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Patanga japonica (Bolívar) shows various body colors in the field. Most nymphs are green in the summer, but some develop non-green colors, such as yellow, white, brown, reddish, and black, in the fall. Nymphs individually reared in white, yellow-green, and black containers showed green, light-green, white, and reddish body colors, and the substrate color significantly influenced the proportions of green nymphs. A few individuals developed black spots and patterns, and such individuals were most frequently observed in the black containers. Nymphs with distinct black patterns were observed when reared in a group of five individuals per container, and the proportion of such individuals varied slightly depending on the brightness of the substrate color. Singly kept nymphs that were allowed to see five other nymphs in another container turned darker than those that were only allowed to see an empty container, suggesting that visual stimuli without mechanical stimulation induced black patterns. In outdoor cages, nymphs tended to develop more pronounced black patterns during their last instar when the hatching date was delayed and the temperature during the later stages of development was decreased. The effect of temperature during the late stadia was tested by transferring a group of third-stadium nymphs from outdoor cool conditions to a high temperature, while other nymphs were continuously maintained outdoors. Markedly melanized individuals were observed in the outdoor cage, whereas the appearance of such individuals was strongly suppressed at a high temperature. Green nymphs injected with synthetic [His⁷]-corazonin developed black patterns after ecdysis to the following instars and to the adult stage, and some looked indistinguishable in body color from group-reared nymphs. Nymphs injected with this hormone developed black patterns even at a high temperature. Adults looked similar in body coloration with some variation. Their hindwings turned reddish after overwintering. These results demonstrate that P. japonica exhibits body-color pholyphenism.

This theme issue pursues an exploration of the potential of taking into account the environmental sensitivity of development to explaining the evolution of metazoan life cycles, with special focus on complex life cycles and the role of developmental plasticity. The evolution of switches between alternative phenotypes as a response to different environmental cues and the evolution of the control of the temporal expression of alternative phenotypes within an organism's life cycle are here treated together as different dimensions of the complex relationships between genotype and phenotype, fostering the emergence of a more general and comprehensive picture of phenotypic evolution through a quite diverse sample of case studies. This introductory article reviews fundamental facts and concepts about phenotypic plasticity, adopting the most authoritative terminology in use in the current literature. The main topics are types and components of phenotypic variation, the evolution of organismal traits through plasticity, the origin and evolution of phenotypic plasticity and its adaptive value.

One hundred years ago in 1921, Sir Boris Uvarov recognized that two locust species are one species but appearing in two different phases, a solitarious and a gregarious phase. As locust swarms are still a big problem affecting millions of people, basic research has tried to understand the causes for the transition between phases. This phenomenon of phase polymorphism, now called polyphenism, is a very complex multifactorial process and this short review will draw attention to this important aspect of insect research.

Some insects have alternative wing morphs, one that is long-winged and changes habitat to follow resources, and one that is short-winged and flightless but has high fertility; here, the molecular details of this switch are revealed, with opposite effects of two insulin receptors controlling the development of different wing morphs in the planthopper. Insect double identity is insulin-linked Some types of insect can exist in two forms, both as long-winged morphs that can move from habitat to habitat to follow resources, and as short-winged flightless morphs with high fertility, but the molecular details of this switch have remained unclear. One species that leads this double life is the migratory brown planthopper Nilaparvata lugens , a serious pest in rice-growing regions of Asia. Chuan-Xi Zhang and colleagues show that long-wing versus short-wing development in N. lugens is controlled through the opposing effects of two insulin receptors, InR1 and InR2, on the activity of the forkhead transcription factor Foxo. Wing polyphenism is an evolutionarily successful feature found in a wide range of insects 1 . Long-winged morphs can fly, which allows them to escape adverse habitats and track changing resources, whereas short-winged morphs are flightless, but usually possess higher fecundity than the winged morphs 1 , 2 , 3 . Studies on aphids, crickets and planthoppers have revealed that alternative wing morphs develop in response to various environmental cues 1 , 2 , 4 , 5 , 6 , 7 , 8 , and that the response to these cues may be mediated by developmental hormones, although research in this area has yielded equivocal and conflicting results about exactly which hormones are involved 4 , 8 , 9 , 10 . As it stands, the molecular mechanism underlying wing morph determination in insects has remained elusive. Here we show that two insulin receptors in the migratory brown planthopper Nilaparvata lugens , InR1 and InR2, have opposing roles in controlling long wing versus short wing development by regulating the activity of the forkhead transcription factor Foxo. InR1, acting via the phosphatidylinositol-3-OH kinase (PI(3)K)–protein kinase B (Akt) signalling cascade, leads to the long-winged morph if active and the short-winged morph if inactive. InR2, by contrast, functions as a negative regulator of the InR1–PI(3)K–Akt pathway: suppression of InR2 results in development of the long-winged morph. The brain-secreted ligand Ilp3 triggers development of long-winged morphs. Our findings provide the first evidence of a molecular basis for the regulation of wing polyphenism in insects, and they are also the first demonstration—to our knowledge—of binary control over alternative developmental outcomes, and thus deepen our understanding of the development and evolution of phenotypic plasticity.

Eusocial insects can be defined as those that live in colonies and have distinct queens and workers. For most species, queens and workers arise from a common genome, and so caste-specific developmental trajectories must arise from epigenetic processes. In this review, we examine the epigenetic mechanisms that may be involved in the regulation of caste dimorphism. Early work on honeybees suggested that DNA methylation plays a causal role in the divergent development of queen and worker castes. This view has now been challenged by studies that did not find consistent associations between methylation and caste in honeybees and other species. Evidence for the involvement of methylation in modulating behaviour of adult workers is also inconsistent. Thus, the functional significance of DNA methylation in social insects remains equivocal. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

The scope of adaptive phenotypic change within a lineage is shaped by how functional traits evolve. Castes are defining functional traits of adaptive phenotypic change in complex insect societies, and caste evolution is expected to be phylogenetically conserved and developmentally constrained at broad phylogenetic scales. Yet how castes evolve at the species level has remained largely unaddressed. Turtle ant soldiers (genus Cephalotes), an iconic example of caste specialization, defend nest entrances by using their elaborately armored heads as living barricades. Across species, soldier morphotype determines entrance specialization and defensive strategy, while head size sets the specific size of defended entrances. Our species-level comparative analyses of morphotype and head size evolution reveal that these key ecomorphological traits are extensively reversible, repeatable, and decoupled within soldiers and between soldier and queen castes. Repeated evolutionary gains and losses of the four morphotypes were reconstructed consistently across multiple analyses. In addition, morphotype did not predict mean head size across the three most common morphotypes, and head size distributions overlapped broadly across all morphotypes. Concordantly, multiple model-fitting approaches suggested that soldier head size evolution is best explained by a process of divergent pulses of change. Finally, while soldier and queen head size were broadly coupled across species, the level of head size disparity between castes was decoupled from both queen head size and soldier morphotype. These findings demonstrate that caste evolution can be highly dynamic at the species level, reshaping our understanding of adaptive morphological change in complex social lineages.

The English grain aphid, Sitobion avenae (Fabricius), exhibits classic and dramatic phenotypic plasticity in wing development. Both genetic and environmental inputs contribute to the wing polyphenism in aphids, an extreme form of phenotypic plasticity in which a single genotype produces discrete winged and wingless morphs. Validated reference genes are needed to accurately normalize temporal and spatial variation in gene expression estimates by RT-qPCR. In this research, the stability of 11 candidate reference genes selected from S. avenae transcriptomes was evaluated under an array of abiotic and biotic conditions relevant to wing development. RefFinder, a comprehensive software integrating rankings from delta Ct, BestKeeper, NormFinder, and geNorm, offered a series of reference genes for every experimental condition. Overall, helicase (HEL) and ubiquitin ribosomal protein S27A fusion protein (RpS27) are suited for most of the conditions examined in this study, although exceptions do exist. Specifically, NADH dehydrogenase (Ap-NADH) and 28S ribosomal RNA (28S) are recommended for insecticide and antibiotic treatments, while ribosomal RNA L14 (RPL14) and 18S ribosomal RNA (18S) are selected for density treatment, respectively. This study provides a suite of reference genes to investigate the wing polyphenism in S. avenae, and is important for application of RT-qPCR in future experiments of novel tactics to control aphids.

In angiosperms, there are two pathways of reproduction through seeds: sexual, or amphimictic, and asexual, or apomictic. The essential feature of apomixis is that an embryo in an ovule is formed autonomously. It may form from a cell of the nucellus or integuments in an otherwise sexual ovule, a process referred to as adventitious embryony. Alternatively, the embryo may form by parthenogenesis from an unreduced egg that forms in an unreduced embryo sac. The latter may form from an ameiotic megasporocyte, in which case it is referred to as diplospory, or from a cell of the nucellus or integument, in which case it is referred to as apospory. Progeny of apomictic plants are generally identical to the mother plant. Apomixis has been seen over the years as either a gain- or loss-of-function over sexuality, implying that the latter is the default condition. Here, we consider an additional point of view, that apomixis may be anciently polyphenic with sex and that both reproductive phenisms involve anciently canalized components of complex molecular processes. This polyphenism viewpoint suggests that apomixis fails to occur in obligately sexual eukaryotes because genetic or epigenetic modifications have silenced the primitive sex apomixis switch and/or disrupted molecular capacities for apomixis. In eukaryotes where sex and apomixis are clearly polyphenic, apomixis exponentially drives clonal fecundity during reproductively favorable conditions, while stress induces sex for stresstolerant spore or egg formation. The latter often guarantees species survival during environmentally harsh seasons.

Abstract Phenotypic plasticity is defined as the property of organisms to produce distinct phenotypes in response to environmental variation. While for more than a century, biologists have proposed this organismal feature... Phenotypic plasticity is defined as the property of organisms to produce distinct phenotypes in response to environmental variation. While for more than a century, biologists have proposed this organismal feature to play an important role in evolution and the origin of novelty, the idea has remained contentious. Plasticity is found in all domains of life, but only recently has there been an increase in empirical studies. This contribution is intended as a fresh view and will discuss current and future challenges of plasticity research, and the need to identify associated molecular mechanisms. After a brief summary of conceptual, theoretical, and historical aspects, some of which were responsible for confusion and contention, I will formulate three major research directions and predictions for the role of plasticity as a facilitator of novelty. These predictions result in a four-step model that, when properly filled with molecular mechanisms, will reveal plasticity as a major factor of evolution. Such mechanistic insight must be complemented with comparative investigations to show that plasticity has indeed created novelty and innovation. Together, such studies will help develop a true developmental evolutionary biology.

Developmental plasticity generates phenotypic variation, but how it contributes to evolutionary change is unclear. Phenotypes of individuals in caste-based (eusocial) societies are particularly sensitive to developmental processes, and the evolutionary origins of eusociality may be rooted in developmental plasticity of ancestral forms. We used an integrative genomics approach to evaluate the relationships among developmental plasticity, molecular evolution, and social behavior in a bee species (Megalopta genalis) that expresses flexible sociality, and thus provides a window into the factors that may have been important at the evolutionary origins of eusociality. We find that differences in social behavior are derived from genes that also regulate sex differentiation and metamorphosis. Positive selection on social traits is influenced by the function of these genes in development. We further identify evidence that social polyphenisms may become encoded in the genome via genetic changes in regulatory regions, specifically in transcription factor binding sites. Taken together, our results provide evidence that developmental plasticity provides the substrate for evolutionary novelty and shapes the selective landscape for molecular evolution in a major evolutionary innovation: Eusociality.