6.8.2. Environmental polymorphism, or polyphenism

A phenotypic difference between generations that lacks a genetic basis and is determined entirely by the environment often is termed polyphenism. An example is the temperate to tropical Old World pierid butterfly Eurema hecabe, which shows a seasonal change in wing color between summer and fall morphs. Photo-period induces morph change, with a dark-winged summer morph induced by a long day of greater than 13 h. A short day of less than 12 h induces the paler-winged fall morph, particularly at temperatures of under 20°C, with temperature affecting males more than females.

Amongst the most complex polyphenisms are those seen in the aphids. Within parthenogenetic lineages (i.e. in which there is absolute genetic identity) the females may show up to eight distinct phenotypes, in addition to polymorphisms in sexual forms. These female aphids may vary in morphology, physiology, fecundity, offspring timing and size, development time, longevity, and host-plant choice and utilization. Environmental cues responsible for alternative morphs are similar to those that govern diapause and migration in many insects (sections 6.5 & 6.7), including photo-period, temperature, and maternal effects, such as elapsed time (rather than number of generations) since the winged founding mother. Overcrowding triggers many aphid species to produce a winged dispersive phase. Crowding also is responsible for one of the most dramatic examples of polyphenism, the phase trans- formation from the solitary young locusts (hoppers) to the gregarious phase (section 6.10.5). Studies on the physiological mechanisms that link environmental cues to these phenotype changes have implicated JH in many aphid morph shifts.

If aphids show the greatest number of polyphenisms, the social insects come a close second, and undoubtedly have a greater degree of morphological differentiation between morphs, termed castes. This is discussed in more detail in Chapter 12; suffice it to say that maintenance of the phenotypic differences between castes as different as queens, workers, and soldiers includes physiological mechanisms such as pheromones transferred with food, olfactory and tactile stimuli, and endocrine control including JH and ecdysone. Super-imposed on these polyphenisms are the dimorphic differences between the sexes, which impose some limits on variation.

Chapter 6