Is melanism in pygmy grasshoppers induced by crowding?

Color polymorphisms in animals may result from plasticity of the developmental system in response to genetic cues in the form of allelic variation at polymorphic loci, environmental cues, or a combination of genetic and environmental cues. An increased understanding of the evolution of color polymorphisms requires better knowledge of when we should expect genetic and environmental cues respectively to influence phenotype determination. Theory posits that the developmental systems of organisms should evolve sensitivity to such cues that most accurately predict coming selective conditions. Pygmy grasshoppers (Orthoptera, Tetrigidae) vary in color pattern within and among populations and show fire melanism, i.e., an increased frequency of black and dark colored phenotypes in high density populations inhabiting fire-ravaged areas. We examined if the population density experienced by individuals during development influenced the phenotypic expression of color pattern in Tetrix subulata. Individuals were experimentally reared either in solitude, at intermediate density or under crowded conditions. We found that color patterns of experimental individuals were independent of rearing density but strongly influenced by maternal color pattern. High population density and crowding may not constitute reliable predictors of the selective regime that characterizes post-fire environments.     

Environmental and genetic cues in the evolution of phenotypic polymorphism

Phenotypic polymorphism is a consequence of developmental plasticity, in which the trajectories of developing organisms diverge under the influence of cues. Environmental and genetic phenotype determination are the two main categories of polymorphic development. Even though both may evolve as a response to varied environments, they are traditionally regarded as fundamentally distinct phenomena. They can however be joined into a single framework that emphasizes the parallel roles of environmental and genetic cues in phenotype determination. First, from the point of view of immediate causation, it is common that phenotypic variants can be induced either by environmental or by allelic variation, and this is referred to as gene-environment interchangeability. Second, from the point of view of adaptation, genetic cues in the form of allelic variation at polymorphic loci can play similar roles as environmental cues in providing information to the developmental system about coming selective conditions. Both types of cues can help a developing organism to fit its phenotype to selective circumstances. This perspective of information in environmental and genetic cues can produce testable hypotheses about phenotype determination, and can thus increase our understanding of the evolution of phenotypic polymorphism.     

The Information Value of Non-Genetic Inheritance in Plants and Animals

Parents influence the development of their offspring in many ways beyond the transmission of DNA. This includes transfer of epigenetic states, nutrients, antibodies and hormones, and behavioural interactions after birth. While the evolutionary consequences of such non-genetic inheritance are increasingly well understood, less is known about how inheritance mechanisms evolve. Here, we present a simple but versatile model to explore the adaptive evolution of non-genetic inheritance. Our model is based on a switch mechanism that produces alternative phenotypes in response to different inputs, including genes and non-genetic factors transmitted from parents and the environment experienced during development. This framework shows how genetic and non-genetic inheritance mechanisms and environmental conditions can act as cues by carrying correlational information about future selective conditions. Differential use of these cues is manifested as different degrees of genetic, parental or environmental morph determination. We use this framework to evaluate the conditions favouring non-genetic inheritance, as opposed to genetic determination of phenotype or within-generation plasticity, by applying it to two putative examples of adaptive non-genetic inheritance: maternal effects on seed germination in plants and transgenerational phase shift in desert locusts. Our simulation models show how the adaptive value of non-genetic inheritance depends on its mechanism, the pace of environmental change, and life history characteristics.     

Do genetic structure and landscape heterogeneity impact color morph frequency in a polymorphic salamander?

Landscape heterogeneity plays an important role in population structure and divergence, particularly for species with limited vagility. Here, we used a landscape genetic approach to identify how landscape and environmental variables affect genetic structure and color morph frequency in a polymorphic salamander. The eastern red‐backed salamander, Plethodon cinereus, is widely distributed in northeastern North America and contains two common color morphs, striped and unstriped, that are divergent in ecology, behavior, and physiology. To quantify population structure, rates of gene flow, and genetic drift, we amplified 10 microsatellite loci from 648 individuals across 28 sampling localities. This study was conducted in northern Ohio, where populations of P. cinereus exhibit an unusually wide range of morph frequency variation. To test whether genetic distance was more correlated with morph frequency, elevation, canopy cover, waterways, ecological niche or geographic distance, we used resistance distance and least cost path analyses. We then examined whether landscape and environmental variables, genetic distance or geographic distance were correlated with variation in morph frequency. Tests for population structure revealed three genetic clusters across our sampling range, with one cluster monomorphic for the striped morph. Rates of gene flow and genetic drift were low to moderate across sites. Genetic distance was most correlated with ecological niche, elevation and a combination of landscape and environmental variables. In contrast, morph frequency variation was correlated with waterways and geographic distance. Thus, our results suggest that selection is also an important evolutionary force across our sites, and a balance between gene flow, genetic drift and selection interact to maintain the two color morphs.     

The evolution of environmental and genetic sex determination in fluctuating environments

Twenty years ago, Bulmer and Bull suggested that disruptive selection, produced by environmental fluctuations, can result in an evolutionary transition from environmental sex determination (ESD) to genetic sex determination (GSD). We investigated the feasibility of such a process, using mutation-limited adaptive dynamics and individual-based computer simulations. Our model describes the evolution of a reaction norm for sex determination in a metapopulation setting with partial migration and variation in an environmental variable both within and between local patches. The reaction norm represents the probability of becoming a female as a function of environmental state and was modeled as a sigmoid function with two parameters, one giving the location (i.e., the value of the environmental variable for which an individual has equal chance of becoming either sex) and the other giving the slope of the reaction norm for that environment. The slope can be interpreted as being set by the level of developmental noise in morph determination, with less noise giving a steeper slope and a more switchlike reaction norm. We found convergence stable reaction norms with intermediate to large amounts of developmental noise for conditions characterized by low migration rates, small differential competitive advantages between the sexes over environments, and little variation between individual environments within patches compared to variation between patches. We also considered reaction norms with the slope parameter constrained to a high value, corresponding to little developmental noise. For these we found evolutionary branching in the location parameter and a transition from ESD toward GSD, analogous to the original analysis by Bulmer and Bull. Further evolutionary change, including dominance evolution, produced a polymorphism acting as a GSD system with heterogamety. Our results point to the role of developmental noise in the evolution of sex determination.     

Phenotypic plasticity in insects: the effects of substrate color on the coloration of two ground-hopper species

SUMMARY The question of how phenotypic variation is maintained within populations has long been a central issue in evolutionary biology. Most of these studies focused on the maintenance of genetic variability, but the phenotype of organisms may also be influenced by environmental cues experienced during ontogeny. Color polymorphism has received particular attention in evolutionary studies as it has strong fitness consequences. However, if body coloration is influenced by the environment, any conclusions on evolutionary consequences of fitness trade-offs can be misleading. Here we present data from a laboratory experiment on the influence of substrate color on three aspects of the coloration of two ground-hopper species, Tetrix subulata and Tetrix ceperoi . We reared hatchlings either on dark or on light substrates, using a split-brood design. Although the type of pronotal pattern changed mainly in response to nymphal development, the basic color was strongly influenced by the substrate color. In both species, black and dark olive color morphs were found more frequently on the dark substrate, whereas the gray color morph dominated on the light substrate. These findings have considerable implications for our understanding of color morph evolution as they show that color polymorphism may not only be maintained by natural selection acting on discrete color morphs, but also by phenotypic plasticity, which enables organisms to adjust to the environmental conditions experienced during ontogeny. This facultative morphology is opposing to the prevailing view of color morph adaptation, which assumes a purely genetic determination and co-evolution of discrete color morphs with life history traits.     

Ecological developmental biology: environmental signals for normal animal development

The environment plays instructive roles in development and selective roles in evolution. This essay reviews several of the instructive roles whereby the organism has evolved to receive cues from the environment in order to modulate its developmental trajectory. The environmental cues can be abiotic (such as temperature or photoperiod) or biotic (such as those emanating from predators, conspecifics, or food), and the “alteration” produces a normal, not a pathological, phenotype, that is appropriate for the environment. In addition, symbiotic organisms can produce important signals during normal development. Environmental cues can be obligatory, such that the organism cannot develop without the environmental cue. These cues often permit and instruct the organism to proceed from one developmental stage to another, as when larvae receive cues to settle and undergo metamorphosis from substrates. Such obligatory cues can also be given by symbionts, as when Wolbachia bacteria prevent apoptosis in developing ovaries of some wasps. Other environmental cues can be used facultatively, allowing organisms to follow different developmental trajectories depending on whether the cue is present or not. This can be seen in the temperature‐dependent determination of sex in many reptiles and in the determination of thermotolerance in aphids by their symbiotic bacteria. Signaling from the environment is essential in development, and co‐development appears to be normative between symbionts and their hosts. Here, one sees the reciprocal induction of gene expression, just as within the embryonic organism. The ability of organisms to respond to environmental cues by producing different phenotypes may be critically important in evolution, and it may be an essential feature that can facilitate or limit evolution.     

The mechanisms of morph determination in the amphipod Jassa: implications for the evolution of alternative male phenotypes

The proximal basis for and the maintenance of alternative male reproductive strategies and tactics are generally not understood in most species, despite the occurrence of male polymorphism across many taxa. In the marine amphipod Jassa marmorata, males differ in morphology as well as behaviour. This dimorphism corresponds to two contrasting reproductive strategies: small sneaker males or 'minors', and large fighter males or 'majors'. This study uses quantitative genetic analyses in conjunction with experimental manipulations to assess the relative importance of genetic versus environmental factors in the determination and maintenance of these alternative mating strategies. Heritability analyses indicated the reproductive phenotypes do not reflect genetic differences between dimorphic males. By contrast, morph determination was significantly affected by diet quality. Majors essentially only developed on high-protein diets. Field studies also identified a strong correlation between seasonal shifts in the relative proportions of morphs and changes in food (i.e. phytoplankton) quantity and composition, corroborating that diet cues the switch between alternative reproductive tactics. Moreover, the comparison of major and minor growth trajectories identified a heterochronic shift in maturation times between morphs, indicating that ecological selective pressures, rather than just sexual selection, may be involved in the maintenance of this conditional strategy.     

Aphid wing dimorphisms: linking environmental and genetic control of trait variation

Both genetic and environmental factors underlie phenotypic variation. While research at the interface of evolutionary and developmental biology has made excellent advances in understanding the contribution of genes to morphology, less well understood is the manner in which environmental cues are incorporated during development to influence the phenotype. Also virtually unexplored is how evolutionary transitions between environmental and genetic control of trait variation are achieved. Here, I review investigations into molecular mechanisms underlying phenotypic plasticity in the aphid wing dimorphism system. Among aphids, some species alternate between environmentally sensitive (polyphenic) and genetic (polymorphic) control of wing morph determination in their life cycle. Therefore, a traditional molecular genetic approach into understanding the genetically controlled polymorphism may provide a unique avenue into not only understanding the molecular basis of polyphenic variation in this group, but also the opportunity to compare and contrast the mechanistic basis of environmental and genetic control of similar dimorphisms.     

How accurate is the phenotype? – An analysis of developmental noise in a cotton aphid clone

Background  

The accuracy by which phenotype can be reproduced by genotype potentially is important in determining the stability, environmental sensitivity, and evolvability of morphology and other phenotypic traits. Because two sides of an individual represent independent development of the phenotype under identical genetic and environmental conditions, average body asymmetry (or "fluctuating asymmetry") can estimate the developmental instability of the population. The component of developmental instability not explained by intrapopulational differences in gene or environment (or their interaction) can be further defined as internal developmental noise. Surprisingly, developmental noise remains largely unexplored despite its potential influence on our interpretations of developmental stability, canalization, and evolvability. Proponents of fluctuating asymmetry as a bioindicator of environmental or genetic stress, often make the assumption that developmental noise is minimal and, therefore, that phenotype can respond sensitively to the environment. However, biologists still have not measured whether developmental noise actually comprises a significant fraction of the overall environmental response of fluctuating asymmetry observed within a population.     

A sex-linked locus controls wing polymorphism in males of the pea aphid,Acyrthosiphon pisum (Harris)

Discrete variation in wing morphology is a very common phenomenon in insects and has been used extensively in the past 50 years as a model to study the ecology and evolution of dispersal. Wing morph determination can be purely genetic, purely environmental, or some combination of the two. The precise genetic determinants of genetically based wing morph variation are unknown. Here we explore the genetic basis of wing polymorphism in the pea aphid, which can produce either winged or wingless males. We confirm that three types of pea aphid clones coexist in natural populations, those producing winged males only, those producing wingless males only, and those producing a mixture of both. A Mendelian genetic analysis reveals that male wing polymorphism in pea aphids is determined by a single locus, two alleles system. Using microsatellite loci of known location, we show that this locus is on the X chromosome. The existence of a simple genetic determinism for wing polymorphism in a system in which genetic investigation is possible may help investigations on the physiological and molecular mechanisms of genetically-based wing morph variation. This locus could also be used in the search for genes involved in the wing polyphenism described in parthenogenetic females and to investigate the interplay between polymorphisms and polyphenisms.     

Stark sexual display divergence among jumping spider populations in the face of gene flow

Gene flow can inhibit evolutionary divergence by eroding genetic differences between populations. A current aim in speciation research is to identify conditions in which selection overcomes this process. We focused on a state of limited differentiation, asking whether selection enables divergence with gene flow in a set of Habronattus americanus jumping spider populations that exhibit three distinct male sexual display morphs. We found that each population is at high frequency or fixed for a single morph. These strong phenotypic differences contrast with low divergence at 210 AFLP markers, suggesting selection has driven or maintains morph divergence. Coinciding patterns of isolation by distance and ‘isolation by phenotype’ (i.e. increased genetic divergence among phenotypically contrasting populations) across the study area support several alternative demographic hypotheses for display divergence, each of which entails gene flow. Display‐associated structure appears broadly distributed across the genome and the markers producing this pattern do not stand out from background levels of differentiation. Overall, the results suggest selection can promote stark sexual display divergence in the face of gene flow among closely related populations.     

Cumulative frequency-dependent selective episodes allow for rapid morph cycles and rock-paper-scissors dynamics in species with overlapping generations

Rock-paper-scissors (RPS) dynamics, which maintain genetic polymorphisms over time through negative frequency-dependent (FD) selection, can evolve in short-lived species with no generational overlap, where they produce rapid morph frequency cycles. However, most species have overlapping generations and thus, rapid RPS dynamics are thought to require stronger FD selection, the existence of which yet needs to be proved. Here, we experimentally demonstrate that two cumulative selective episodes, FD sexual selection reinforced by FD selection on offspring survival, generate sufficiently strong selection to generate rapid morph frequency cycles in the European common lizard Zootoca vivipara, a multi-annual species with major generational overlap. These findings show that the conditions required for the evolution of RPS games are fulfilled by almost all species exhibiting genetic polymorphisms and suggest that RPS games may be responsible for the maintenance of genetic diversity in a wide range of species.     

Gene expression and the evolution of insect polyphenisms

Polyphenic differences between individuals arise not through differences at the genome level but as a result of specific cues received during development. Polyphenisms often involve entire suites of characters, as shown dramatically by the polyphenic castes found in many social insect colonies. An understanding of the genetic architecture behind polyphenisms provides a novel means of studying the interplay between genomes, gene expression and phenotypes. Here we discuss polyphenisms and molecular genetic tools now available to unravel their developmental bases in insects. We focus on several recent studies that have tracked gene-expression patterns during social insect caste determination. BioEssays 23:62-68, 2001. Published 2001 John Wiley & Sons, Inc.     

植物开花时间：自然变异与遗传分化

开花时间是植物的重要生活史性状。对模式植物的研究表明： 从感受内外环境信号开始到最终分化形成功能性花器官的过程涉及复杂的信号转导途径和调控网络; 开花时间受多种因子的调控, 而FT基因作为整合途径成分起到非常关键的作用。植物的花期变异在物种、群体和个体水平上具有复杂的自然变异模式, 且不同植物的花期变异随全球环境变化而具有不同的变异趋势。植物个体之间通过传粉进行的基因交流需要功能性开花时间的一致或重叠, 而花期变异会导致群体之间或群体内部亚群体之间的基因流障碍和遗传分化, 并可能导致邻域或同域的物种形成。该文分析了植物花期变异与群体遗传分化的关系, 认为决定开花时间的基因在物种分化中可能起到关键的作用, 而对开花时间自然变异模式的研究对于揭示晚近分化快速辐射物种的进化模式具有重要意义。     

NEGATIVE FREQUENCY‐DEPENDENT SELECTION IN FEMALE COLOR POLYMORPHISM OF A DAMSELFLY

Negative frequency‐dependent selection (NFDS) is one of the most powerful selective forces maintaining genetic polymorphisms in nature. Recently many prospective cases of polymorphisms by NFDS have been reported. Some of them are very complicated, although strongly supportive of the NFDS. Here we investigate NFDS in wild populations of the dimorphic damselfly Ischnura senegalensis, in which females occur as andromorphs and gynomorphs. Specifically, we (1) test fitness responses to morph frequencies, (2) built a simple population genetic model, and (3) compare the observed and predicted morph‐frequency dynamics. Fitnesses of the two morphs are an inverse function of its own frequency in a population, and are about equal when their frequencies are similar. Thus the conditions necessary for NFDS are satisfied. The long‐term field surveys show that the morph frequencies oscillate with a period of two generations. Morph frequencies in a small population undergo large oscillations whereas those in a large population do small oscillations. The demographic properties of the observed dynamics agree well with those of our model. This example is one of the simplest confirmed cases of NFDS maintaining genetic polymorphisms in nature.     

Levels of selection on threshold characters

Threshold models are useful for understanding the evolution of dimorphic traits with polygenic bases. Selection for threshold characters on individuals is expected to be frequency dependent because of the peculiar way that selection views underlying genetic and environmental factors. Selection among individuals is inefficient because individual phenotypes fall into only two discrete categories that map imperfectly to the underlying genes. Incidence, however, can be continuously distributed among groups, making among-group selection relatively more efficient. Differently put, the group-mean phenotype can be a better predictor of an individual's genotype than that individual's own phenotype. Because evolution in group-structured populations is governed by the balance of selection within and between groups, we can expect threshold traits to evolve in fundamentally different ways when group mean fitness is a function of morph frequency. We extend the theory of selection on threshold traits to include group selection using contextual analysis. For the simple case of linear group-fitness functions, we show that the group-level component of selection, like the individual-level component, is frequency dependent. However, the conditions that determine which component dominates when levels of selection are in conflict (as described by Hamilton's rule) are not frequency dependent. Thus, enhanced group selection is not an inherent property of threshold characters. Nevertheless, we show that predicting the effects of multiple levels of selection on dimorphic traits requires special considerations of the threshold model.