Genetic variation of density responses in relation to wing polymorphism in the oriental chinch bug,Cavelerius saccharivorus Okajima (Heteroptera: Lygaeidae)

Responses to nymphal density in the determination of wing form were compared between the offspring from brachypter x brachypter crosses and those from macropter x macropter crosses of the oriental chinch bug, Cavelerius saccharivorus. In the offspring from crosses between macropters, there was a strong tendency for macropters to increase to a rather high level with increasing nymphal density in both sexes. In contrast to this, in the offspring from crosses between brachypters, the appearance rate of macropters attained a maximum value in moderately crowded conditions and conversely decreased to a lower level in more crowded conditions in both sexes. Thus density response patterns concerning the determination of wing form were quite different between the offspring from different crosses in the wing form, indicating that there is a genetic basis underlying wing polymorphism in this species. As for the body size of emerged adults, macropters tended to be larger than brachypters in the same crowded condition. Moreover, the rate of decrease of body size with nymphal density was lower in the offspring from crosses between macropters than in the offspring from crosses between brachypters. This indicated that the former offspring are more tolerant of nymphal crowding than the latter. The difference in such a tolerance against nymphal crowding between the offspring from different crosses was considered to be related to the difference in the appearance of macropters in the crowded conditions between them.     

Adaptive significance of wing dimorphism in the absence of dispersal: a comparative study of wing morphs in the waterstrider, Gerris remigis

ABSTRACT.

• 1 Gerris remigis Say (Hemiptera; Gerridae) is primarily apterous, but populations with up to 33% macropters have been reported. The macropters seldom fly, and field studies have revealed no detectable differences between wing morphs in movement or survival at any time of year.
• 2 In this paper, life history traits of macropterous and apterous G. remigis are compared in an attempt to determine the mechanisms responsible for the maintenance of macroptery in this species in spite of the very low flight capacity and infrequent flight of macropters.
• 3 Development time, proportion breeding without diapause, and overwinter survival do not differ between morphs. However, pre-diapause macropterous females have a significantly shorter pre-oviposition period than apterous females. In contrast, post-diapause macropters begin reproducing later than apters, and have a lower cumulative fecundity.
• 4 These results suggest that macropters may be at a selective advantage in warm habitats which favour pre-diapause reproduction, but that apters should be favoured in the preferred, cool, lotic habitats.
• 5 However, crossing and rearing experiments indicate that wing morphology is primarily environmentally controlled in this species, and that the heritability of wing morphology is low, at best. In light of this, the relative impacts of purely phenotypic (environmental) variation, random effects, and the observed fitness differences on the maintenance of macroptery in this species are discussed.

     

Ecological significance of the wing polymorphism of the oriental chinch bug,Cavelerius saccharivorus Okajima (Heteroptera: Lygaeidae)

The wing polymorphism of the oriental chinch bug, Cavelerius saccharivorus, was studied in relation to its reproductive strategy. The frequency distribution of wing form in terms of the relative wing length (R.W.L.) changed with increase in the population density from a single modality biased toward the brachypterous form to bimodality with both macropterous and more extreme brachypterous forms. Since some evidences in the field showed that fliers are limited to macropters, such a bimodality of wing form indicated that C. saccharivorus employs a mixed strategy with some portion of adults staying on in the same habitat, whereas the rest disperse. Moreover, macropters were larger in body size than brachypters emerging in high density populations, although smaller than brachypters emerging in low density populations. Larger body size as well as the delay of ovarian maturation in macropters was considered to be a beneficial trait for migratory behaviour.     

Comparative studies on the seasonal occurrence,nymphal development and food menu in two giant water bugs,Diplonychus japonicus vuillefroy andDiplonychus major esaki (Hemiptera: Belostomatidae)

We compared the seasonal changes in population density and nymphal development at different water temperatures and under different food conditions between two giant water bugs, Diplonychus japonicus and D. major, in Okayama, Japan. D. japonicus produced 1–2 generations a year, while D. major was strictly inivoltine. The developmental velocity was higher in D. japonicus than in D. major. The thermal constant of D. japonicus was less than that of D. major. These results suggest that D. japonicus is adapted, to higher water temperature than D. major. In the field, D. japonicus preyed predominately on Lymnaeidae and Physidae snails, while D. major preyed on aquatic insects such as dragonfly nymphs.     

Phenological studies on the wing dimorphism of a semi-aquatic bug,Microvelia douglasi (Heteroptera: Veliidae)

Seasonal changes in the percentage of macropterous individuals in natural population of Microvelia douglasi in permanent habitats were investigated in Shimane, western part of Japan. Macropterous forms of 5th instar nymphs occurred most frequently from late June to early July. However, the seasonal appearance of macropterous adults differed between habitats and years. There were significant effects of density, temperature, photoperiod and food availability on the percentage of macropters under laboratory conditions. The percentage of macropterous forms generally increased with density, but the mode of the response was modified by temperature, photoperiod and food availability. When insects were reared under long day photperiod (16L8D) with abundant food (0.5–1.0 g/container/day), the percentage of macroptery varied most among the different density treatments at the intermediate temperature (24°C) and the difference was reduced at higher temperature (28° and 32°C). Effects of temperature were small under the short day photoperiod (12L12D). Under 12L12D photoperiod sensitivity of density responses were higher at 32°C and lower at 24°C than those under 15L8D photoperiod. When the insects were supplied with food every other day, the density response became obscure. Ecological significance of environmental responses in the wing morph determination was discussed in relation to the heterogeneous environment of water surface.     

Differences in haemolymph proteins in relation to diapause and wing dimorphism inPyrrhocoris apterus (L.) (Heteroptera: Pyrrhocoridae)

Three different cultures of dimorphic bug,Pyrrhocoris apterus, were analyzed concerning diapause and its relation to wing morph pattern. The proportion of macropterous bugs was considerably higher (36%) in the Mediterranean culture from Israel than that (1.3%) in the temperate culture from the Czech Republic. The macropterous morph- and brachypterous morphrelated types of reproduction arrest, differing by the length of pre-oviposition period, were distinguished in cultures analyzed. The reproduction arrest with an average pre-oviposition period of 38.2 days in Mediterranean macropters and 18.5 days in macropters from selected macropterous strain, was found to be typical for macropterous morph. Two different photoperiodic conditions induced macropterous morph-related reproduction arrest, the long-day (18 h light-6 h dark) photoperiod in macropterous strain macropters and the short-day (12 h light-12 h dark) photoperiod in Mediterranean macropters. The brachypterous morph-related reproduction arrest, characterized by pre-oviposition period longer than 90 days, occurred predominantly in diapausing brachypterous bugs. While the hibernal diapause of brachypterous bugs was characterized by a very high level of the 78- and 82-kDa proteins in haemolymph, their content in haemolymph of macropters during macropterous morph-related reproduction arrest was almost as low as in the reproductive adults. The variation of reproduction arrest in relation to wing dimorphism represents an important feature in the life strategy ofP. apterus.     

Adaptive significance of wing dimorphism in males ofNilaparvata lugens

The trade-offs associated with macroptery in males were investigated in a wing-dimorphic planthopper, Nilaparvata lugens (Stål) (Delphacidae, Hemiptera). Developmental time of brachypters was significantly shorter than that of macropters across a wide range of population densities, while no within-population differences between wing morphs in body weight were found. In mating experiments, brachypterous males 12 to 24 h old were almost three times more successful in competition for females than macropterous males of the same age, while in five days old males, macropters were twice as successful as brachypters. In competition for females involving direct body contact between males, brachypterous and macropterous males were equally successful. It is suggested that macroptery and delayed peak in sexual activity in males may both be influenced by the titre of juvenile hormone, as is known from females. The combination of prolonged development and delayed maximum competitive ability for mates, associated with macroptery, is likely to be disad vantageous, representing probably a penalty associated with macroptery in males in undisturbed habitats.     

Factors and consequences of a non-functional alary polymorphism inPyrrhocoris apterus (Heteroptera: Pyrrhocoridae)

Functional alary polymorphisms have been studied rather extensively in several insect species. This review article deals with factors controlling wing polymorphism in a flightless species,Pyrrhocoris apterus (L.), and discusses its adaptive significance and mechanisms for their persistence under natural conditions. The macropterous morph is determined by a recessive allele whose penetrance depends on photoperiod and temperature. Natural populations of this species contain a small fraction of flightless macropters. The disadvantages of being a macropter (increase of development time, decrease of fecundity) are minimal, while the benefit may consist in the tendency to prereproductive arrest of ovarian development in teneral females. It prevents establishing a second generation which would mostly die during the next winter. The mechanism of alary morph regulation may be an ancestral trait linkingP. apterus with other polymorphic Heteroptera, while its decreased penetrance may be a derivative character. Variation in fitness due to alary morphs is small compared to the one associated with differences in body size. The latter is environmentally determined, and not linked to the genetic basis of wing polymorphism. In the “mosaic” of phenotypes of various size the significance of the genetic macroptery may be close to neutral.     

Density‐dependent sex‐biased development of macroptery in a water strider

In wing‐polymorphic insects, wing morphs differ not only in dispersal capability but also in life history traits because of trade‐offs between flight capability and reproduction. When the fitness benefits and costs of producing wings differ between males and females, sex‐specific trade‐offs can result in sex differences in the frequency of long‐winged individuals. Furthermore, the social environment during development affects sex differences in wing development, but few empirical tests of this phenomenon have been performed to date. Here, I used the wing‐dimorphic water strider Tenagogerris euphrosyne to test how rearing density and sex ratio affect the sex‐specific development of long‐winged dispersing morphs (i.e., sex‐specific macroptery). I also used a full‐sib, split‐family breeding design to assess genetic effects on density‐dependent, sex‐specific macroptery. I reared water strider nymphs at either high or low densities and measured their wing development. I found that long‐winged morphs developed more frequently in males than in females when individuals were reared in a high‐density environment. However, the frequency of long‐winged morphs was not biased according to sex when individuals were reared in a low‐density environment. In addition, full‐sib males and females showed similar macroptery incidence rates at low nymphal density, whereas the macroptery incidence rates differed between full‐sib males and females at high nymphal density. Thus complex gene‐by‐environment‐by‐sex interactions may explain the density‐specific levels of sex bias in macroptery, although this interpretation should be treated with some caution. Overall, my study provides empirical evidence for density‐specific, sex‐biased wing development. My findings suggest that social factors as well as abiotic factors can be important in determining sex‐biased wing development in insects.     

小翅稻蝗(Oxya yezoensis Shiraki)翅多型现象浅释

对小翅稻蝗翅型构成的季节变化,长翅型、短翅型产卵前期间的差异及光周期对翅型分化的影响进行了调查,结果表明,小翅稻蝗存在着明显的翅多型现象,相对翅长(前翅长/后足腿节长)最长可达1.49,最短只有0.50,翅长呈连续性双模态分布。早期羽化的个体大多为长翅型,随着季节的推移,羽化成虫短翅率逐渐增加。长翅型雌成虫的产卵前期间为42.7±13.1日,短翅型为15.3±6.4日,短翅型雌成虫的产卵前期间显著短于长翅型。不同的光周期短翅成虫出现的比例不同,光照短于LD1212时,几乎全为短翅型,LD1410短翅率为66.8%,LD168则长翅率达62.1%,光照长于LD186时,几乎全为长翅型。     

Coevolution of traits determining migratory tendency: correlated response of a critical enzyme,juvenile hormone esterase,to selection on wing morphology

Migratory tendency in insects is a complex trait, composed of a suite of correlated behavioural, physiological, morphological and life history traits. We investigate the genetic and physiological basis of the coevolution of this suite of traits using laboratory lines of the wing dimorphic cricket, Gryllus firmus, selected for increasing and decreasing incidence of macroptery. Selection on wing morphology has produced strong direct responses in proportion macropterous as well as correlated (indirect) responses in wing muscle histolysis, flight propensity and fecundity. We investigate the hypothesis that these responses have been mediated by changes in the metabolism of juvenile hormone (JH) during the final nymphal stadium (the critical period for wing morph determination). Previous studies of Gryllus sp. have established that JH titre in this period is determined primarily by the activity of the degradative enzyme, juvenile hormone esterase (JHE). Assays of JHE activity in the final nymphal stadium of the replicated control and selected lines demonstrate highly significant differences in both mean activity and the probability of macroptery for a given level of activity (i.e., the threshold activity required to induce wing formation). These correlated responses in JH metabolism support the general hypothesis that the correlations among traits determining migratory tendency result at least in part from the common influence of JH during the final nymphal stadium. We discuss these results in the context of the quantitative genetic model for the evolution of polygenic, dichotomous traits (the threshold model), and present four general predictions concerning the coevolution of traits associated with ecological (i.e., trophic, life history, behavioural) dimorphisms.     

Dispersal capability in a habitat specialist bush cricket: the role of population density and habitat moisture

1. In fragmented landscapes many insect species depend on a regular exchange of individuals between subpopulations to ensure the persistence of the population. Thus, the ability to disperse is of particular relevance. 2. However, in some insect species mobility is not a fixed trait. Hence, knowing the causes of phenotypic plasticity is of great importance when evaluating whether a species is able to survive in fragmented landscapes or not. 3. A multi‐year field study was conducted to identify possible causes of macroptery in the wing‐dimorphic habitat specialist Metrioptera brachyptera L. and to quantify its dispersal capability (% macropters). Therefore, 746 individuals of the species were caught on 135 plots. Additionally, environmental variables that possibly induce the development of macropters (population density and habitat moisture) were recorded. 4. Dispersal capability of M. brachyptera was very low. Less than 3% were long‐winged. The statistical analysis revealed that the proportion of long‐winged M. brachyptera was strongly correlated with high bush‐cricket densities and not with habitat moisture. 5. The low dispersal capability of M. brachyptera leads to the conclusion that individual exchange between isolated populations is limited or even impossible. Habitat specialists, like M. brachyptera, may thus be unable to respond to rapid changes in the availability of suitable habitats by dispersing, and hence may be especially dependent on habitat management activities that promote the long‐term stability of existing habitat patches.     

Sensitive periods for wing development and precocious metamorphosis after precocene treatment of the brown planthopper,Nilaparvata lugens

The critical periods for juvenile hormone suppression of wing development and metamorphosis were examined in a pure brachypterous line of Nilaparvata lugens following topical application of Precocene II (PII) to various stages of the third and fourth nymphal stadia. When PII, in doses ranging from 10 pg to 100 ng, was applied to 12-h-old third or 6-h-old fourth stadium nymphs, long-wing formation (macroptery) was induced. Macropter induction ranged from 5 to 50% in females and from 30 to 50% in males, the effect being more prominent in males. The sensitive period for macropter induction lasted from the early second stadium through to day one of the fourth (penultimate) stadium. Beyond that period nymphs required sensitivity to PII and the numbers of macropters increased towards the final nymphal molt, again males were more sensitive. On the other hand, periods sensitive to PII for precocious metamorphosis induction appeared from early second stadium to 18 h after molting to the third nymphal stadium. Overlapping application of JH-III with PII at early third stadium could totally inhibit the occurrence of precocious metamorphosis and significantly rescue the macropter induction. From the data described, a scheme is presented for the control of wing development and metamorphosis by juvenile hormone, assuming two types of threshold value determining wing form and metamorphosis.     

Effect of photoperiod on the size–temperature relationship in a pentatomid bug, Dolycoris baccarum

(1) The effects of photoperiod and temperature on nymphal growth, development and adult size in a pentatomid bug, Dolycoris baccarum, were examined. (2) The temperature–size relationship was not stable but was highly affected by the photoperiod, and showed a geographical variation. (3) There were variations also in the developmental period and the growth rate. (4) The responses to temperature and photoperiod can explain the seasonal and geographical adaptations of the insect, and thus are considered to be important seasonal adaptations in D. baccarum.     

Wing and flight muscle polymorphism in a lygaeid bug, Horvathiolus gibbicollis: determinants and life history consequences

ABSTRACT.

• 1 Horvathiolus gibbicollis (Costa), a ground-living seed-feeding bug of the mediterranean region, has two wing morphs. In macrop-terous bugs both pairs of wings are fully developed. In brachypterous ones forewings are reduced to about two-thirds and hindwings to less than a third of their length in macropters.
• 2 Each morph bred true with regard to wing length when reared under variable density, food and temperature conditions for several generations.
• 3 All F₁ offspring between crosses of the two morphs were brachypterous. In F₂ approximately 25% were macropters and 75% brachypters implying monogenic control of wingform.
• 4 Flight muscles in macropters vary from fully developed to totally reduced. This variation is determined by environmental conditions during adult life. Most young adult bugs have flight muscles, and totally starved or unmated bugs retain their flight muscles. Fed and mated females histolyse flight muscles as they start laying eggs, while most males of the same group retain their muscles.
• 5 Brachypterous bugs have a smaller thorax and larger abdomen than macropterous ones.
• 6 Brachypterous bugs reach adulthood slightly before macropterous ones, and they have a distinctly shorter adult preoviposition period.
• 7 Lifetime egg production does not differ significantly between the two wing morphs. However, the temporal pattern of egg laying is different in the two morphs. The m_x-curve of macropters starts later, then attains a higher peak and finally decreases faster than that of brachypters.
• 8 Initially, macropters lay smaller eggs than brachypters, but egg volume increases with age in macropters and eventually approaches that of brachypters.
• 9 The initial increase in reproductive effort (egg volumexegg number) of macropters is concomitant with wing muscle histolysis and the mobilization of thorax space for reproduction.
• 10 Adult survival rate does not differ between the morphs.

     

Genetic correlation of wing polymorphism between females and males in the oriental chinch bug,Cavelerius saccharivorus Okajima (Heteroptera: Lygaeidae)

Genetic correlations of the wing form and the relative wing length between females and males were estimated in the oriental chinch bug, Cavelerius saccharivorus, by calculating the correlation between the sexes of the proportion macropterous or the mean relative wing length in full-sib families obtained from different wing forms of parents emerged in a high density population. There was a significantly positive genetic correlation between the sexes in both the proportion macropterous and the mean relative wing length. However, the appearance rate of macropters tended to be much lower in males than in females under the rearing conditions which promote the appearance of macropters. This was evident especially in the offspring of brachypterous parents. These indicated that in C. saccharivorus the wing polymorphism of males is not a simple result of the genetic correlation of wing morphology between the two sexes. It was considered that both of the female and male fitness advantages to wing reduction, as well as the genetic correlation between the sexes, would influence the evolution of wing polymorphism in this species.     

Reproduction and egg diapause of the oriental chinch bug,Cavelerius saccharivorus okajima (Heteroptera: Lygaeidae), in the subtropical winter season in relation to its wing polymorphism

Reproduction and egg diapause of the oriental chinch bug, Cavelerius saccharivorus, in the subtropical winter season were investigated in relation to its wing polymorphism. Macropterous females collected from the southern part of Okinawa Is. in the autumn season delayed their oviposition and were less fecund early in the adult life period, but survived much longer than brachypterous females collected from the same locality. The total fecundity was not significantly different between wing morphs. The diapause of eggs laid by brachypters tended to be terminated more easily at a high temperature than that of eggs laid by macropters. This indicated that the eggs laid by macropters were more intense in their diapause than those laid by brachypters. However, irrespective of the parental wing form, diapause showed considerable variation in its intensity within and between clutches.     

The role of circadian clock genes in the photoperiodic timer of the linden bug Pyrrhocoris apterus during the nymphal stage

Many insects survive seasonal adversities during diapause, a form of programmed developmental and metabolic arrest. Photoperiodically regulated entry into diapause allows multivoltine insect species to optimize the number of generations. The molecular mechanism of the photoperiodic timer is unknown in insects. In the present study, we take advantage of the robust reproductive diapause response in the linden bug Pyrrhocoris apterus and explore the fifth‐instar nymphal stage, which is the most photoperiod‐sensitive stage. The nymphs display daily changes in locomotor activity during short days; this differs from the activity observed during long days. We find evidence of cyclical expression of the circadian clock genes, per and cyc, in nymphal heads; in addition, per expression is also photoperiod‐dependent. The RNA interference‐mediated knockdown of the two circadian clock genes, Clk and cyc, during the nymphal stage results in reproductive arrest in adult females. Furthermore, Clk and cyc knockdown induces the expression of the storage protein hexamerin in the fat body, whereas the expression of vitellogenin diminishes. Taken together, these data support the involvement of circadian clock genes in photoperiodic timer and/or diapause induction.     

Reduced flight capability in British Virgin Island populations of a wing-dimorphic insect: the role of habitat isolation, persistence, and structure

1. The effects of habitat isolation, persistence, and host‐plant structure on the incidence of dispersal capability (per cent macroptery) in populations of the delphacid planthopper Toya venilia were examined throughout the British Virgin Islands. The host plant of this delphacid is salt grass Sporobolus virginicus, which grows either in undisturbed habitats (large expanses on intertidal salt flats and around the margins of salt ponds, or small patches of sparse vegetation on sand dunes along the shore), or in less persistent, disturbed habitats (managed lawns). 2. Both sexes of T. venilia were significantly more macropterous in disturbed habitats (77.1% in males, 12.5% in females) than in more persistent, undisturbed habitats (19.2% in males, < 1% in females). 3. Males exhibited significantly higher levels of macroptery (26.9 ± 7.6%) than did females (2.0 ± 1.7%), and per cent macroptery was positively density dependent for both sexes in field populations. 4. There was no evidence that the low incidence of female macroptery in a subset of island populations inhabiting natural habitats (1.7 ± 1.2%) was attributable to the effects of isolation on oceanic islands. The incidence of macroptery in British Virgin Island populations of T. venilia was not different from that observed in mainland delphacid species existing in habitats of similar duration. 5. Rather, the persistence of most salt grass habitats throughout the British Virgin Islands best explains the evolution of flight reduction in females of this island‐inhabiting delphacid. 6. Males were significantly more macropterous in populations occupying dune vegetation (37.6 ± 9.8%) than in populations occupying salt flat–pond margin habitats (7.6 ± 5.6%). By contrast, females exhibited low levels of macroptery in both dune (0%) and salt flat–pond margin (< 1%) habitats. Variation in salt‐grass structure probably underlies this habitat‐related difference in macroptery because flight‐capable males of planthoppers are better able to locate females in the sparse‐structured grass growing on dunes. This habitat‐related difference in male macroptery accounted for the generally higher level of macroptery observed in males than in females throughout the islands. 7. The importance of habitat persistence and structure in explaining the incidence of dispersal capability in T. venilia is probably indicative of the key role these two factors play in shaping the dispersal strategies of many insects.     

Enhancement of short wing formation and ovarian growth in the genetically defined macropterous strain of the brown planthopper,Nilaparvata lugens

When JH II, III or methoprene was applied in the nymphal stages to two different strains of the brown planthopper which were selected to produce long (macropterous) or short (brachypterous) wing forms, no effect was observed on the molting profile or metamorphosis. Brachypterization of a majority of the presumptive macropters was, however, observed by application of these chemicals, although there was no effect on wing form in the presumptive brachypters. The results show that the sensitive periods for the brachypterization of the presumptive macropters falls between early antepenultimate instar and within 1 or 2days of the penultimate instar, and that the chemicals were effective, in the following order of potency: methoprene>JH III>JH II. Ovarian growth was greatly enhanced in the presumptive macropters when JH III or methoprene was applied twice, within 12h of the 3rd or 4th nymphal instar and 6h before adult emergence. JH II on the other hand had no effect on ovarian growth when applied to the presumptive macropters at any of the nymphal stages. None of the chemicals had any effect on ovarian growth in the presumptive brachypters.