Polyphenism

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Definition

A polyphenic trait is a trait for which multiple, discrete phenotypes can arise from a single genotype as a result of differing environmental conditions. 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.^[1]

When polyphenic forms exist at the same time in the same panmictic (interbreeding) population they can be compared to genetic polymorphism^[2]. 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 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.

Examples of Polyphenism

Sex determination

Sex-determining polyphenisms allow a species to benefit from sexual reproduction while permitting gender ratios other than unity. This is beneficial to the 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.^[4]

Population-dependent and reversible sex determination, found in animals such as the blue wrasse fish, has 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. Furthermore, this system is inherently unstable because a single mutation 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.

The caste system in insects

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.

Seasonal pigmentation changes

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. 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.^[8]

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 a disadvantage if the predator evolves to stop producing the kairomone 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.^[9]

Cannibalistic Polyphenism

The spadefoot toad’s polyphenism 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.^[10]

Evolution of Polyphenisms

A mechanism has been proposed for the development of polyphenisms:^[11]

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 of novel polyphenisms through this mechanism has been demonstrated in the laboratory. Suzuki and Nijhout^[12] 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 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, resulting in a polyphenism.

References

1. ^ Woodward, D.E. and Murray, J.D. (1993). On the effect of temperature-dependent sex determination on sex ratio and survivorship in crocodilians. Proc. R. Soc. Lond. [B] 252:149-155.
2. ^ Ford E.B. 1975. Ecological genetics. 4th ed, Chapman & Hall, London
3. ^ Woodward, D.E. and Murray, J.D. (1993). On the effect of temperature-dependent sex determination on sex ratio and survivorship in crocodilians. Proc. R. Soc. Lond. [B] 252:149-155.
4. ^ Gilbert, S.F. (2003). Developmental Biology, 7 edn (Sunderland, Massachusetts, Sinauer Associates, Inc.). pp.731.
5. ^ Gilbert, S.F. (2003). Developmental Biology, 7 edn (Sunderland, Massachusetts, Sinauer Associates, Inc.). pp.732
6. ^ Abouheif, E., and Wray, G.A. (2002). Evolution of the gene network underlying wing polyphenism in ants. Science 297, 249-252.
7. ^ Braendle, C., and Flatt, T. (2006). A role for genetic accommodation in evolution? Bioessays 28, 868-873.
8. ^ Gilbert, S.F. (2003). Developmental Biology, 7 edn (Sunderland, Massachusetts, Sinauer Associates, Inc.). pp.727.
9. ^ Gilbert, S.F. (2003). Developmental Biology, 7 edn (Sunderland, Massachusetts, Sinauer Associates, Inc.). pp.735.
10. ^ Storz, B.L. (2004). Reassessment of the environmental mechanisms controlling developmental polyphenism in spadefoot toad tadpoles. Oecologia 141, 402-410.
11. ^ Braendle, C., and Flatt, T. (2006). A role for genetic accommodation in evolution? Bioessays 28, 868-873.
12. ^ Suzuki, Y., and Nijhout, H.F. (2006). Evolution of a polyphenism by genetic accommodation. Science 311, 650-652.

Note: The seventh edition of Gilbert's Developmental Biology is available online: DevBio.