Hormones

Part 2: Polyphenisms

Stages of spermathecal development in Drosophila melanogaster: Images: Mark Mayhew (UQ)

Activities

Go to the journal Science and read the following paper by Suzuki and Nijhout

Suzuki, Y. and Nidjout, H.F. 2006. Evolution of a polyphenism by genetic accommodation. Science vol. 311, no. 5761, pp 650-652.

What is Polyphenism?

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

All individuals have the capacity to become any variant. They are channelled by geneticswitches into one or another pathway. Possession of polyphenic traits means that distinct forms can develop in response to some internal or environmental cue. The term polyphenism tends to be used to describe traits that are present in distinct states rather than showing a continuum. For example, size—although environmentally influenced—would not usually be described as a polyphenic trait.

See Chapter 15

Note on terminology

Polymorphism and polyphenism are often confused. Strictly, in polymorphisms, the alternative phenotypes are due to genetic differences between individuals, for example populations of a species of butterfly at high altitudes may have different colouration to populations at low altitudes. The colouration itself is a heritable trait, so the different phenotypes arise from different genotypes and the trait would be described as a polymorphism.

The “reaction norm” is the range of the trait.
Note that Chapman uses the term “polymorphism” where Nijhout uses “polyphenism”.
Nijhout gives metamorphosis as an example of “sequential” polyphenism.

Examples of polyphenisms in insects include:

chromatic adaptation
seasonal morphs
castes in social insects
phase polyphenism in locusts

Chromatic Adaptation

It is rare for insects to be able to undergo physiological colour change. More often colour change is slow to develop and requires a moult to register.

The green vs brown polyphenism in Orthoptera is usually a response to either photoperiod or humidity. Juvenile hormone levels appear to be responsible because in the limited number of experiments that have been carried out, CA implantation or JH application causes colour switches at the next moult.

Suzuki and Nijhout used artificial selection to produce a temperature-sensitive polyphenism in the tobacco hornworm, Manduca sexta.

A naturally occurring mutant, called black, has dark pigmentation in place of the normal green colour.
Two lines artificially selected for (1) high response (incr green) or (2) low response (incr black) and (3) unselected (but heat-shocked) line as a control for treatment and rearing. 13 generations leads to no variability in monophenic line 2 and high levels green in polyphenic line

Phase change

Locusts are grasshoppers that swarm. To swarm, they switch from solitary to gregarious phase. Phase change associated with different colour, morphology and behaviour.

Population growth leads to high density and “jostling”. Olfactory, visual and tactile cues than cause an elevation of serotonin levels in the ventral nerve cord. Leads to behavioural change from solitary (move away from others) to gregarious (move toward others). Once gregarized the resulting swarm must move because stationary locusts are preyed upon and consumed by others. Leads to mass synchronised behaviour.

Solitary: cryptic colouration, avoid others, low activity levels

Gregarious: conspicuous colouration, aggregate, are mobile

Nymphs form bands

Phase change can occur within life of an individual or phase can be determined by parental traits.

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Topic Review

You should have read the following paper by Suzuki and Nijhout: