Polyphenism

Type of polymorphism where different forms of an animal arise from a single genotype

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. For example, crocodiles possess a temperature dependent sex determining polyphenism, where sex is the trait influenced by variations in nest temperature. When polyphenic forms exist at the same time in the same panmictic (interbreeding) population they can be compared to genetic polymorphism. 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. 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. 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. 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. 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.

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.

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.