Dispersal polymorphism

One source of variability in dispersal within populations is a somatic polymorphism amongst the progeny of a single parent. This is typically associated with habitats that are variable or unpredictable. A classic example is the desert annual plant Gymnarrhena micrantha. This bears very few (one to three) large seeds (achenes) in flowers that remain unopened below the soil surface, and these seeds germinate in the original site of the parent. The root system of the seedling may even grow down through the dead parent's root channel. But the same plants also produce above-ground, smaller seeds with a feathery pappus, and these are wind dispersed. In very dry years only the undispersed underground seeds are produced, but in wetter years the plants grow vigorously and produce a large number of seeds above ground, which are released to the hazards of dispersal (Koller & Roth, 1964).

There are very many examples of such seed dimorphism amongst the flowering plants. Both the dispersed and the 'stay at home' seeds will, in their turn, produce both dispersed and 'stay at home' progeny. Moreover, the 'stay at home' seed is often produced from self-pollinated flowers below ground or from unopened flowers, whereas the seeds that are dispersed are more often the product of cross-fertilization. Hence, the tendency to disperse is coupled with the possession of new, recombinant ('experimental') genotypes, whereas the 'stay at home' progeny are more likely to be the product of self-fertilization.

A dimorphism of dispersers and nondispersers is also a common phenomenon amongst aphids (winged and wingless progeny). As this differentiation occurs during the phase of population growth when reproduction is parthenogenetic, the winged and wingless forms are genetically identical. The winged morphs are clearly more capable of dispersing to new habitats, but they also often have longer development times, lower fecundity, shorter lifespans and hence a reduced intrinsic rate of increase (Dixon, 1998). It is perhaps no surprise, therefore, that aphids may modify the proportions of winged and wingless morphs in immediate response to the environments in which they find themselves. The pea aphid, Acyrthosiphon pisum, for example, produces more winged morphs in the presence of predators (Figure 6.11), presumably as an escape response from an adverse environment.

Figure 6.11 The mean proportion (± SE) of winged morphs of the pea aphid, Acyrthosiphon pisum, produced after two separate periods of exposure to each of two predators: (a) hoverfly larvae and (b) lacewing larvae. Dark bars, predator treatment; light bars, control. (After Kunert & Weisser, 2003.)

dispersal dimorphisms

Figure 6.11 The mean proportion (± SE) of winged morphs of the pea aphid, Acyrthosiphon pisum, produced after two separate periods of exposure to each of two predators: (a) hoverfly larvae and (b) lacewing larvae. Dark bars, predator treatment; light bars, control. (After Kunert & Weisser, 2003.)

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