Phenotypic Plasticity and Somatic Polymorphism

Phenotypic plasticity is treated in broader terms in Chap. 7 but brief comment here should be made with respect to the role that it plays specifically in the biology of modular organisms. As noted previously, modular organisms, especially those that are clonal, tend to be highly plastic in size, shape, and fecundity because the number of modules can change readily by birth and death processes. Harper and Bell (1979) state in part (p. 31) “the placement of modular units determines the form of the organism. Form is a consequence of the dynamics.” This is particularly evident in the case of plants where the iterative process determines the general size, shape, arrangement of leaves, branch angles, etc. The modular growth and death process is analogous to a child playing with Lego blocks, adding pieces in some places and removing them in others.

Sessile modular organisms tend to grow either predominantly vertically or horizontally. For many plants and in particular trees, the competitive race may be to stack modules vertically and to retain the genet intact thereby shading one’s competing neighbors. Alternatively, for clonal plants such as clover or bracken fern, it may be to capture light resources by lateral expansion and perhaps in so doing avoid some of the mechanical limitations of vertical expansion (recall scaling in Chap. 4; also Watkinson and White 1985). A compromise between the two would be a clonal tree like quaking aspen (Populus tremuloides), which has an erect form but spreads laterally by root suckers (Mitton and Grant 1996). Other modular organisms may have a dissociated genet as in the case of floating aquatic plants such as Lemna noted earlier. In rhizomatous or stoloniferous plants new root systems typically are produced at nodes.

The intervening segments may rot, disintegrate, or break under the influence of trampling hooves, so that the original zygote becomes represented by physiologically independent individuals. Harper worded these processes imaginatively as follows: “A clone of Lemna or Pistia expresses itself as a genetic individual by continually falling to pieces” (1978, p. 28) and “The ability of some plant species to form fragmented phenotypes of a single genotype is just one of the variety of successful ways of playing the game of being a plant (1977, p. 27) ... a zygote of Hydra does just the same, and, at the end of a season of growth, many daughter zygotes may be the descendants of a single parental zygote but formed from a fragmented phenotype of independently living polyps” (Harper 1978, p. 28).

So, ultimately the number of modules produced—whether they remain attached or become separate—will affect the number of progeny produced, in turn the number of descendants, and hence fitness. In some circumstances, evidently fitness is increased by retention of the intact genet; in others, by a dissociated genet (Harper 1981a). Whether modules remain intact and integrated, or become dissociated and function independently, is an interesting developmental question that has received relatively little attention (however, see later comments in later section, When to Divide?).

Iteration means that different parts of the same genet are affected by different environments and consequently are subjected to different selection pressures (Harper 1985; Harper et al. 1986). One segment of the genet may be expanding while the other is contracting and progeny typically are produced from those portions of the genet that are most successful. This situation is not possible for unitary organisms. The portions might be segments that have adequate nutrition or avoid being eaten by predators or, in the case of a pathogenic fungus, those clonal spores that contact a suitable host. Thus, for modular organisms, the testing of a particular gene combination is achieved by growth, i.e., by a particular architecture or geometry, whereas for unitary organisms it is frequently by mobility within the population. Phrased differently, growth amounts to movement for modular organisms.

Somatic polymorphism (a type of phenotypic plasticity; Chap. 7) has little real meaning in unitary organisms. It does occur modular organisms to the extent that a given genotype presents different phenotypes adapted to different conditions. Examples include the aerial versus submerged leaves of the same aquatic plant, or differences in leaf morphology between wetter and drier seasons (large vs. small leaf sets, respectively) of some desert plants. The form and arrangement of leaves may be quite different on juvenile and mature branches of Eucalyptus.

There are at least two important consequences of somatic polymorphism: The first is on the variable number of modules allocated to a particular reproductive or vegetative role at a given time. The second is that the phenotypic plasticity that results from this allocation pattern also influences the behavior of associated organisms. Insofar as parasites and herbivores are concerned, flowers are quite different from leaves or carpels or roots. Rather than the entire genet being directly affected (as is the case with unitary organisms), the potentially damaging activity is concentrated on the module, which may be destroyed and replaced, potentially without great consequence to the organism. Here is yet another example of the issue of trade-offs—how natural selection balances among the available polymorphisms to maximize fitness of the organism given competing demands and with any option entailing costs in terms of resource allocation.