Reactive Plant Tissue Sites and the Population Biology of Gall Makers

Plant galls, one of the great curiosities of the insect world, have fascinated naturalists from the time of Theophrastus (57). A century ago, the debate over natural selection briefly focused on insect-induced galls because, according to Romanes (51), they seemed to be , .. the one and only case in the whole range of organic nature where it can be truly said that we have unequivocal evidence of a structure occurring in one species for the exclusive benefit of another. Earlier, Darwin (18) had admitted that if any such case were uncovered it would annihilate his theory. However, Romanes saw an explana­ tion for galls that was consistent with the emerging evolutionary orthodoxy; although direct selection on the plant cannot lead to the evolution of galls, ... natural selection may operate to this end on the plants indirectly through the insect, viz. by always selecting those individual larvae the character of whose excitatory emanations is such as will best cause the plant to grow the kind of morphological abnormality that is required (51). In the present day it is appreciated that selection acts on gall makers to manipulate host-plant development (19, 37, 63) for nutritiOn and for protection from both the physical environment and some natural enemies (1, 45, 63). The evolution of the plant-gall maker interaction is complicated by the fact that galls are phenotypic entities that develop under the influence of both plant and insect genotypes (Figure 1). From the plant's perspective, the gall is a developmental abnormality, induced by an environmental stimulus, i.e. the insect. When viewed from the insect's perspective, the gall is a phenotypic