Polymorphism And Polyphenism

The existence of several generations per year often is associated with morphological change between generations. Similar variation may occur contemporaneously within a population, such as the existence simultaneously of both winged and flightless forms ("morphs"). Sexual differences between males and females and the existence of strong differentiation in social insects such as ants and bees are further obvious examples of the phenomenon. The term polymorphism encompasses all such discontinuities, which occur in the same life-history phase at a frequency greater than might be expected from repeated mutation alone. It is defined as the simultaneous or recurrent occurrence of distinct morphological differences, reflecting and often including physiological, behavi oral, and/or ecological differences among conspecific individuals.

6.8.1 Genetic polymorphism

The distinction between the sexes is an example of a particular polymorphism, namely sexual dimorphism, which in insects is almost totally under genetic determination. Environmental factors may affect sexual expression, as in castes of some social insects or in feminization of genetically male insects by mermithid nematode infections. Aside from the dimorphism of the sexes, different genotypes may co-occur within a single species, maintained by natural selection at specific frequencies that vary from place to place and time to time throughout the range. For example, adults of some ger-rid bugs are fully winged and capable of flight, whereas other coexisting individuals of the same species are brachypterous and cannot fly. Intermediates are at a selective disadvantage and the two genetically determined morphs coexist in a balanced polymorphism. Some of the most complex, genetically based, polymorphisms have been discovered in butterflies that mimic chemically protected butterflies of another species (the model) for purposes of defense from predators (section 14.5). Some butterfly species may mimic more than one model and, in these species, the accuracy of the several distinct mimicry patterns is maintained because inappropriate intermediates are not recognized by predators as being distasteful and are eaten. Mimetic polymorphism predominantly is restricted to the females, with the males generally monomorphic and non-mimetic. The basis for the switching between the different mimetic morphs is relatively simple Mendelian genetics, which may involve relatively few genes or supergenes.

It is a common observation that some individual species with a wide range of latitudinal distributions show different life-history strategies according to location. For example, populations living at high latitudes (nearer the pole) or high elevation may be univoltine, with a long dormant period, whereas populations nearer the equator or lower in elevation may be multi-voltine, and develop continuously without dormancy. Dormancy is environmentally induced (sections 6.5 & 6.10.2), but the ability of the insect to recognize and respond to these cues is programmed genetically. In addition, at least some geographical variation in life histories results from genetic polymorphism.

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 transformation 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. Superimposed on these polyphenisms are the dimorphic differences between the sexes, which impose some limits on variation.

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