A polyphenic trait is a trait for which multiple, discrete phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

s can arise from a single genotype

Genotype

The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...

 as a result of differing environmental conditions.

Definition

A polyphenism is a biological mechanism that causes a trait to be polyphenic. For example, crocodiles possess a sex-determining polyphenism, and therefore their gender is a polyphenic trait.

When polyphenic forms exist at the same time in the same panmictic (interbreeding) population they can be compared to genetic polymorphism

Polymorphism (biology)

Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...

. With polyphenism the switch between morphs is environmental, but with genetic polymorphism with 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

Ontogeny

Ontogeny is the origin and the development of an organism – for example: from the fertilized egg to mature form. It covers in essence, the study of an organism's lifespan...

 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

Pheromone

A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

 cues, kairomonal

Kairomone

A kairomone is a semiochemical, emitted by an organism, which mediates interspecific interactions in a way that benefits an individual of another species which receives it, without benefiting the emitter. This "eavesdropping" is often disadvantageous to the producer...

 cues (signals released from one species that can be recognized by another), and nutritional cues.

Sex determination

Sex-determining polyphenisms allow a species to benefit from sexual reproduction

Sexual reproduction

Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...

 while permitting an unequal gender

Gender

Gender is a range of characteristics used to distinguish between males and females, particularly in the cases of men and women and the masculine and feminine attributes assigned to them. Depending on the context, the discriminating characteristics vary from sex to social role to gender identity...

 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. Temperature-dependent sex determination has been proposed as an explanation for the extinction of the dinosaurs.

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: larva

Larva

A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

e 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. 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. Furthermore, this system is inherently unstable on a small scale because a single mutation

Mutation

In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

 causing a fish to remain permanently male would spread quickly through the population (due to high female availability) and might eventually cause loss of females in the species, and therefore extinction

Extinction

In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

.

The caste system in insects

The caste system of insects enables eusociality

Eusociality

Eusociality is a term used for the highest level of social organization in a hierarchical classification....

, 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

Ant

Ants are social insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from wasp-like ancestors in the mid-Cretaceous period between 110 and 130 million years ago and diversified after the rise of flowering plants. More than...

, P. morrisi, an embryo must develop under certain temperature and photoperiod conditions in order to become a reproductively-active queen. This allows for control of the mating season, but like sex determination, limits the spread of the species into certain climate

Climate

Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

s.
In bees, royal jelly

Royal jelly

Royal jelly is a honey bee secretion that is used in the nutrition of larvae, as well as adult queens. It is secreted from the glands in the hypopharynx of worker bees, and fed to all larvae in the colony....

 provided by worker bee

Worker bee

A Worker bee is any female eusocial bee that lacks the full reproductive capacity of the colony's queen bee; under most circumstances, this is correlated to an increase in certain non-reproductive activities relative to a queen, as well...

s causes a developing larva to become a queen

Queen bee

The term queen bee is typically used to refer to an adult, mated female that lives in a honey bee colony or hive; she is usually the mother of most, if not all, the bees in the hive. The queens are developed from larvae selected by worker bees and specially fed in order to become sexually mature...

. 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.

Seasonal pigmentation changes

Polyphenic pigmentation

Biological pigment

Biological pigments, also known simply as pigments or biochromes are substances produced by living organisms that have a color resulting from selective color absorption. Biological pigments include plant pigments and flower pigments...

 is adaptive for insect species that undergo multiple mating seasons each year. Different pigmentation patterns provide appropriate camouflage

Camouflage

Camouflage is a method of concealment that allows an otherwise visible animal, military vehicle, or other object to remain unnoticed, by blending with its environment. Examples include a leopard's spotted coat, the battledress of a modern soldier and a leaf-mimic butterfly...

 throughout the seasons, as well as alter heat retention as temperatures change. 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. Birds and mammals, however, are capable of continued physiological changes in adulthood, and some display reversible seasonal polyphenisms, such as coat color in the Arctic fox

Arctic fox

The arctic fox , also known as the white fox, polar fox or snow fox, is a small fox native to Arctic regions of the Northern Hemisphere and is common throughout the Arctic tundra biome. The Greek word alopex, means a fox and Vulpes is the Latin version...

.

Predator-Induced Polyphenisms

Predator-induced polyphenisms are advantageous because they 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 advantageous polyphenism can quickly become neutral or a disadvantage if the predator evolves to stop producing the kairomone

Kairomone

A kairomone is a semiochemical, emitted by an organism, which mediates interspecific interactions in a way that benefits an individual of another species which receives it, without benefiting the emitter. This "eavesdropping" is often disadvantageous to the producer...

 to which the prey responds. For example, the fly larvae that feed on Daphnia cucullata (a water flea) release a kairomone that Daphnia can detect. When the fly 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.

Cannibalistic Polyphenism

The spadefoot toad

Spadefoot Toad

Spadefoot refers to the following toads:* Scaphiopodidae - American spadefoot toads* Pelobates - European spadefoot toads* Notaden - genus that includes Australian spadefoot...

’s polyphenism maximizes its reproductive capacity in temporary desert

Desert

A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...

 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.

Dauer diapause in nematodes

Under conditions of stress such as crowding and high temperature, L1 larvae of some free living nematodes such as C. elegans

Caenorhabditis elegans

Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...

 can switch development to the so called dauer larva

Dauer larva

Dauer describes an alternative developmental stage of nematode worms, particularly Caenorhabditis elegans whereby the larva goes into a type of stasis and can survive harsh conditions. It may also be equivalent to the infective stage of parasitic nematode larvae...

 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.

Evolution of Polyphenisms

A mechanism has been proposed for the development of polyphenisms:

1. A mutation results in a novel, heritable trait.
2. The trait’s frequency expands in the population, creating a population on which selection can act.
3. Pre-existing (background) genetic variation in other genes results in phenotypic differences in expression of the new trait.
4. These phenotypic differences undergo selection; as genotypic differences narrow, the trait becomes:
   1. Genetically fixed (non-responsive to environmental conditions)
   2. Polyphenic (responsive to environmental conditions)

Evolution

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 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 (M. 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

Instar

An instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...

 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.

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

Allele

An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

, resulting in a polyphenism.