The genus Tamalia comprises upwards of six described species of aphid occupying galls on shrubs in the Heath family (Ericaceae). Our research in western North America concerns the evolution and ecology of the social habits of Tamalia coweni on ericaceous shrubs. A newly described congener, Tamalia inquilinus, co-occupies the galls of T. coweni, presumably as a socially parasitic inquiline. We undertook a phylogenetic analysis of Tamalia aphids with the goal of testing whether the pattern of coevolution between T. coweni and T. inquilinus lineages fits Emery's rule, which holds that socially parasitic species and their hosts tend to share immediate common ancestors.
We sampled Tamalia aphids on various host plant taxa and constructed a phylogeny based on the cytochrome oxidase I mtDNA region, using maximum likelihood, maximum parsimony, and neighbor joining. Tamalia inquilinus on various host plant species constitutes a monophyletic group distinct from, but sister-group to, its gall-inducing hosts. It exhibits a single origin from a galling ancestor, possibly in association with a host-plant shift. These data do not support Emery's rule and instead suggest that a single ancestral inquiline lineage radiated to exploit galling aphids on many different host plants. Additionally, the inquilines exhibit much stronger genetic differentiation among host plant species than do the gall-inducers, indicating a greater degree of host race or sibling species formation in the inquilines.
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