Citation. Craig, T. P.; Itami, J. K.; Horner, J. D.; Abrahamson, W. G. 1994. Host shifts and speciation in gall-forming insects. Pages 194-207 in P. W. Price, W. J. Mattson, and Y. N. Baranchikov, Eds., The Ecology and Evolution of Gall-forming Insects. U.S. Dept. of Agriculture, Forest Service, North Central Forest Experiment Station. St. Paul, MN. General Technical Report NC-174.   [1030  LTERCC]

Abstract. The phylogenetic constraint of gall-forming results in an adaptive syndrome that has emergent properties that determine which modes of speciation are possible. Gall-forming is an adaptive syndrome that entails enophagy and resoursce manipulation. This adaptive syndrome leads to a high degree of inter- and intro-specific host specialization by the gall former. Because of this syndrome the herbivore acquires host-plant characteristics that influence its ecological interactions. We consider how this adaptive syndrome influences emergent properties that could influence the probability of a population of herbivores colonizing a new host and upon the probability that the colonists will become reproductively isolated. We conclude that the adaptive syndrome of gall-formers has the following emergent properties that increase the probability of colonization of a new host plant: they may form polymorphisms in response to intraspecific host-plant variation that may preadapt them for host shifts, and they may be subject to frequency-dependent selection so that individuals in a low-density colonizing population may have relatively high fitness. Gall-formers probably lack the ability to shift hosts on a non-genetic basis, such as experience, that would further facilitate host shifts. The adaptive syndrome of gall-formers has several emergent properties that would facilitate rapid reproductive isolation of populations once a host shift was initiated. 1) There is likely to be a negative correlation of performance traits (survival, reproduction and mating success) required to survive on any pair of host plants. 2) Gall-formers have a genetically based host preference, that is not easily altered by experience. 3) Mating occurs on the host plant. 4) Gall-formers are under strong selection to adapt to the phenology of the host plant on which they ovipost. A key determinant of both the probability of a host shift andof subsequent reproductive isolation is the number of genes involved in shifting host preference and the ability to survie on a new host. This has not been determined for any fall-forming species. An examination of the assumptions of sympatric and allopatric models of speciation shows that gall-formers are more likely to speciate sympatrically.

Keywords. allopatric speciation, Eurosta solidaginis, gall-former, host race, host shift, Solidago gigantea, sympatric speciation