昆虫细胞生长之调控机制的探讨

细胞生长调控机制的探讨是近年来发育生物学中一个十分热门的研究领域。生物体内之细胞是如何得知何时该生长及分裂?何时该停止生长?何时该死亡?对生物体来说至关重大。研究显示调控细胞生长之信息传递系统出现差错,将导致生长异常,从而产生组织细胞之异常增生而诱发癌症或产生其它重大疾病。而人类也只有在充分理解细胞生长之机制的基础上,我们才能了解癌症等重大疾病发生的细胞生物学基础。在探讨细胞生长调控机制的研究中,昆虫特别是果蝇Drosophilamelanogaster一直是一个十分理想的实验材料。文章介绍了如何从昆虫看细胞之生长调控机制。     

Testing mechanistic models of growth in insects

Insects are typified by their small size, large numbers, impressive reproductive output and rapid growth. However, insect growth is not simply rapid; rather, insects follow a qualitatively distinct trajectory to many other animals. Here we present a mechanistic growth model for insects and show that increasing specific assimilation during the growth phase can explain the near-exponential growth trajectory of insects. The presented model is tested against growth data on 50 insects, and compared against other mechanistic growth models. Unlike the other mechanistic models, our growth model predicts energy reserves per biomass to increase with age, which implies a higher production efficiency and energy density of biomass in later instars. These predictions are tested against data compiled from the literature whereby it is confirmed that insects increase their production efficiency (by 24 percentage points) and energy density (by 4 J mg⁻¹) between hatching and the attainment of full size. The model suggests that insects achieve greater production efficiencies and enhanced growth rates by increasing specific assimilation and increasing energy reserves per biomass, which are less costly to maintain than structural biomass. Our findings illustrate how the explanatory and predictive power of mechanistic growth models comes from their grounding in underlying biological processes.     

Evolutionary causes of male-biased sexual size dimorphism from a female perspective in the seed bug Togo hemipterus (Heteroptera: Lygaeidae)

Sexual size dimorphisms (SSDs) in body size are expected to evolve when selection on female and male sizes favors different optima. Many insects show female-biased SSD that is usually explained by the strong fecundity advantage of larger females. However, in some insects, males are as large as or even larger than females. The seed bug Togo hemipterus (Scott) also exhibits a male-biased SSD in body size. Many studies that have clarified the evolutionary causes of male-biased SSD have focused only on male advantages due to male–male competition. To clarify the evolutionary causes of male-biased SSD in body size, we should examine the degree of not only the sexual selection that favors larger males but also natural selection that is acting on female fecundity. The obtained results, which showed higher mating acceptance rates to larger males, implies that females prefer larger males. No significant relationship was detected between female body size and fecundity; body size effects on female fecundity were weak or undetectable. We conclude that male-biased SSD in T. hemipterus can be accounted for by a combination of sexual selection through male–male competition and female choice favoring large males, plus weak or undetectable natural selection that favors large females due to a fecundity advantage.     

The control of body size in insects

Control mechanisms that regulate body size and tissue size have been sought at both the cellular and organismal level. Cell-level studies have revealed much about the control of cell growth and cell division, and how these processes are regulated by nutrition. Insulin signaling is the key mediator between nutrition and the growth of internal organs, such as imaginal disks, and is required for the normal proportional growth of the body and its various parts. The insulin-related peptides of insects do not appear to control growth by themselves, but act in conjunction with other hormones and signaling molecules, such as ecdysone and IDGFs. Size regulation cannot be understood solely on the basis of the mechanisms that control cell size and cell number. Size regulation requires mechanisms that gather information on a scale appropriate to the tissue or organ being regulated. A new model mechanism, using autocrine signaling, is outlined by which tissue and organ size regulation can be achieved. Body size regulation likewise requires a mechanism that integrates information at an appropriate scale. In insects, this mechanism operates by controlling the secretion of ecdysone, which is the signal that terminates the growth phase of development. The mechanisms for size assessment and the pathways by which they trigger ecdysone secretion are diverse and can be complex. The ways in which these higher-level regulatory mechanisms interact with cell- and molecular- level mechanisms are beginning to be elucidated.     

What keeps insects small? Time limitation during oviposition reduces the fecundity benefit of female size in a butterfly

Laboratory studies of insects suggest that female fecundity may increase very rapidly with adult size and that mass may often increase close to exponentially with time during larval growth. These relationships make it difficult to see how realistic levels of larval mortality can outweigh the fecundity benefit of prolonged growth. Hence, it is unclear why many insects do not become bigger. In this study, we experimentally explore the relationship between female size and fecundity in the butterfly Pararge aegeria and show that thermally induced time limitation during oviposition may substantially reduce the fecundity benefit of larger females. We model time-limited oviposition under natural temperature conditions and show that fecundity is also likely to increase asymptotically with female size in the field. With realistic estimates of juvenile mortality, the model predicts optimal body sizes within the observed range even when larvae grow exponentially. We conclude that one important reason for why insects with a high capacity of larval growth do not evolve toward larger sizes may be that the fecundity benefit is in fact relatively limited under natural conditions. If so, these results may help resolve some of the inconsistencies between theory and empirical patterns in explaining optimal size in insects.     

Plasticity of size and growth in fluctuating thermal environments: comparing reaction norms and performance curves

Ectothermic animals exhibit two distinct kinds of plasticityin response to temperature: Thermal performance curves (TPCs),in which an individual's performance (e.g., growth rate) variesin response to current temperature; and developmental reactionnorms (DRNs), in which the trait value (e.g., adult body sizeor development time) of a genotype varies in response to developmentaltemperatures experienced over some time period during development.Here we explore patterns of genetic variation and selectionon TPCs and DRNs for insects in fluctuating thermal environments.First, we describe two statistical methods for partitioningtotal genetic variation into variation for overall size or performanceand variation in plasticity, and apply these methods to availabledatasets on DRNs and TPCs for insect growth and size. Our resultsindicate that for the datasets we considered, genetic variationin plasticity represents a larger proportion of the total geneticvariation in TPCs compared to DRNs, for the available datasets.Simulations suggest that estimates of the genetic variationin plasticity are strongly affected by the number and rangeof temperatures considered, and by the degree of nonlinearityin the TPC or DRN. Second, we review a recent analysis of fieldselection studies which indicates that directional selectionfavoring increased overall size is common in many systems—thatbigger is frequently fitter. Third, we use a recent theoreticalmodel to examine how selection on thermal performance curvesrelates to environmental temperatures during selection. Themodel predicts that if selection acts primarily on adult sizeor development time, then selection on thermal performance curvesfor larval growth or development rates is directly related tothe frequency distribution of temperatures experienced duringlarval development. Using data on caterpillar temperatures inthe field, we show that the strength of directional selectionon growth rate is predicted to be greater at the modal (mostfrequent) temperatures, not at the mean temperature or at temperaturesat which growth rate is maximized. Our results illustrate someof the differences in genetic architecture and patterns of selectionbetween thermal performance curves and developmental reactionnorms.     

Social structure and winter survival in acorn ants

Fluctuating selection is a major theme in the evolutionary and ecological literature, yet attempts to measure how differential selection across time or space affects long-term change in life history traits or behaviors are still rare. Social evolution among the insects has been broadly studied with respect to how key parameters such as queen number and relatedness vary to influence colony fitness. However, a primary focus on fertility selection in the warm months must be complemented by parallel investigations on survivorship selection during the cold months. Here we provide the first assessment of social structure and overwintering survivorship in the field. We studied the acorn ant, which stays aboveground throughout the cold winters in North America, by varying queen number and colony size over two consecutive winters. We found that winter survival was quite low but unconnected to variable colony structure. Therefore previous studies on how social structure affects fertility selection in acorn ants have not been confounded by countervailing selection during the cold months. Our data support the assumption of the larger literature that selective forces molding social behavior in ants act primarily on fertility selection during the reproductive season.     

What keeps insects small?—Size dependent predation on two species of butterfly larvae

Insect size usually increases greatly in the latter stages of development, while reproductive value increases strongly with adult size. Mechanisms that can balance the benefits associated with increased growth are poorly understood, raising the question: what keeps insects from becoming larger? If predation risk was to increase with juvenile size, it would make an extension of development very risky, favouring smaller final sizes. But field measures of juvenile mortality seldom show any general patterns of size dependence. We here therefore try to estimate a mechanistic relationship between juvenile size and predation risk by exposing the larvae of two closely related butterflies to a generalist invertebrate predator in a laboratory experiment. Predation risk increased with larval size but was not affected by the species-specific growth rate differences. These results indicate that predation risk may increase with the size of the juvenile even when predators are relatively small. By basing a model simulation on our data we also show that size dependent predation of the kind found in this study has potential to stabilise selection on body size in these species. Thus, these findings suggest that more detailed studies of the size dependence of predation risk on juvenile instars will increase the understanding of what it is that keeps insects small.     

Multiple Ecological Factors Influence the Location of Proboscis Monkey (<Emphasis Type="Italic">Nasalis larvatus</Emphasis>) Sleeping Sites in West Kalimantan,Indonesia

Multiple ecological factors have been hypothesized to influence primate sleeping site selection. Testing multiple hypotheses about sleeping site selection permits examination of the relative strength of distinct ecological factors and expands our ability to understand how selection pressures influence primate sleeping behavior. Here we examine how avoidance of biting insects, thermoregulation, foraging efficiency, tree stability, and interspecific competition influence selection of sleeping sites by proboscis monkeys (Nasalis larvatus) in Indonesian Borneo. We collected data on relative insect abundance, temperature, rainfall, food availability, group size, sleeping site location, and presence of other primates for 12 mo. Using formal model comparison and information criteria, we analyzed the relative importance of these ecological factors in determining one aspect of sleeping site location: distance from the river. Our models supported the avoidance of biting insects and the foraging efficiency hypotheses. Proboscis monkeys slept further inland on nights when the abundance of sandflies was high along the river, and when less food was available along the river. Many studies suggest that primates select sleeping trees and locations to reduce predation risk; our study indicates that additional factors may also be important in determining sleeping site selection.     

Natural selection on floral volatile production in Penstemon digitalis: Highlighting the role of linalool

Natural selection is thought to have shaped the evolution of floral scent; however, unlike other floral characters, we have a rudimentary knowledge of how phenotypic selection acts on scent. We found that floral scent was under stronger selection than corolla traits such as flower size and flower color in weakly scented Penstemon digitalis. Our results suggest that to understand evolution in floral phenotypes, including scent in floral selection, studies are crucial. For P. digitalis, linalool was the direct target of selection in the scent bouquet. Therefore, we determined the enantiomeric configuration of linalool because interacting insects may perceive the enantiomers differentially. We found that P. digitalis produces only (S)-(+)-linalool and, more interestingly, it is also taken up into the nectar. Because the nectar is scented and flavored with (S)-(+)-linalool, it may be an important cue for pollinators visiting P. digitalis flowers.     

Life History Consequences of Bioenergetic and Biomechanical Constraints on Migration

In this paper I test the hypothesis that bioenergetic and biomechanicalconstraints to migration play a pivotal role in shaping thelife history characteristics of migrants. Firstly, I examinebioenergetic constraints on the ability to migrate by activetransport and how they shape the life histories of insects andfish, and, secondly, the consequences of biomechanical constraintsto the migration by passive transport in insects and spiders. In both insects and fish the mass-specific energetic costs ofactive transport (flight and swimming, respectively) decreasewith body size, and hence selection should favor large sizein migrant species. Because their habitats are ephemeral, migrantinsects must grow rapidly. In fish, migrant species are ableto exploit resources unavailable to more sedentary species andhence should also show an enhanced rate of growth. These predictionsare supported by comparisons within populations, between populations,and among species in both groups. In contrast to the above, biomechanical factors limit the uppersize at which insects and spiders can migrate by passive transport.Theory predicts that ballooning will be most likely in spidersconsiderably less than 6 mm in length. Therefore, species thatmigrate as adults are predicted to be smaller than those thatdo not. This prediction is supported by a comparison of migratoryand non-migratory spider species from the United Kingdom. Theaverage length of species that migrate as adults, and of migratingyoung of spiders too large to balloon as adults, is about 2mm. Further, within this geographic species assemblage, thesize distribution of adult spiders is markedly peaked in the2 mm region, suggesting that biomechanical constraints on ballooningmay have a major influence on the evolution of body size inspiders.     

Tooth crown heights, tooth wear, sexual dimorphism and jaw growth in hominoids

The aim of this review is to bring together data that link tooth morphology with tooth function and tooth growth: We aim to show how the microanatomy of hominoid teeth is providing evidence about rates of tooth growth that are likely to be a consequence of both masticatory strategy and social behaviour. First, we present data about incisor and molar tooth wear in wild short chimpanzees that demonstrate how crown heights are likely to be related to relative tooth use in a broad sense. Following this we review recent studies that describe the microanatomy of hominoid tooth enamel and show how these studies are providing evidence about tooth crown formation times in hominoids, as well as improving estimates for the age at death of certain juvenile fossil hominids. Next, we outline what is known about the mechanisms of tooth growth in the sexually dimorphic canine teeth of chimpanzees and compare these patterns of growth with tooth growth patterns in the canines of three fossil hominids from Laetoli, Tanzania. Finally, we discuss how selection pressures that operate to increase or reduce the size of anterior teeth interact with jaw size. We argue that the space available to grow developing teeth in the mandibles of juvenile hominoids is determined by the growth patterns of the mandibles, which in turn reflect masticatory strategy. The consequences of selection pressure to grow large or small anterior teeth are likely to be reflected in the times at which these teeth are able to emerge into occlusion.     

A Test of Genital Allometry Using Two Damselfly Species does not Produce Hypoallometric Patterns

It is widely admitted that sexual selection is the responsible force behind genital traits. However, the particular mechanisms of genital evolution are still debated. Recently, studies of genital static allometry in insects have been used to elucidate such mechanisms. Insect genital traits are often reported to show negative allometry (i.e., a slope < 1), which has generated a number of ideas on how genital traits are selected. However, many studies that have inferred selection mechanisms have omitted consideration of the function of genital traits, used unreliable indicators of body size, and only rarely included female genitalia in their analysis. We investigated whether negative allometry operates for genitalia in two damselfly species (Protoneura cara and Ischnura denticollis). Damselflies are suitable for genital allometry tests as their genital function and body size indicators (wing length and head width) are relatively well known and established. First, we show that the aedeagus is used to physically remove sperm from both sperm storage organs (bursa and spermatheca) and that wing length and head width correlate positively with other morphological traits for the two study species. Second, we estimated genital allometry by measuring aedeagal length, vaginal length, bursal volume, and spermathecal volume. Our results indicate no consistent allometric pattern. Allometry for aedeagal length and vaginal width was not the same. Thus, there was no support for a negative allometric relationship. We urge researchers investigating allometry to look directly at how genitalia function rather than inferring function from allometric relationships only.     

An integrated analysis of phenotypic selection on insect body size and development time

Most studies of phenotypic selection do not estimate selection or fitness surfaces for multiple components of fitness within a unified statistical framework. This makes it difficult or impossible to assess how selection operates on traits through variation in multiple components of fitness. We describe a new generation of aster models that can evaluate phenotypic selection by accounting for timing of life‐history transitions and their effect on population growth rate, in addition to survival and reproductive output. We use this approach to estimate selection on body size and development time for a field population of the herbivorous insect, Manduca sexta (Lepidoptera: Sphingidae). Estimated fitness surfaces revealed strong and significant directional selection favoring both larger adult size (via effects on egg counts) and more rapid rates of early larval development (via effects on larval survival). Incorporating the timing of reproduction and its influence on population growth rate into the analysis resulted in larger values for size in early larval development at which fitness is maximized, and weaker selection on size in early larval development. These results illustrate how the interplay of different components of fitness can influence selection on size and development time. This integrated modeling framework can be readily applied to studies of phenotypic selection via multiple fitness components in other systems.     

Antagonistic natural and sexual selection on wing shape in a scrambling damselfly

Wings are a key trait underlying the evolutionary success of birds, bats, and insects. For over a century, researchers have studied the form and function of wings to understand the determinants of flight performance. However, to understand the evolution of flight, we must comprehend not only how morphology affects performance, but also how morphology and performance affect fitness. Natural and sexual selection can either reinforce or oppose each other, but their role in flight evolution remains poorly understood. Here, we show that wing shape is under antagonistic selection with regard to sexual and natural selection in a scrambling damselfly. In a field setting, natural selection (survival) favored individuals with long and slender forewings and short and broad hindwings. In contrast, sexual selection (mating success) favored individuals with short and broad forewings and narrow‐based hindwings. Both types of selection favored individuals of intermediate size. These results suggest that individuals face a trade‐off between flight energetics and maneuverability and demonstrate how natural and sexual selection can operate in similar directions for some wing traits, that is, wing size, but antagonistically for others, that is, wing shape. Furthermore, they highlight the need to study flight evolution within the context of species’ mating systems and mating behaviors.     

The regulation of organ size in Drosophila: physiology, plasticity, patterning and physical force

The correct regulation of organ size is a fundamental developmental process, the failure of which can compromise organ function and organismal integrity. Consequently, the mechanisms that regulate organ size have been subject to intense research. This research has highlighted four classes of mechanism that are involved in organ size regulation: physiology, plasticity, patterning and physical force. Nevertheless, how these mechanisms are integrated and converge on the cellular process that regulate organ growth is unknown. One group of animals where this integration is beginning to be achieved is in the insects. Here, I review the different mechanisms that regulate organ size in insects, and describe our current understanding of how these mechanisms interact. The genes and hormones involved are remarkably conserved in all animals, so these studies in insects provide a precedent for future research on organ size regulation in mammals.     

Artificial selection on larval growth curves in Tribolium: correlated responses and constraints

Body size is often constrained from evolving. Although artificial selection on body size in insects frequently results in a sizable response, these responses usually bear fitness costs. Further, these experiments tend to select only on size at one landmark age, rather than selecting for patterns of growth over the whole larval life stage. To address whether constraints may be caused by larval growth patterns rather than final size, we implemented a function‐valued (FV) trait method of selection, in which entire larval growth curves from Tribolium were artificially selected. The selection gradient function used was previously predicted to give the maximal response and was implemented using a novel selection index in the FV framework. Results indicated a significant response after one generation of selection, but no response in subsequent generations. Correlated responses included increased mortality, increased critical weight, and decreased development time (DT). The lack of response in size and development time after the first generation was likely caused by increased mortality suffered in selected lines; we demonstrated that the selection criterion caused both increased body size and increased mortality. We conclude that artificial selection on continuous traits using FV methods is very efficient and that the constraint of body size evolution is likely caused by a suite of trade‐offs with other traits.     

Host-plant selection by insects – a theory based on `appropriate/inappropriate landings'' by pest insects of cruciferous plants

Seven hypotheses, including the `Resource Concentration Hypothesis' and the `Enemies Hypothesis', have been put forward to explain why fewer specialist insects are found on host plants growing in diverse backgrounds than on similar plants growing in bare soil. All seven hypotheses are discussed and discounted, primarily because no one has used any of them to produce a general theory of host plant selection, they still remain as hypotheses. However, we have developed a general theory based on detailed observations of insect behaviour. Our theory is based on the fact that during host plant finding the searching insects land indiscriminately on green objects such as the leaves of host plants (appropriate landings) and non-host plants (inappropriate landings), but avoid landing on brown surfaces, such as soil. The complete system of host plant selection involves a three-link chain of events in which the first link is governed by cues from volatile plant chemicals, the central link by visual stimuli, and the final link by cues from non-volatile plant chemicals. The previously `missing' central link, which is based on what we have described as `appropriate/inappropriate landings', is governed by visual stimuli. Our theory explains why attempts to show that olfaction is the crucial component in the central link of host plant selection proved intractable. The `appropriate/inappropriate landings' theory is discussed to indicate the type of work needed in future studies to improve our understanding of how intercropping, undersowing and companion planting can be used to optimum effect in crop protection. The new theory is used also to suggest how insect biotypes could develop and to describe why pest insects do not decimate wild host plants growing in `natural' situations.     

Preference–performance relationship in the gall midge Rabdophaga rosaria: insights from a common‐garden experiment with nine willow clones

1. Oviposition preferences of herbivorous insects are predicted to match offspring performance on different host taxa or on conspecific host genotypes. In gall‐inducing insects, host‐plant properties such as growth rate and gall size, which are determined by plant genotype and growing conditions, may have a significant impact on offspring performance and, hence, should influence oviposition site selection. 2. The present study investigated host preference of the European rosette willow gall midge Rabdophaga rosaria (Loew) in relation to offspring success on seven clones of Salix myrsinifolia Salisb. and two naturally hybridised S. myrsinifolia × phylicifolia L. clones growing in a replicated design in an experimental field under two fertilisation regimes. For each clone, the average growth rate, number of shoot tips, and leaf and gall size were determined, and their effects on midge preference and larval survival were examined. 3. Main shoot height, number of shoot tips, and gall size were significantly affected by clone. The midges clearly preferred certain clones over the others, but preferences were not related to willow growth traits or to gall size. Survival probability was higher in large than in small galls, but females did not prefer large‐leaved clones that produced the biggest rosette galls. Midge oviposition was also uncorrelated with prior rates of leaf‐rust infection and with feeding preferences of voles and folivorous insects. 4. The weak preference–performance relationship of R. rosaria within S. myrsinifolia is probably explained by evolutionary constraints that prevent generalist insects from achieving an ability to discriminate among conspecific hosts of variable quality.     

Body-size control: how an insect knows it has grown enough

Insulin signaling controls organ growth and final body size in insects. Recent results have begun to clarify how insulin signaling drives organ growth to match nutrient levels, but have not yet elucidated how insulin signaling controls final body size.