of Chorthippus albomarginatus was induced by injections of acetylcholine agonists into the protocerebrum. Pharmacologically induced stridulation, in many parameters, was quite similar to the natural behavior. However, the order of the courtship element alternation was different from that of the natural song. In some cases the pharmacologically induced stridulation included only one or two courtship elements. Based on the exclusive stimulation of a particular element of courtship songs and similarity of its movement pattern with the calling song, both patterns of Ch. albomarginatus stridulation appear to be homologous. The results obtained on this species confirm the idea of a hierarchic organization of the central nervous control of stridulation in gomphocerine grasshoppers and indicate participation of certain protocerebrum structures in this control.     

Song recognition in the grasshopper Chorthippus biguttulus is not impaired by shortening song signals: implications for neuronal encoding

The communication signals in many grasshopper species are composed of multiple repetitions of highly stereotyped subunits, and thus provide redundancy. In a behavioural paradigm, we tested the ability of males of the grasshopper Chorthippus biguttulus to recognize shortened versions of the communication signals of conspecific females. Males reliably responded to a three-subunits signal (250 ms), i.e. to a signal of less than a quarter of the natural duration. This performance is remarkable in view of the substantial adaptation and the variability present in the spiking responses of auditory interneurones. These behavioural results will impose constraints for investigating possible encoding mechanisms used by the grasshoppers' auditory system. Accepted: 1 September 1998     

Environmental stress and fluctuating asymmetry in the grasshopper Chorthippus parallelus (Acrididae: Gomphocerinae)

We examined the effect of agrochemical pollution/fertilisation of the soil and climatic conditions at high altitudes on fluctuating asymmetry (FA) level in hindfemur and forewing lengths of the grasshopper Chorthippus parallelus (Zetterstedt). All traits and samples exhibited ideal FA. Forewings, which are less functionally significant in this species, generally exhibited greater FA than the more functionally significant femora. Forewing FA was also more affected by stress induced by climatic conditions at high altitudes than femur FA. The high altitude sample was the most asymmetric sample for both traits, followed by the two control samples. The two samples from agrochemically contaminated habitats showed the lowest asymmetry in femora and forewings of C. parallelus. These results suggest that high altitude imposes more stress on this grasshopper species than does agrochemical contamination although the stress response was more dramatic in the less functional trait.     

Regeneration of axotomized tympanal nerve fibres in the adult grasshopper Chorthippus biguttulus (L.) (Orthoptera: Acrididae) correlates with regaining the localization ability

Adult males of the grasshopper Chorthippus biguttulus exhibit a stereotyped turning behaviour towards the direction of the female song. This behaviour has been used to study the regeneration of synaptic connections used for pattern recognition and sound localization. Unilaterally deafened animals are not able to localize the sound direction and turn exclusively towards the intact side, regardless of the speaker position. This behaviour does not change with postoperative time. After an axotomy of the tympanic nerve fibres the sensory axons regrow and regenerate their synaptic contacts which is deduced from the recovery of the ability to localize sound. The behavioural threshold for stimulation from the operated side is increased by approx. 5 dB SPL. The probability of correct turning towards the operated side increases with postoperative time. The ability for lateralization improves with postoperative time and may reach values of intact animals (discrimination of 1–2 dB SPL difference. Animals with two operations (axotomy of one tympanal nerve and blocking of the other ear) do not react to the female song, which suggests that recognition of the species-specific song pattern is not possible with the regenerated fibres and their synaptic connections alone. Neuroanatomical studies show that the regeneration of localization ability is correlated with an ingrowth of sensory fibres into the frontal auditory neuropil of the metathoracic ganglion.     

Prevalence of a Nosema sp. (Microsporida: Nosematidae) in Natural Populations of Chorthippus brunneus (Orthoptera: Gomphocerinae) in Southern Spain

A microsporidium Nosema sp. was isolated from the grasshopper Chorthippus brunneus (Thunberg) collected in Spain. Infected individuals were found throughout most of the season when nymphs and adults of C. brunneus were present (March-June). Other acridoids collected in the same area were not found to be infected, indicating that this microsporidium is probably host specific. Attempts to artificially infect Dociostarus maroccanus (Thunberg) nymphs failed.     

Inheritance of song and stridulatory peg number divergence between Chorthippus brunneus and C. jacobsi, two naturally hybridizing grasshopper species (Orthoptera: Acrididae)

Knowledge of the genetic basis of divergence in mating signal characters that contribute to reproductive isolation is critical to understanding speciation. Here, we describe a semi-automated system for characterizing grasshopper acoustic signals. We used this system to study the genetic basis of divergence in three male calling song components [echeme (EL), syllable (SL) and phrase (PL) lengths] between Chorthippus brunneus and C. jacobsi, two species of grasshoppers that hybridize in northern Spain. We also studied the number of pegs in the stridulatory file. For all characters, additive effects accounted for most of the genetic differentiation between species. However, the three song components also showed small but significant epistatic effects. No sex linkage was detected. Wright-Castle-Lande estimates of the minimum numbers of genetic factors underlying song and peg number divergence were low: peg number (n(e)=5.87+/-5.84), SL (n(e)=2.37+/-4.79) and PL (n(e)=0.87+/-0.86). On the other hand, EL appeared to be controlled by many genes. These results suggest that divergence in SL and PL might be driven by sexual selection whereas EL might not be under selection. This is consistent with experimental results on female song preference in related species. However, the fact that few factors appear to underlie the differences in peg number is surprising. Peg number is not closely related to song characteristics. It often varies between closely related grasshopper species and it has been assumed to be a neutral character. The biometrical approaches used here tend to underestimate the number of factors influencing a trait but provide valuable background for subsequent quantitative trait loci analyses.     

Oviposition date and pattern of embryogenesis in the grasshopper Chorthippus brunneus (Orthoptera, Acrididae)

The morphological development of eggs of the grasshopper Chorthippus brunneus , laid during successive periods through a single season, was closely monitored in the field at a site in NW England. Eggs within the same clutch developed synchronously. However, there were significant differences between clutches laid at the same time, reflecting spatial variation in microclimate (and possibly maternal effects and/or genetic differences). Eggs laid early in the season entered diapause and hatched relatively synchronously in the following spring. Eggs laid later failed to reach the diapause-stage before winter, but hatched, albeit late and with greater variance. Diapause was averted in these eggs. Plasticity in the nature of diapause has been documented from laboratory studies of many insect species (including C. brunneus ,) but this study directly relates the incidence of such developmental plasticity to oviposition date and seasonal temperature cycles in the field. The adaptive significance of diapause aversion and the evolutionary consequences for female reproductive behaviour are considered.     

Modification of the courtship song by visual stimuli in the grasshopper Gomphocerus rufus (Acrididae)

ABSTRACT. Males of Gomphocerus rufus L. perform a courtship song consisting of repetitive units, each of which is composed of three subunits (S1, S2, S3). S1 is characterized mainly by slow and fast head rolling; S2 and S3 are distinguished by different types of leg-stridulation. These movements and the associated sounds were recorded during presentation of visual stimuli, either linear displacement of a living female or optomotor stimuli generated by a striped drum. Females moved artificially through the binocular visual field of a courting male with a velocity of 1 cm/s or more are mounted by the male from any subunit S1, S2 or S3, although under natural conditions mounting occurs only from S2. Thus above a critical velocity the courtship programme can be modified. Rotation of a striped drum about the yaw axis of the male during the slow S1 induces asymmetrical leg position, following movements of the head, and prolongation of S1. During S2 the male is especially sensitive to optomotor stimuli and responds with marked changes in body position. In S3 the intensity of the song is reduced, and its duration shortened. Fast drum movements interrupt the courtship programme. Rotation of the drum about the roll axis elicits optomotor head turning that interferes with the head rolling of S1. The fast phase of S1 and the frequency of head-rolling during S1 cannot be modified by optomotor stimulation. The results can be interpreted by assuming certain interactions between three central nervous elements: a calling-song generator, a head-rolling generator, and an optomotor centre.     

Assortative preferences and discrimination by females against hybrid male song in the grasshoppers Chorthippus brunneus and Chorthippus jacobsi (Orthoptera: Acrididae)

The grasshoppers Chorthippus brunneus and Chorthippus jacobsi are highly differentiated for male mating signals, and form a mosaic hybrid zone in northern Spain. At some sites within this zone, many hybrids are observed. At others, few hybrids are observed. Such bimodal sites may reflect recent contacts between parental genotypes, or local variation in levels of assortative mating or selection against hybrids. Playback of 12 parental and F1 male songs to 296 parental and hybrid females revealed positive assortative preferences in C. brunneus and C. jacobsi females, supporting a direct role of male mating signals in female choice. However, all female genotypic classes showed reduced responsiveness to F1 male songs. Such sexual selection against hybrids is consistent with the narrow cline width observed in the field for song characters relative to morphology. These results have implications for the genetic structure of the hybrid zone and for models of speciation by reinforcement.     

Effects of forest-dune ecotone management on the endangered heath grasshopper, Chorthippus vagans (Orthoptera: Acrididae)

Dry, oligotrophic ecosystems are highly threatened in Europe due to massive changes in land use and eutrophication. The conservation of these xeric habitats has received much attention, whereas the ecotones between xeric habitats and other habitat types are often disregarded. One species which mainly inhabits the transition zone between pine forests and adjacent xeric habitats is the heath grasshopper, Chorthippus vagans. This species is endangered in large parts of Europe. One of the largest populations in northern Germany is found on a degraded inland dune near Hanover. This population is threatened by dense growth of deciduous trees and litter accumulation. We analyzed changes in the distribution of this population after the implementation of conservation measures (thinning out the forest and removal of leaf litter). Moreover, we examined dispersal distances of the species in order to assess its colonization potential. We also studied the microhabitat preferences of C. vagans to assess key factors influencing its local distribution. Our data show a substantial growth in population size, which might be a consequence of the conservation measures. New patches on the dune were colonized, promoting dispersal between the subpopulations. We propose that restoration of forest-dune ecotones should be considered more often in landscape planning and conservation management.     

Chirp rate variability in male song ofEphippigerida taeniata (Orthoptera: Ensifera)

Variability in the chirp rate of the male song of the ephippigerine speciesEphippigerida taeniata during intraspecific communication was investigated in the laboratory. Conspecific chirps were used as auditory stimuli. The stimulus rate was controlled by computer. Experiments were carried out at 19, 27, and 35°C. Acoustically isolated males ofE. taeniata sang with a relatively constant chirp rate, which depended on the ambient temperature. Chirp rate significantly increased with rising temperature from 19 to 27°C, whereas at 35°C the chirp rate did not differ significantly from that at 27°C. Male chirp rates were affected by stimulus rates. Males significantly increased their chirp rate in response to increasing stimulus rates at temperatures of 19 and 27°C. At 35°C the increase in the chirp rate was not significant. At 27 and 35°C males sang with a higher chirp rate than the stimulus rate within a certain range. Evaluating stimulus and response chirp pattern when the males increased their chirp rate in response to the stimulus rate showed that an alternation pattern was established. More than 50% of the male chirps occurred at a characteristic time range at around 40% of the interstimulus interval, which was slightly affected by temperature.     

Divergence in cuticular hydrocarbons between parapatric subspecies of the meadow grasshopper, Chorthippus parallelus (Orthoptera, Acrididae)

Two subspecies of the grasshopper Chorthippus parallelus (Orthoptera; Acrididae) meet and form a hybrid zone in the Pyrenees. Here we investigate whether the subspecies differ in the blend of cuticular hydrocarbons. Such differences may function as chemical signals, being detected via contact chemoreception and used during mate choice as in other insects. Gas chromatography shows that the subspecies have diverged in the composition of their cuticular hydrocarbons. This difference between subspecies exists over and above variation in cuticular hydrocarbons among individual populations. The majority of the difference can be attributed to the relative amounts of a small group of compounds present in the cuticle. The possible consequences of this chemical divergence for mating encounters between the subspecies are discussed.     

Super-normal length song preferences of female zebra finches (Taeniopygia guttata) and a theory of the evolution of bird song

Two way choice tests show a preference of female zebra finches for male songs four standard deviations longer than normal song. Further tests show the ontogeny of this preference to parallel song learning in general as well as a preference for songs with entirely heterogeneous notes compared to songs with four note repeats. These findings are discussed in relation to a theory of the evolution of bird song from bird calls due to female preferences for longer, more complex vocalizations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Calling song and selective phonotaxis in the field crickets,Gryllus firmus andG. pennsylvanicus (Orthoptera: Gryllidae)

The field cricket species, Gryllus firmusand G. pennsylvanicus,occur in a mosaic hybrid zone that roughly parallels the eastern slope of the Appalachian mountains in the northeastern United States. It is important to know what role, if any, the calling song plays in mate choice in sympatric and allopatric populations. In this report, we present results on the variability of calling song properties along transects across this hybrid zone. We also present the results of experiments on phonotactic selectivity of females from an allopatric population of G. firmus.The male calling song of allopatric G. firmuswas significantly slower in temporal rhythm (i. e., chirp and pulse repetition rates) and lower in pitch (i.e., dominant frequency) than that of allopatric G. pennsylvanicus.Calling song properties of males recorded in the hybrid zone varied considerably in temporal and spectral properties. In two-stimulus (choice) phonotaxis experiments, allopatric females of G. firmuspreferred synthetic calling songs with conspecific pulse repetition rates over songs that had lower and higher pulse rates. This preference persisted even when the sound pressure levels of alternative stimuli were unequal. Therefore, allopatric females of G. firmuscan discriminate between conspecific and heterospecific calling songs. Whether or not this same selectivity is present in sympatric populations remains unclear. Investigations of phonotactic selectivity in other allopatric and sympatric populations of both species are currently under way.