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. The expression of a particular phenotype depends upon one or more genes that are triggered by an environmental cue. An example is the temperate to tropical Old World butterfly Eurema hecabe (Lepidoptera: Pieridae), which shows a seasonal change in wing color between summer and fall morphs. Photoperiod induces morph change, with a dark-winged summer morph induced by a long day of greater than 13 hours. A short day of less than 12 hours induces the paler-winged fall morph, particularly at temperatures of under 20°C, with temperature affecting males more than females. A second example, is the color polyphenism of the caterpillars of the American peppered moth Biston betularia cognataria (Lepidoptera: Geometridae), which are generalist herbivores with a body color that varies to match the green or brown twigs of their host plants. Experiments have shown that the caterpillars change the pigments in their epidermal cells in response primarily to their visual experience, rather than to their diet. This larval color polyphenism is not related to the genetic polymorphism for melanic forms seen in adult moths (Box 14.1).

Amongst the most complex polyphenisms are those seen in the aphids (Hemiptera: Aphidoidea). 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 photoperiod, 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 are 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.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook

Post a comment