The physiological costs of flight capability in wing-dimorphic crickets

Nutritional indices, triglyceride levels and flight muscle developmental profiles were compared between long-winged (LW) and short-winged (SW; flightless) morphs of the cricketsGryllus rubens Scudder andG. firmus Scudder. This was done to identify potential physiological costs of flight capability in adults. The LW morph of each species converted a lower proportion of assimilated nutrients into biomass (reduced ECD) than did the SW morph. This documents increased respiratory metabolism in the LW morph. Triglyceride concentration was higher in LW vs. SW adults. This suggests that the elevated respiration in the LW morph may be at least partially due to the increased biosynthesis of this high energy substance. Preliminary data indicate higher respiration rates of LW functional vs. SW vestigial flight muscles. Collectively, these data suggest that the energetic cost of flight capability in adults results from biosynthesis of triglyceride flight fuel and flight muscle maintenance but not flight muscle growth. No flight muscle growth was observed in adults.     

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

Summary 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 inC. 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.     

The evolutionary significance of wing dimorphism in carabid beetles (Coleoptera: Carabidae)

A review of data on the background of wing dimorphism in carabid beetles (Coleoptera: Carabidae) and especially of the closely relatedCalathus cinctus andC. melanocephalus is given. In bothCalathus species wing dimorphism is inherited in a simple Mendelian fashion with the brachypterous condition dominant, but inC. melanocephalus the expression of the long winged genotype is under environmental control as well. The development of long winged phenotypes in the latter species is favoured by relatively favourable environmental conditions, such as high temperatures and a high food-supply. The higher fecundity of the larger and heavier long winged females of both species may compensate for losses of long winged phenotypes by flight activities. The evolutionary significance of both types of inheritance is discussed in relation to dispersal. The ‘fixed type’ as found inC. cinctus is considered an opportunistic short term ‘between sites strategy’, whereas the ‘dynamic type’ ofC. melanocephalus represents a flexible long term ‘within sites strategy’.     

Genetic analysis of the wing-form determination of the small brown planthopper,Laodelphax striatellus (Hemiptera: Delphacidae)

Summary The mechanism of the genetic control of wing form determination of the small brown planthopperLaodelphax striatellus was analyzed by selection and crossing experiments. The selection experiment for brachypterous and macropterous forms for 13 generations showed that the wing form is under some genetic control. The crossing experiment showed that the wing form polymorphism was not a simple Mendelian character but was under polygenic control. The data from the crossing experiment were applied to the liability method for the quantitative genetics by Falconer (1981). The analysis suggested that wing form determination was a threshold character under polygenic control, and the estimated minimum number of loci concerned was 2.07. The realized heritability estimated from the data of selection experiment was 0.36 for macroptery. On the other hand, the heritability in broad sense estimated on the basis of the variance of F₁ and F₂ in crossing experiment was 0.81.     

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

Summary 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.     

Why is there so much genetic variation for wing dimorphism?

Wing dimorphism appears in general to be determined either by a single locus, 2 allele system in which brachyptery is dominant, or by the additive action of numerous loci. In the latter case studies indicate that the heritability is typically quite large. It is generally postulated that wing dimorphism is under strong selection: why then is genetic variation not eroded? In this paper I consider three possible explanations. First, genetic variation may not be exposed to selection because environmental heterogeneity effectively makes heritability zero. Because wing dimorphisms are known to evolve it seems unlikely that this is the primary factor. Second, directional selection on a threshold trait may push the population almost to monomorphism but erodes genetic variance at a very slow rate. This mechanism cannot preserve variation but makes it possible for other factors to more easily maintain variability. Finally, I demonstrate that in a heterogeneous environment spatio-temporal variation in fitness will itself maintain a genetic polymorphism for wing dimorphism. This paper is dedicated to Professor Sinzo Masaki, whose work on the evolution of wing dimorphism and diapause has been so important to our understanding of the evolution of dimorphic variation.     

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

Summary 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 thatC. 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.     

Variability in wing form of crickets

Loss of functional hindwings is observed in most subfamilies of Japanese crickets. Habitat, behaviour, body size and phylogenetic factors might be involved, but interactions among them may obscure the general trend. Wing dimorphism is common among the relatively small-sized members of Gryllinae, and the two small-sized subfamilies, Nemobiinae and Trigonidiinae. Both environmental cues (e. g. photoperiod) and genetic factors affect the wing form. InDianemobius fascipes (Nemobiinae), the percentage macroptery was drastically changed by selection for macroptery or microptery. Crossing experiments indicated polygenic control of wing form as well asX-chromosomal and maternal effects. Neither the long-winged nor short-winged line bred true after 40 generations of selection. Full-sib families revealed a large genetic variation in frequency of macropterous forms within a population. The genetic determination of the propensity for macropterism did not seem to be directly coupled to the mechanism performing the photoperiodic time-measurement.     

Studies on the adult polymorphism in the southern green stink bug,Nezara viridula (hemiptera: Pentatomidae)

Summary Polymorphism in adult colour pattern ofNezara viridula is determined on the genetic basis. The basic colour patterns of adult are classified into four types, i. e. G, O, R and F. No appreciable differences between these types were observed in respect to various physiological traits of nymphs and adults, except that type G seems to be superior in reproductive ability but to be inferior in ability of surviving winters at least to types R and F. Inter-generation changes in percentage frequency of G type were examined from 1959 to 1967 covering more than 30 generations. Percentage frequencies of G types prior to the severe winter of 1962–3 fluctuated greatly around a mean of 87.9%, while they did to a lesser extent than before with a mean of 85.0% after the winter. Unexpected high percentages of G type were recorded frequently in summer generations, viz. 1st and 2nd. On the other hand, the relative frequencies of G and O types decreased after hibernation in contrast to the increases in those of F and R. This sort of changes in genetic composition related to the winter of 1962–3 was observed in several populations segregated from each other. This seasonal alternation of selective activity in the environments is considered to be responsible for retention of the polymorphism. Alternative possible causes, i. e. difference in habitat preference, non-random mating and selective predation by predators among polymorphs, may safely be rejected as irrelevant to the mechanism in maintaining polymorphism. The polymorphism of this insect seems to be in a transient state rather than balanced one contributing little to population regulation, but the persistence ofN. viridula in the periphery range may be assisted by retention of the polymorphism.     

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

Summary Seasonal changes in the percentage of macropterous individuals in natural population ofMicrovelia 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.     

Hybridization and speciation in the carabid beetles of the subgenusOhomopterus (Coleoptera, Carabidae, genusCarabus)

Natural hybridization among wingless carabid beetles of the subgenusOhomopterus (Carabidae, genusCarabus) is reviewed, and its significance in the evolution of this subgenus discussed. Natural hybridization occurs between parapatric species of similar size. Two case studies of natural hybridization suggest that natural hybridization could have affected the evolution of this subgenus in different ways. When there is a large difference in genital morphology between hybridizing species, interspecific copulation often results in genital injuries that causes mortality of copulating individuals, and hence reduces the fitness of hybridizing individuals greatly. In such a case, hybridization may be effective in maintaining the parapatric distribution of the two species, and in the long term, may promote reinforcement selection for traits which are effective in prezygotic reproductive isolation. When the morphological difference in genitalia is not so large as to cause genital injury, a hybrid population may be established at the intermediate zone between two parental species, provided that the immigration rates of the two species into the intermediate zone are small. Thus, natural hybridization may have contributed to both divergence and reticulate evolution in this subgenus.     

Polymorphic fundatrices in thimbleberry aphid — ecology and maintenance

The thimbleberry aphid,Masonaphis maxima (Mason) lives on patches of plants that support 3,4 or 5 generations depending on site and weather. The life cycle requires sexual females and males to produce overwintering eggs. The eggs hatch in the spring to produce the first ’fundatrix’ generation; subsequent generations are produced parthenogenically. Males and other morphs are produced by wingless virginoparae, but sexual females are produced by ’gynoparae’, a winged morph that is specialized to produce only sexual females. The fundatrices have no indication of the number of generations that the plants will support in the current year. There are two fundatrix types that coexist in different ratios depending on the number of generations supported by the patch the previous year. One type produces sexual females in generations 3 and 5, and males in generations 4 and 5; the other type produces sexual females in generations 4 and 5, and males in generations 3, 4 and 5. The dimorphism adapts the aphid to its heterogeneous and somewhat unpredictable environment. The role of sex in the maintenance of the dimorphism is discussed. This is the first report of fundatrix polymorphism and consequent differential sex expression in aphids.     

Group predatory behavior by the assassin bugAgriosphodrus dohrniSignoret (Hemiptera: Reduviidae)

Summary Nymphs ofAgriosphodrus dohrni Signoret (Reduviidae) have a strong gregariousness and show group predatory behavior. This study was conducted to clarify adaptive significance of group predation of this species, including laboratory observations and 6-year field surveys. In the laboratory, observations on both solitary and group attacking against armyworms were made at varying prey size classes to compare the capture success rate by solitary predators with that by groups. The efficiency in capturing the prey was significantly higher in group attacking at any prey size class compared. Data obtained from the field surveys indicated the tnedency for searching nymphs to feed in group and to increase the number of predators feeding per prey item with increasing prey size. Average sizes of prey captured were also larger in group feeding throughout the nymphal stage. In particular, it was remarkable that, when prey were “creeping” types, the upper size limit of prey eaten was dramatically increased.     

Genetic variance and covariance patterns of larval development in the small white butterflyPieris rapae crucivora Boisduval

Summary Quantitative genetic theory indicates that genetic covariance patterns among life history characters should have played an important role as genetic constraint in life history evolution. Highly positve (and negative) genetic correlations between larval development time (or larval growth rate) and adult size characters were detected by means of sib analysis for the small white butterflyPieris rapae crucivora. The genetic associations suggested that evolution of developmental characteristics and adult phenotypic traits were constrained by pleiotropy. The positive genetic correlations between development time and adult body size may be compatible with the trade-off between them, but the negative genetic correlations between larval growth rate and adult body size are not predicted from theories of optimal energy allocation. That phenotypic correlations drastically differed from the genetic correlations indicates limitations of evolutionary inferences based only on phenotypic variation.     

Wing dimorphism and the migratory syndrome: Correlated traits for migratory tendency in wing dimorphic insects

The hypothesis that the morphological, physiological, and behavioral traits comprising the migratory syndrome in insects are genetically correlated through pleiotropic effects of genes controlling the titre of a common hormonal determinant is explored. Evidence that juvenile hormone (JH) influences the component traits of the migratory syndrome is presented, and thus JH is assumed to be the underlying, common determinant. However, readers are cautioned that this does not imply that JH is solely responsible for these traits, nor is this necessary for the arguments presented. For wing dimorphic taxa, the “correlated traits hypothesis” predicts covariance within wing morphs between JH titre and the proportion winged. Four simple genetic models for wing-morph determination are considered: single-locus with short-winged (SW) dominant; single-locus with long-winged (LW) dominant; polygenic, fixed threshold, shifting distribution; and polygenic, shifting threshold, fixed distribution. In each case, wing morphology is assumed to be a threshold trait with the liability being JH titre at some critical stage of development. All models predict covariation between %LW and the mean JH titre of at least one of the wing morphs, but the form and direction of the relationship depends critically on the genetic model used. The results suggest that we should expect the traits associated with the migratory syndrome, and hence the trade-offs associated with the evolution of wing dimorphism, to be correlated with proportion winged and, in this sense, to be frequency-dependent.     

Origin and evolution of endemic plants of the Bonin (Ogasawara) Islands

The Bonin Islands are typical oceanic islands, located at the western part of the North Pacific Ocean and approximately 1,000 km south of mainland Japan. This archipelago consists of about 20 small islands. Although floristic diversity is low due to the small area and limited environmental diversity, the Bonin Islands harbor unique endemic flora as in other well-known oceanic islands. This paper presents a brief summary of the results obtained from recent studies on the endemic flora of the Bonin Islands. The results are reviewed in relation to the four stages of the evolution of endemic flora in the oceanic islands; migration, establishment, enlargement and diversification. The ancestors of the flora originated mostly from tropical and subtropical Southeast Asia or mainland Japan by rare events of long distance dispersal. The proportion of bird-dispersed species is relatively high as for other oceanic islands. Genetic data sets obtained from allozyme variation in some endemic species suggest that migration occurred several million years ago and genetic diversity is correlated with current population size. At the time of establishment, self-compatible plants are expected to have an advantage. However, the percentage of dioecious plants is relatively high. This is partly due to evolutionary changes from hermaphroditic ancestors to dioecy which occurred in two genera in the Bonin Islands. In addition, there are some examples of evolutionary changes from herbaceous ancestors to woody endemics. Adaptive radiation is found in some genera, although the number of congeneric endemic species is less than five. Studies of allozyme variation inPittosporum, Symplocos andCrepidiastrum showed that genetic identity is generally very high between congeneric species in spite of their distinct morphologies. This result suggests that divergence of these species occurred rather recently and distinct morphological differences are based on a limited number of genetic changes.     

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

Summary 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 biology of the voleClethrionomys rufocanus: A review

The biology of the gray-sided voleClethrionomys rufocanus in Hokkaido, concerning taxonomy, morphology, phylogeny, distribution, and natural history, is reviewed. Applied issues in forest management (damage, control and census) are also mentioned. AlthoughClethrionomys rufocanus of Hokkaido was originally identified as a distinct species,Evotomys (=nowClethrionomys) bedfordiae Thomas, 1905, current literature generally refers to the gray-sided vole of Hokkaido asClethrionomys rufocanus or asC. rufocanus bedfordiae (vernacular name, the Bedford’s red-backed vole). The gray-sided vole is the most common small mammal in Hokkaido. It inhabits open areas as well as forests, and mainly feeds on green plants. The gray-sided vole has a high reproductive potential; litter size: 4–7; gestation period: 18–19 days; maturation age: 30–60 days old. Although spring-born individuals usually attain sexual maturity in their summer/fall of birth, their maturation is sometimes suppressed under high densities. The breeding season is generally from April to October, but with some regional variation.Clethrionomys rufocanus has a rather specialized diet (folivorous), particularly during winter when it feeds on bamboo grass. Many predators specialize on the grey-sided vole in Hokkaido; even the red fox, which is a typical generalist predator, selectively feeds on this vole. Damage by voles’ eating bark used to be sever on forest plantations in Hokkaido. Censuses of small rodents have been carried out for management purpose since 1954.     

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

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

Evolution of litter size

Summary Theories on the evolution of litter size among organisms were reexamined. The competition theories, including that based on ther−K-selection hypothesis, could not explain well why low-fecundity strategies have often evolved in stressful environments such as mountain streams, deep sea and the antarctic, where interspecies competition is considered to be lax. The theory, based on It?'s (1980) concept of theprocurability of food by the young, was considered to have greater generality because it could explain not only the above-mentioned cases but also those where small litter size is observed in habitats with high species diversity (where interspecific competition may be keen), such as tropical rain forest. Examination of the process of selection of high-fecundity and low-fecundity genotypes also suggested that the procurability of food by the young can best explain the evolution of low-fecundity. The concept of density-induced dispersal and a distinction between density-dependent and density-independent predation pressures should be incorporated into our discussions on the evolution of reproductive rates. This work was supported in part by Grant-in-Aid No. 439017 from the Ministry of Education, Science and Culture.