The genus Tamalia comprises upwards of six species of aphid occupying galls on shrubs in the Heath family (Ericaceae). My research at Blodgett Forest Research Station and elsewhere in California and Arizona has investigated the social habits of Tamalia coweni (the Manzanita Leaf-gall Aphid) on several species of Arctostaphylos (manzanita) including their consequences and possible causes, in the context of the evolution of social behavior. I have identified and formally described a congener, Tamalia inquilinus, co-occupying galls of T. coweni obligately, so acting as a social parasite. Generally, I wish to know whether the pattern of coevolution between T. coweni and T. inquilinus fits Emory's Rule, viz., that socially parasitic species and their hosts share immediate common ancestors, as opposed to the alternative scenario of a distinct origin of the social parasite clade.
I have undertaken a molecular phylogeny of the genus Tamalia with Dr. Bernie Crespi of Simon Fraser University and we have preliminary results. We sampled Tamalia aphids of four putative species on seven host plant taxa and constructed a phylogeny based on the cytochrome oxidase I mtDNA region, using the neighbor-joining bootstrap method with outgroup to generate a 50% majority-rule consensus tree. These results give both a phylogram and estimates of genetic distance among the aphid taxa.
Tamalia inquilinus in various species of host galls appears to constitute a monophyletic group distinct from its gall-causing hosts, and may have had a single origin from a gall-causing ancestor. Therefore, Emory's Rule is NOT supported by these preliminary data and instead an ancestral inquiline taxon may have radiated to at least three distinct types of host gall, one on Arctostaphylos viscida and two on Arctostaphylos patula. Further, the inquiline taxa appear to have evolved much faster than their hosts, perhaps because of a history of genetic bottlenecks. More data will give us further confidence in interpreting correctly Emory's Rule in light of the origins of social behavior and social parasitism in the genus Tamalia on its host plants Arctostaphylos and Arbutus.
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Figure 1. Two species of aphid co-occupying a gall on Arctostaphylos patula (Green-leaf Manzanita) at Blodgett Forest Research Station, July 1996. The aphid below is a wingless female foundress of Tamalia coweni, the Manzanita Leaf-gall Aphid. The aphid above is a wingless female of a newly described socially parasitic species, Tamalia inquilinus. |
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