Relatedness among galling aphids as determined by AFLP analysis

Cooperative or eusocial behavior is not unknown among gall-inducing insects, but the ecological and evolutionary contexts which produce such behaviors vary in their details. In Northern California, the manzanita leaf-gall aphid Tamalia coweni has shown a tendency to communally occupy leaf galls, in contrast to solitary gall occupation, the mode more typical for galling aphids. One potential explanation for this behavior is that kin selection may make communal gall occupation a viable alternative to solitary gall occupation, due to a high degree of relatedness between gall co-occupants and increased numbers of offspring in communally occupied galls. To evaluate this, the “kin selection hypothesis,” my study involved recording the frequency of communal behavior in a local population of T. Coweni on Arctostaphylos patula, and evaluated differences in reproductive rates between single and multiple-occupant galls and the degree of relatedness among gall co-occupants. Out of 375 galls examined, 11 percent were communally occupied, with double-foundress galls being the most common type of communal gall. The maximum number of communal foundresses found in any single gall was five. The productivity of communal galls (measured by numbers of offspring per gall) was higher than for single-foundress galls on a per-gall basis but lower per capita. AFLP-PCR was used to gentotype individual aphids from communal galls. Analysis of genetic distance between individuals revealed that relatedness among gall cohabitants was higher, on average, than for non-gall cohabitants. Parsimony analysis of the AFLP profiles found a number of highly parsimonious pairs cohabiting in galls. These results support the kin selection hypothesis, and suggest that communal gall occupation in this species represents a form of cooperative behavior, which may facilitate elaboration of cooperation, and, potentially, the origins of caste and eusociality in this species.