Phenotypic plasticity

Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompasses all types of environmentally induced changes (e.g. morphological, physiological, behavioural, phenological) that may or may not be permanent throughout an individual's lifespan. The term was originally used to describe developmental effects on morphological characters, but is now more broadly used to describe all phenotypic responses to environmental change, such as acclimation (acclimatization), as well as learning. The special case when differences in environment induce discrete phenotypes is termed polyphenism. Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompasses all types of environmentally induced changes (e.g. morphological, physiological, behavioural, phenological) that may or may not be permanent throughout an individual's lifespan. The term was originally used to describe developmental effects on morphological characters, but is now more broadly used to describe all phenotypic responses to environmental change, such as acclimation (acclimatization), as well as learning. The special case when differences in environment induce discrete phenotypes is termed polyphenism. Generally, phenotypic plasticity is more important for immobile organisms (e.g. plants) than mobile organisms (e.g. most animals), as mobile organisms can often move away from unfavourable environments. Nevertheless, mobile organisms also have at least some degree of plasticity in at least some aspects of the phenotype. One mobile organism with substantial phenotypic plasticity is Acyrthosiphon pisum of the aphid family, which exhibits the ability to interchange between asexual and sexual reproduction, as well as growing wings between generations when plants become too populated. Phenotypic plasticity in plants includes the timing of transition from vegetative to reproductive growth stage, the allocation of more resources to the roots in soils that contain low concentrations of nutrients, the size of the seeds an individual produces depending on the environment, and the alteration of leaf shape, size, and thickness. Leaves are particularly plastic, and their growth may be altered by light levels. Leaves grown in the light tend to be thicker, which maximizes photosynthesis in direct light; and have a smaller area, which cools the leaf more rapidly (due to a thinner boundary layer). Conversely, leaves grown in the shade tend to be thinner, with a greater surface area to capture more of the limited light. Dandelion are well known for exhibiting considerable plasticity in form when growing in sunny versus shaded environments. The transport proteins present in roots also change depending on the concentration of the nutrient and the salinity of the soil. Some plants, Mesembryanthemum crystallinum for example, are able to alter their photosynthetic pathways to use less water when they become water- or salt-stressed. Because of phenotypic plasticity, it is hard to explain and predict the traits when plants are grown in natural conditions unless a explicit environment index can be obtained to quantify environments. Identification of photothermal time from a critical growth periods being highly correlated with sorghum flowering time enables such predictions. Leaves are very important to a plant in that they create an avenue where photosynthesis and thermoregulation can occur. Evolutionarily, the environmental contribution to leaf shape allowed for a myriad of different types of leaves to be created in the means of balancing energy production with the plants’ fitness. The shape of the leaf is important and not only depends on genetics, but also the environment that the plant lives in. Environmental factors, such as light and humidity have been shown to affect leaf morphology, giving rise to the question of how this shape change is controlled at the molecular level. Genotypes often give rise to specific phenotypes, however leaves have phenotypic plasticity. This means that different leaves could have the same gene but present a different form every time based on environmental factors. Plants are sessile beings so this phenotypic plasticity allows the plant to take in information from its environment and respond without changing its location and simultaneously increasing its fitness. In order to understand how leaf morphology works, the anatomy of a leaf must be understood. The main part of the leaf, the blade or lamina, consists of the epidermis, mesophyll, and vascular tissue. The epidermis contains stomata which allows for gas exchange and controls perspiration of the plant. The mesophyll contains most of the chloroplast where photosynthesis can occur. Developing a wide blade/lamina can maximize the amount of light hitting the leaf, thereby increasing photosynthesis, however too much sunlight can damage the plant. Wide lamina can also catch wind easily which can cause stress to the plant, so finding a happy medium is imperative to the plants’ fitness. The Genetic Regulatory Network (GRN) is responsible for creating this phenotypic plasticity and involves a variety of genes and proteins regulating leaf morphology.Phytohormones have been shown to play a key role in signaling throughout the plant, and changes in concentration of the phytohormones can cause a change in development. Studies on the aquatic plant species Ludwigia arcuata have been done to look at the role of Abscisic acid (ABA), as L. arcuata is known to exhibit phenotypic plasticity and has two different types of leaves, the aerial type (leaves that touch the air) and the submerged type (leaves that are underwater). ABA is a phytohormone responsible for regulating stress responses and is known to be specifically involved in drought stress response. When adding ABA to the underwater shoots of L. arcuata, the plant was able to produce aerial type leaves underwater, suggesting that increased concentrations of ABA in the shoots, likely caused by air contact or a lack of water, triggers the change from the submerged type of leaf to the aerial type. This suggests ABA’s role in leaf phenotypic change and its importance in regulating stress through environmental change (such as adapting from being underwater to above water). In the same study, another phytohormone, ethylene, was shown to induce the submerged leaf phenotype unlike ABA, which induced aerial leaf phenotype. Because ethylene is a gas, it tends to stay endogenously within the plant when underwater – this growth in concentration of ethylene induces a change from aerial to submerged leaves and has also been shown to inhibit ABA production, further increasing the growth of submerged type leaves. These factors (temperature, water availability, and phytohormones) contribute to changes in leaf morphology throughout a plants lifetime and are vital to maximize plant fitness. The developmental effects of nutrition and temperature have been demonstrated. The gray wolf (Canis lupus) has wide phenotypic plasticity. Additionally, male speckled wood butterflies have two morphs: one with three dots on its hindwing, and one with four dots on its hindwings. The development of the fourth dot is dependent on environmental conditions – more specifically, location and the time of year. In amphibians, Pristimantis mutabilis has remarkable phenotypic plasticity. Another example is the southern rockhopper penguin. Rockhopper penguins are present at a variety of climates and locations; Amsterdam Island's subtropical waters, Kerguelen Archipelago's subarctic coastal waters, and Crozet Archipelago's subantarctic coastal waters. Due to the species plasticity they are able to express different strategies and foraging behaviors depending on the climate and environment. A main factor that has influenced the species' behavior is where food is located. Plastic responses to temperature are essential among ectothermic organisms, as all aspects of their physiology are directly dependent on their thermal environment. As such, thermal acclimation entails phenotypic adjustments that are found commonly across taxa, such as changes in the lipid composition of cell membranes. Temperature change influences the fluidity of cell membranes by affecting the motion of the fatty acyl chains of glycerophospholipids. Because maintaining membrane fluidity is critical for cell function, ectotherms adjust the phospholipid composition of their cell membranes such that the strength of van der Waals forces within the membrane is changed, thereby maintaining fluidity across temperatures. Phenotypic plasticity of the digestive system allows some animals to respond to changes in dietary nutrient composition, diet quality, and energy requirements. Changes in the nutrient composition of the diet (the proportion of lipids, proteins and carbohydrates) may occur during development (e.g. weaning) or with seasonal changes in the abundance of different food types. These diet changes can elicit plasticity in the activity of particular digestive enzymes on the brush border of the small intestine. For example, in the first few days after hatching, nestling house sparrows (Passer domesticus) transition from an insect diet, high in protein and lipids, to a seed based diet that contains mostly carbohydrates; this diet change is accompanied by two-fold increase in the activity of the enzyme maltase, which digests carbohydrates. Acclimatizing animals to high protein diets can increase the activity of aminopeptidase-N, which digests proteins.