A polyphenic trait is a trait for which multiple, discrete phenotypes can arise from a single genotype as a result of differing environmental conditions. It is therefore a special case of phenotypic plasticity.
There are several types of polyphenism in animals, from having sex determined by the environment to the castes of honey bees and other social insects. Some polyphenisms are seasonal, as in some butterflies which have different patterns during the year, and some Arctic animals like the snowshoe hare and Arctic fox, which are white in winter. Other animals have predator-induced or resource polyphenisms, allowing them to exploit variations in their environment. Some nematode worms can develop either into adults or into resting dauer larvae according to resource availability.
A polyphenism is the occurrence of several phenotypes in a population, the differences between which are not the result of genetic differences. [2] For example, crocodiles possess a temperature-dependent sex determining polyphenism, where sex is the trait influenced by variations in nest temperature. [3]
When polyphenic forms exist at the same time in the same panmictic (interbreeding) population they can be compared to genetic polymorphism. [4] With polyphenism, the switch between morphs is environmental, but with genetic polymorphism the determination of morph is genetic. These two cases have in common that more than one morph is part of the population at any one time. This is rather different from cases where one morph predictably follows another during, for instance, the course of a year. In essence the latter is normal ontogeny where young forms can and do have different forms, colours and habits to adults.
The discrete nature of polyphenic traits differentiates them from traits like weight and height, which are also dependent on environmental conditions but vary continuously across a spectrum. When a polyphenism is present, an environmental cue causes the organism to develop along a separate pathway, resulting in distinct morphologies; thus, the response to the environmental cue is “all or nothing.” The nature of these environmental conditions varies greatly, and includes seasonal cues like temperature and moisture, pheromonal cues, kairomonal cues (signals released from one species that can be recognized by another), and nutritional cues.
Sex-determining polyphenisms allow a species to benefit from sexual reproduction while permitting an unequal gender ratio. This can be beneficial to a species because a large female-to-male ratio maximizes reproductive capacity. However, temperature-dependent sex determination (as seen in crocodiles) limits the range in which a species can exist, and makes the species susceptible to endangerment by changes in weather pattern. [3] Temperature-dependent sex determination has been proposed as an explanation for the extinction of the dinosaurs. [5]
Population-dependent and reversible sex determination, found in animals such as the blue wrasse fish, have less potential for failure. In the blue wrasse, only one male is found in a given territory: larvae within the territory develop into females, and adult males will not enter the same territory. If a male dies, one of the females in his territory becomes male, replacing him. [5] While this system ensures that there will always be a mating couple when two animals of the same species are present, it could potentially decrease genetic variance in a population, for example if the females remain in a single male's territory.
The caste system of insects enables eusociality, the division of labor between non-breeding and breeding individuals. A series of polyphenisms determines whether larvae develop into queens, workers, and, in some cases soldiers. In the case of the ant, P. morrisi, an embryo must develop under certain temperature and photoperiod conditions in order to become a reproductively-active queen. [6] This allows for control of the mating season but, like sex determination, limits the spread of the species into certain climates. In bees, royal jelly provided by worker bees causes a developing larva to become a queen. Royal jelly is only produced when the queen is aging or has died. This system is less subject to influence by environmental conditions, yet prevents unnecessary production of queens.
Polyphenic pigmentation is adaptive for insect species that undergo multiple mating seasons each year. Different pigmentation patterns provide appropriate camouflage throughout the seasons, as well as alter heat retention as temperatures change. [7] Because insects cease growth and development after eclosion, their pigment pattern is invariable in adulthood: thus, a polyphenic pigment adaptation would be less valuable for species whose adult form survives longer than one year. [5]
Wet season | Dry season |
---|---|
upper side | upper side |
underside | underside |
Birds and mammals are capable of continued physiological changes in adulthood, and some display reversible seasonal polyphenisms, such as in the Arctic fox, which becomes all white in winter as snow camouflage. [5]
Predator-induced polyphenisms allow the species to develop in a more reproductively-successful way in a predator's absence, but to otherwise assume a more defensible morphology. However, this can fail if the predator evolves to stop producing the kairomone to which the prey responds. For example, the midge larvae (Chaoborus) that feed on Daphnia cucullata (a water flea) release a kairomone that Daphnia can detect. When the midge larvae are present, Daphnia grow large helmets that protect them from being eaten. However, when the predator is absent, Daphnia have smaller heads and are therefore more agile swimmers. [5]
Organisms with resource polyphenisms show alternative phenotypes that allow differential use of food or other resources. One example is the western spadefoot toad, which maximizes its reproductive capacity in temporary desert ponds. While the water is at a safe level, the tadpoles develop slowly on a diet of other opportunistic pond inhabitants. However, when the water level is low and desiccation is imminent, the tadpoles develop a morphology (wide mouth, strong jaw) that permits them to cannibalize. Cannibalistic tadpoles receive better nutrition and thus metamorphose more quickly, avoiding death as the pond dries up. [8]
Among invertebrates, the nematode Pristionchus pacificus has one morph that primarily feeds on bacteria and a second morph that produces large teeth, enabling it to feed on other nematodes, including competitors for bacterial food. In this species, cues of starvation and crowding by other nematodes, as sensed by pheromones, trigger a hormonal signal that ultimately activates a developmental switch gene that specifies formation of the predatory morph. [9]
Density-dependent polyphenism allows species to show a different phenotype based on the population density in which it was reared. In Lepidoptera, African armyworm larvae exhibit one of two appearances: the gregarious or solitary phase. Under crowded or "gregarious" conditions, the larvae have black bodies and yellow stripes along their bodies. However, under solitary conditions, they have green bodies with a brown stripe down their backs. The different phenotypes emerge during the third instar and remain until the last instar. [10]
Under conditions of stress such as crowding and high temperature, L2 larvae of some free living nematodes such as Caenorhabditis elegans can switch development to the so-called dauer larva state, instead of going the normal molts into a reproductive adult. These dauer larvae are a stress-resistant, non-feeding, long-lived stage, enabling the animals to survive harsh conditions. On return to favorable conditions, the animal resumes reproductive development from L3 stage onwards.
A mechanism has been proposed for the evolutionary development of polyphenisms: [7]
Evolution of novel polyphenisms through this mechanism has been demonstrated in the laboratory. Suzuki and Nijhout used an existing mutation (black) in a monophenic green hornworm ( Manduca sexta ) that causes a black phenotype. They found that if larvae from an existing population of black mutants were raised at 20˚C, then all the final instar larvae were black; but if the larvae were instead raised at 28˚C, the final instar larvae ranged in color from black to green. By selecting for larvae that were black if raised at 20˚C but green if raised at 28˚C, they produced a polyphenic strain after thirteen generations. [11]
This fits the model described above because a new mutation (black) was required to reveal pre-existing genetic variation and to permit selection. Furthermore, the production of a polyphenic strain was only possible because of background variation within the species: two alleles, one temperature-sensitive and one stable, were present for a single gene upstream of black (in the pigment production pathway) before selection occurred. The temperature-sensitive allele was not observable because at high temperatures, it caused an increase in green pigment in hornworms that were already bright green. However, introduction of the black mutant caused the temperature-dependent changes in pigment production to become obvious. The researchers could then select for larvae with the temperature-sensitive allele, resulting in a polyphenism.[ citation needed ]
The genotype–phenotype distinction is drawn in genetics. "Genotype" is an organism's full hereditary information. "Phenotype" is an organism's actual observed properties, such as morphology, development, or behavior. This distinction is fundamental in the study of inheritance of traits and their evolution.
A maternal effect is a situation where the phenotype of an organism is determined not only by the environment it experiences and its genotype, but also by the environment and genotype of its mother. In genetics, maternal effects occur when an organism shows the phenotype expected from the genotype of the mother, irrespective of its own genotype, often due to the mother supplying messenger RNA or proteins to the egg. Maternal effects can also be caused by the maternal environment independent of genotype, sometimes controlling the size, sex, or behaviour of the offspring. These adaptive maternal effects lead to phenotypes of offspring that increase their fitness. Further, it introduces the concept of phenotypic plasticity, an important evolutionary concept. It has been proposed that maternal effects are important for the evolution of adaptive responses to environmental heterogeneity.
The red-backed salamander is a small, hardy woodland salamander species in the family Plethodontidae. The species inhabits wooded slopes in eastern North America, west to Missouri, south to North Carolina, and north from southern Quebec and the Maritime provinces in Canada to Minnesota. It is also known as the redback salamander, eastern red-backed salamander, or the northern red-backed salamander to distinguish it from the southern red-backed salamander. It is one of 56 species in the genus Plethodon. Red-backed salamanders are notable for their color polymorphism and primarily display two color morph varieties, which differ in physiology and anti-predator behavior.
In biology, polymorphism is the occurrence of two or more clearly different morphs or forms, also referred to as alternative phenotypes, in the population of a species. To be classified as such, morphs must occupy the same habitat at the same time and belong to a panmictic population.
Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote. Sex determination is often distinct from sex differentiation, sex determination is the designation for the development stage towards either male or female while sex differentiation is the pathway towards the development of the phenotype.
Polygonia c-album, the comma, is a food generalist (polyphagous) butterfly species belonging to the family Nymphalidae. The angular notches on the edges of the forewings are characteristic of the genus Polygonia, which is why species in the genus are commonly referred to as anglewing butterflies. Comma butterflies can be identified by their prominent orange and dark brown/black dorsal wings.
Dauer describes an alternative developmental stage of nematode worms, particularly rhabditids including Caenorhabditis elegans, whereby the larva goes into a type of stasis and can survive harsh conditions. Since the entrance of the dauer stage is dependent on environmental cues, it represents a classic and well studied example of polyphenism. The dauer state is given other names in the various types of nematodes such as ‘diapause’ or ‘hypobiosis’, but since the C. elegans nematode has become the most studied nematode, the term ‘dauer stage’ or 'dauer larvae' is becoming universally recognised when referring to this state in other free-living nematodes. The dauer stage is also considered to be equivalent to the infective stage of parasitic nematode larvae.
The common side-blotched lizard is a species of side-blotched lizard in the family Phrynosomatidae. The species is native to dry regions of the western United States and northern Mexico. It is notable for having a unique form of polymorphism wherein each of the three different male morphs utilizes a different strategy in acquiring mates. The three morphs compete against each other following a pattern of rock paper scissors, where one morph has advantages over another but is outcompeted by the third.
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 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.
In biology, a cline is a measurable gradient in a single character of a species across its geographical range. First coined by Julian Huxley in 1938, the “character” of the cline referred to is usually genetic, or phenotypic. Clines can show smooth, continuous gradation in a character, or they may show more abrupt changes in the trait from one geographic region to the next.
Environmental sex determination is the establishment of sex by a non-genetic cue, such as nutrient availability, experienced within a discrete period after fertilization. Environmental factors which often influence sex determination during development or sexual maturation include light intensity and photoperiod, temperature, nutrient availability, and pheromones emitted by surrounding plants or animals. This is in contrast to genotypic sex determination, which establishes sex at fertilization by genetic factors such as sex chromosomes. Under true environmental sex determination, once sex is determined, it is fixed and cannot be switched again. Environmental sex determination is different from some forms of sequential hermaphroditism in which the sex is determined flexibly after fertilization throughout the organism’s life.
Arctia plantaginis, the wood tiger, is a moth of the family Erebidae. Several subspecies are found in the Holarctic ecozone south to Anatolia, Transcaucasus, northern Iran, Kazakhstan, Mongolia, China, Korea and Japan. One subspecies is endemic to North America.
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Nemoria arizonaria is a species of moth belonging to the family Geometridae. It was first described by Augustus Radcliffe Grote in 1883. It is indigenous to Arizona, New Mexico and the Davis Mountains in Texas.
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An alternative mating strategy is a strategy used by male or female animals, often with distinct phenotypes, that differs from the prevailing mating strategy of their sex. Such strategies are diverse and variable both across and within species. Animal sexual behaviour and mate choice directly affect social structure and relationships in many different mating systems, whether monogamous, polygamous, polyandrous, or polygynous. Though males and females in a given population typically employ a predominant reproductive strategy based on the overarching mating system, individuals of the same sex often use different mating strategies. Among some reptiles, frogs and fish, large males defend females, while small males may use sneaking tactics to mate without being noticed.
Pristionchus pacificus is a species of free-living nematodes (roundworms) in the family Diplogastridae. The species has been established as a satellite model organism to Caenorhabditis elegans, with which it shared a common ancestor 200–300 million years ago. The genome of P. pacificus has been fully sequenced, which in combination with other tools for genetic analysis make this species a tractable model in the laboratory, especially for studies of developmental biology.
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