Chromosomal differentiation through an Alpine hybrid zone in the grasshopper Chorthippus parallelus

The grasshopper Chorthippus parallelus is genetically differentiated over its range into at least five major geographical subdivisions. Two of these subdivisions, designated as subspecies, meet and form a hybrid zone in the Pyrenees. These subspecies differ for a wide range of morphological, behavioural and chromosomal characters, which vary clinally across the zone. A further geographical subdivision exists within Italy. Here we present the first evidence for chromosomal divergence between populations of the grasshopper north and south of the Alps. Across two Alpine cols (Col de Larche, between France and Italy; Passo de Resia, between Austria and Italy), these populations differ in both structural and functional components of the X-chromosome. Northern Alpine individuals possess an active nucleolar organizing region (NOR) at the distal end of the X-chromosome and an associated region of heterochromatin (C-band). Both these features are absent from individuals from the south of the Alps. However, all individuals examined carry distally located rDNA on the X-chromosome. Clinal transition was examined in the distal C-band in transects through the two cols. The cline centres are roughly coincident with the tops of the cols. Both clines were of similar widths (Col de Larche, 21.88 km; Passo de Resia, 24.05 km), and therefore much wider than those for an X-linked distal C-band in the Pyrenean hybrid zone. This suggests that there are different selective pressures on the cytogenetic characters in the Alps. The results are discussed in the context of the historical population dynamics of the species in relation to the climatic changes associated with the Pleistocene ice ages.     

The origins of premating reproductive isolation: testing hypotheses in the grasshopper Chorthippus parallelus

There are many proposed routes for the origin of premating reproductive isolation, but few systematic studies aimed at testing their relative importance. Accumulated information about the biogeographical history of the European meadow grasshopper, Chorthippus parallelus, has allowed us to make a planned series of comparisons among populations aimed at distinguishing the contributions of some of these hypotheses. We have compared the effects on assortative mating of long-term isolation in glacial refugia, founder events during postglacial colonization, and sympatry with a closely related species. A likelihood-based analysis allowed us to separate effects of variation in male and female mating propensity among populations from variation in mate choice leading to assortative mating. All three effects contributed significantly to the overall variation in mating pattern in a set of 21 pairwise comparisons among seven populations. Male cuticular composition, but not other candidate signals, was significantly associated with the level of assortative mating. Of the hypotheses for the origin of reproductive isolation, only the predictions of the founder hypothesis explained a significant amount of the variation in assortative mating. This does not rule out the possiblity that there may be some other explanation. Having established the pattern of divergence, it is possible to generate hypotheses that explain our results at least as well as the founder hypothesis. However, because many such post hoc hypotheses are possible, they cannot be tested with this dataset. On this basis, our results favor the hypothesis that some aspect of the colonization process tends to accelerate divergence in mating signals leading to premating reproductive isolation. This could be accomplished through any one of several mechanisms. Colonization involves many bottlenecks as new populations are established at the edge of the range by long-distance migrants. Genetic effects may be important, but these bottlenecks may also alter the conditions under which mates are found and chosen, as suggested by Kaneshiro. At the same time, the colonizing populations may encounter novel environmental challenges.     

Within- and between-individual sequence variation among ITS1 copies in the meadow grasshopper Chorthippus parallelus indicates frequent intrachromosomal gene conversion

Sequencing multiple copies of the ITS1 region revealed the coexistence of two or more haplotypes within the genome of Chorthippus parallelus. Using a PCR-RFLP approach, the ITS1 numbers and frequencies of haplotypes present in each of 40 individuals were investigated, revealing a consistent lack of homogeneity. For each individual, the level of intra-individual variation was estimated from a sample of 20 ITS1 copies. The level of differentiation in haplotype frequency among individuals was then estimated by maximum likelihood using models based on the Dirichlet distribution. This confirmed the existence of significant levels of variation among individuals within each population studied. The most likely turnover mechanism that could generate this pattern of variation is gene conversion, operating at the intrachromosomal level. Furthermore, the discovery of linkage disequilibrium among the ITS1 haplotypes of C. parallelus suggests that intrachromosomal gene conversion occurs more frequently than interchromosomal recombination. Subspecies of C. parallelus showed significantly different haplotype distributions following about 0.5 Myr of divergence. With respect to the process of concerted evolution, we show that homogenization of repeats is slow relative to speciation, and the standing variation among individuals is sufficient for selection to operate.     

Thermal biology of the meadow grasshopper, Chorthippus parallelus, and the implications for resistance to disease

Abstract.  1. The thermal biology of the meadow grasshopper, Chorthippus parallelus , a common, habitat generalist acridid species found in the U.K., was characterised and the influence of thermoregulatory behaviour for resistance against a temperate ( Beauveria bassiana ) and tropical ( Metarhizium anisopliae var. acridum ) fungal pathogen was determined.
2.  Chorthippus parallelus was found to be an active behavioural thermoregulator, with a preferred temperature range of 32–35 °C.
3. Both pathogens proved lethal to fifth instar and adult grasshoppers. No evidence of behavioural fever in response to infection by either pathogen was found, but normal thermoregulation was found to reduce virulence and spore production of B. bassiana. Normal thermoregulation did not appear to affect M. anisopliae var. acridum .
4. These results suggest that the effects of temperature on host resistance depend on the thermal sensitivity of the pathogen and, in this case, derive from direct effects of temperature on pathogen growth rather than indirect effects mediated by host immune response.
5. The implications for possible risks of exotic pathogens and influence of climate change are discussed.     

Assortative mating across a hybrid zone in Chorthippus parallelus (Orthoptera: Acrididae)

Two subspecies of the grasshopper Chorthippus parallelus meet and form a hybrid zone in the Pyrenees. Both strong hybrid dysfunction and a behavioural difference occur, which would seem to make the zone a suitable candidate for speciation by reinforcement. One of the classic ways of looking for this is to test for increased levels of assortative mating between populations from close to the region of contact. Here we show that, with virgin insects, such assortment decreases as one approaches the centre of the zone in C. parallelus. The pattern is different upon remating, with non-virgins showing a pattern more like that predicted by reinforcement. Overall there is little evidence for reinforcement. We argue that other tests of the model may be more appropriate for stable hybrid zones.     

Fitness consequences of potential assortative mating inside and outside a hybrid zone in Chorthippus parallelus (Orthoptera: Acrididae): implications for reinforcement and sexual selection theory

We have examined the fitness consequences of random and potentially non-random matings within two populations taken from inside, and two from outside a hybrid zone in Chorthippus parallelus. When given the opportunity to mate non-randomly, females from all populations laid egg pods more quickly than females obliged to mate at random. A range of fitness parameters measured on the offspring did not show increased fitness following potential non-random mating for any population. However, in non-hybrid populations, the sons of non-randomly mated females had about twice the mating success of the sons of those females forced to mate at random, suggesting the existence of heritable variation for male reproductive success. Hybrid dysfunction did not occur amongst the offspring of randomly mated hybrid females, demonstrating that the lack of dysfunction within these populations is not due to the evolution of assortative mating within them.     

Causation, fitness effects and morphology of macropterism in Chorthippus parallelus (Orthoptera: Acrididae)

ABSTRACT. 1. Chorthippus parallelus (Zett.) displays an apparent'flight polyphenism'despite the fact that macropterous individuals cannot fly.
2. Photoperiod is used as an environmental cue controlling the polyphenism: long day lengths increasing the proportion of macropterous adults.
3. Macropterous females start mating when older and are less fecund than brachypters.
4. In contrast to other studies the response may not be due to cues associated with a reduced environmental quality and we speculate that macropterism is not a dispersal mechanism in the Gomphocerinae.     

Testing for recombination in a short nuclear DNA sequence of the European meadow grasshopper, Chorthippus parallelus

Single-copy nuclear DNA sequences have high potential as a source of genetic markers for population analyses. However, the difficulties that arise when haplotypes that are the product of recombinational rearrangements are present require additional consideration. Two statistical methods for identifying potential recombinants by detecting anomalies in the distribution of variable sites along sequences were used to screen sequences from a single-copy nuclear DNA fragment, cpnl-1, of the European meadow grasshopper (Chorthippus parallelus). Five of the 71 haplotypes in the cpnl-1 data set showed nonrandom distribution of polymorphic sites using both methods. The second method pinpointed an additional four haplotypes. Estimates of the rate of recombination in the entire data set were obtained using standard methods. It is concluded that cpnl-1 haplotypes have been involved in recombination or gene conversion events at a rate more than twice the mutation rate. This confirms that recombination and gene conversion are significant factors in the generation of haplotype variation in nuclear gene sequences. The cpnl-1 haplotypes identified by the tests were present only in populations that have had recent contact; the Balkan and Turkish refugial populations and their post-glacial colonies to the north. This is discussed in relation to the phylogenetic inferences drawn from the same data in a previous report.     

Genetic divergence in the Chorthippus parallelus species group (Orthoptera: Acrididae)

Specimens of Chorthippus parallelus parallelus, C. p. erythropus, C. montanus, C. curtipennis and C. dorsatus were examined electrophoretically. This analysis indicates that C. curtipennis is very closely related to C. p. parallelus and C.p. erythropus (Nei's I = 0.97) while C. montanus is more distantly related ( I = 0.88). The values are consistent with divergence of C. p. erythropus from C. p. parallelus since its separation by the Pyrenean ice sheet during the Pleistocene and of C. curtipennis since invasion of North America by C. parallelus at the same time. A much earlier divergence of C. montanus is implied.     

Enzyme variation in the grasshopper Chorthippus brunneus (Thunberg)

Forty-two populations of the grasshopper Chorthippus brunneus from four different areas were scored for polymorphism at 15 enzyme loci. The areas sampled were Nottinghamshire, inland Essex, the Essex coast and Dovedale in Derbyshire. Five loci were found to be polymorphic (isocitrate dehydrogenase (2), malate dehydrogenase, glyceroI-3-phosphate dehydrogenase and phosphoglucomutase) but levels of heterozygosity were generally low. Differences in enzyme variation between different areas were demonstrated using multivariate analysis.     

The genetics of colour-patterns in the grasshopper Chorthippus brunneus

Several alleles were found to determine the colour of the dorsal pronotum in Chorthippus brunneus; there was evidence for at least two loci (C and V). Brown (C^B)was the universal recessive and green (C^C) was dominant to all other colours. The white allele (C^W)was codominant with green(C^G)and purple (C^P). Wing-patterns were determined by a separate, probably linked locus (W). A dominant plain wing-pattern (W^P) was associated with colours other than brown. Striped(W^S)and mottled(W^mo) were codominant and a plain recessive allele (W^P) was also found. All three alleles were associated with the brown phenotype. A purple-sided allele (S^Pu) was sometimes obmd with C^pu.. S^Pu was dominant to brown sides (S^B), A series of markings on the dorsal and lateral pronotum (linea intermedia, fascia postocularis, linea media, carina media and zona lateralis) were investigated and found to be controlled at separate loci which may be linked to W. These characters were expressed by dominant alleles. Epistatic effects by modifier loci were shown to have an important effect on the determination of wing phenotype. Allele Wo⁺, for example, suppressed the stripe-wing pattern, linea media, carina media and zona lateralis. It was concluded that colour patterns appear to be under genetic control and that dominant alleles were rare in the wild. Changes in shades of colours were shown to be age-dependent and minor.     

The genetics of colour-patterns in the grasshopper Chorthippus brunneus

Several alleles were found to determine the colour of the dorsal pronotum in Chorthippus brunneus; there was evidence for at least two loci ( C and V ). Brown ( C^B )was the universal recessive and green ( C^C ) was dominant to all other colours. The white allele ( C^W )was codominant with green( C^G )and purple ( C^P ). Wing-patterns were determined by a separate, probably linked locus ( W ). A dominant plain wing-pattern ( W^P ) was associated with colours other than brown. Striped( W^S )and mottled( W^mo ) were codominant and a plain recessive allele ( W^P ) was also found. All three alleles were associated with the brown phenotype. A purple-sided allele ( S^Pu ) was sometimes obmd with C^pu.. S ^Pu was dominant to brown sides ( S^B ), A series of markings on the dorsal and lateral pronotum ( linea intermedia, fascia postocularis, linea media, carina media and zona lateralis ) were investigated and found to be controlled at separate loci which may be linked to W . These characters were expressed by dominant alleles.
Epistatic effects by modifier loci were shown to have an important effect on the determination of wing phenotype. Allele Wo⁺ , for example, suppressed the stripe-wing pattern, linea media, carina media and zona lateralis .
It was concluded that colour patterns appear to be under genetic control and that dominant alleles were rare in the wild. Changes in shades of colours were shown to be age-dependent and minor.     

Postglacial population expansion drives the evolution of long-distance migration in a songbird

The evolution of long-distance migratory behavior from sedentary populations is a central problem in studies of animal migration. Three crucial issues that remain unresolved are: (1) the biotic and abiotic factors promoting evolution of migratory behavior, (2) the geographic origin of ancestral sedentary populations, and (3) the time scale over which migration evolves. We test the role of postglacial population expansions during the Quaternary in driving the evolution of songbird migration against prevailing views favoring the role of intraspecific competition. In contrast to previous attempts to investigate these questions using interspecific phylogenies, we adopt an intraspecific approach and examine the phylogeography of a North American songbird, the chipping sparrow (Spizella passerina), which exhibits both long-distance migratory behavior in temperate North America and sedentary behavior in Mexico and Central America. We show that migratory populations descend from sedentary populations in southern Mexico and that migration has evolved as a result of a northward population expansion into temperate North America since the last glacial maximum 18,000 years ago. Migration appears to have evolved rapidly in some species as populations colonized areas of high seasonality in the temperate zone. The phylogeography of the yellow-eyed junco (Junco phaeonotus), a strictly sedentary species, provides a null model supporting the view that northward range expansions were driven solely by environmental factors and not by a predisposition to evolve migratory behavior. These results provide the strongest evidence to date that historical climate patterns can drive the rapid evolution of avian migration in natural populations, and they suggest a general mechanism for the repeated evolution of migration within and across bird lineages.     

Testicular dysfunction in hybrids between parapatric subspecies of the grasshopper Chorthippus parallelus

Two subspecies of Chorthippus parallelus meet in a narrow hybrid zone in the Pyrenees. Laboratory Fl hybrids between the two races had atrophied testes and were virtually sterile. Backcross males had variable levels of dysfunction but were generally intermediate between Parental and Fl males; there were some reciprocal differences. No other effects on fertility and viability of males and females were apparent, and Fl males had normal accessory glands. These data provide another clear example of Haldane's rule and have implications for the structure of hybrid zones, the evolution of mating isolation and mating strategies.     

Limited condition dependence of male acoustic signals in the grasshopper Chorthippus biguttulus

In many animal species, male acoustic signals serve to attract a mate and therefore often play a major role for male mating success. Male body condition is likely to be correlated with male acoustic signal traits, which signal male quality and provide choosy females indirect benefits. Environmental factors such as food quantity or quality can influence male body condition and therefore possibly lead to condition-dependent changes in the attractiveness of acoustic signals. Here, we test whether stressing food plants influences acoustic signal traits of males via condition-dependent expression of these traits. We examined four male song characteristics, which are vital for mate choice in females of the grasshopper Chorthippus biguttulus. Only one of the examined acoustic traits, loudness, was significantly altered by changing body condition because of drought- and moisture-related stress of food plants. No condition dependence could be observed for syllable to pause ratio, gap duration within syllables, and onset accentuation. We suggest that food plant stress and therefore food plant quality led to shifts in loudness of male grasshopper songs via body condition changes. The other three examined acoustic traits of males do not reflect male body condition induced by food plant quality.     

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.     

Colour morph related performance in the meadow grasshopper Chorthippus parallelus (Orthoptera,Acrididae)

Abstract 1. Polymorphism has been described for a number of herbivorous insects, but little is known about whether differences in body colour cause fitness differences. In Chorthippus parallelus, three main colour morphs occur, namely brown, green, and dorsally striped. 2. The present study examined colour morph abundances and morph‐related differences in body size, oviposition rate, and offspring numbers in females of C. parallelus collected in 15 montane grasslands. The study also examined the effect of plant species richness, composition, community productivity, and solar radiation on colour morph frequency and fitness. 3. The relative frequencies of the three colour morphs was 31.7% (brown), 33.1% (green), and 35.2% (dorsally striped), but the morphs were not evenly distributed across the 15 sites. 4. There was no effect of the habitat variables on the distribution of the green and the striped morph in the study sites, however 80% of the variation in the abundances of the brown morph was explained by plant species richness and composition. 5. Grasshopper size was equal among the morphs. Brown females laid significantly more egg pods than the green and dorsally striped morphs. There were no significant differences in offspring numbers among the colour morphs. 6. Body colour in C. parallelus seems to be a fitness‐relevant trait, raising the question of the evolutionary maintenance of polymorphism.     

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.