Polyphenism

Abstract: SUMMARY Phenotypic plasticity is the primitive characterstate for most if not all traits. Insofar as developmental andphysiological processes obey the laws of chemistry andphysics, they will be sensitive to such environmental vari-ables as temperature, nutrient supply, ionic environment, andthe availability of various macro- and micronutrients. De-pending on the effect this phenotypic plasticity has on fitness,evolution may proceed to select either for mechanisms thatbuffer or canalize the phenotype against relevant environ- mental variation or for a modified plastic response in whichsome ranges of the phenotypic variation are adaptive to partic-ular environments. Phenotypic plasticity can be continuous, in which case it is called a reaction norm, or discontinuous,in which case it is called a polyphenism. Although the mor-phological discontinuity of some polyphenisms is producedby discrete developmental switches, most polyphenisms aredue to discontinuities in the environment that induce only por-tions of what is in reality a continuous reaction norm. In insectpolyphenisms, the environmental variable that induces the al-ternative phenotype is a token stimulus that serves as a pre-dictor of, but is not itself, the environment to which thepolyphenism is an adaptation. In all cases studied so far,the environmental stimulus alters the endocrine mechanismof metamorphosis by altering either the pattern of hormonesecretion or the pattern of hormone sensitivity in different tis-sues. Such changes in the patterns of endocrine interactionsresult in the execution of alternative developmental path-ways. The spatial and temporal compartmentalization ofendocrine interactions has produced a developmental mech-anism that enables substantial localized changes in morphol-ogy that remain well integrated into the structure and functionof the organism.INTRODUCTIONIn most organisms a genotype can produce many differentphenotypes. The exact phenotype that is expressed dependson the environment in which the organism develops. Pheno-typic plasticity can be gradual or discrete. Phenotypes thatchange with small gradual changes in an environmental vari-able are called reaction norms. In addition to these continu-ously variable phenotypes, some organisms can develop twoor more discrete alternative phenotypes, without intermedi-ate forms. This phenomenon is called polyphenism and cancome about in two ways: either when different members of aspecies experience discretely different environments (as inthe case of a bivoltine insect with discrete generations in dif-ferent seasons) or when a continuously variable environmentinduces a discrete threshold-like switch from one develop-mental pathway to another.The mechanisms that mediate these two types of pheno-typic plasticity are beginning to be understood. As we willsee below, the development of alternative phenotypes in re-action norms and polyphenisms can be caused by speciallyevolved mechanisms that are regulated by variation in thepatterns of hormone secretion. Reaction norms can also re-sult from the fact that the rates and timing of developmentalprocesses are affected by such environmental variables astemperature, nutrition, photoperiod, and so on. These envi-ronmental variables affect the underlying chemical and met-abolic processes of development directly, without the inter-vention of a specially evolved mechanism.

Abstract: Wing polyphenism in ants evolved once, 125 million years ago, and has been a key to their amazing evolutionary success. We characterized the expression of several genes within the network underlying the wing primordia of reproductive (winged) and sterile (wingless) ant castes. We show that the expression of several genes within the network is conserved in the winged castes of four ant species, whereas points of interruption within the network in the wingless castes are evolutionarily labile. The simultaneous evolutionary lability and conservation of the network underlying wing development in ants may have played an important role in the morphological diversification of this group and may be a general feature of polyphenic development and evolution in plants and animals.

Abstract: Wing polyphenism is an evolutionarily successful feature found in a wide range of insects. 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. Studies on aphids, crickets and planthoppers have revealed that alternative wing morphs develop in response to various environmental cues, 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. 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.

Abstract: Polyphenisms are adaptations in which a genome is associated with discrete alternative phenotypes in different environments. Little is known about the mechanism by which polyphenisms originate. We show that a mutation in the juvenile hormone-regulatory pathway in Manduca sexta enables heat stress to reveal a hidden reaction norm of larval coloration. Selection for increased color change in response to heat stress resulted in the evolution of a larval color polyphenism and a corresponding change in hormonal titers through genetic accommodation. Evidently, mechanisms that regulate developmental hormones can mask genetic variation and act as evolutionary capacitors, facilitating the origin of novel adaptive phenotypes.