Evolution of wing pigmentation patterns in a tephritid gallmaker: divergence and hybridization

Wing pigmentation patterns are commonly used for taxonomic identification in many groups of gall-making flies in the family Tephritidae; however, neither the functional significance nor the mechanisms of evolutionary change in these characters have been well studied. We applied a quantitative image analysis approach to measure wing-pattern variation and differentiation in populations of the goldenrod gall fly Eurosta solidaginis. A large body of work by Abrahamson and coworkers has established that "host races" have emerged from within the eastern U.S. subspecies, E. solidaginis solidaginis, via a shift from an ancestral host-plant species (Solidago altissima [Asteraceae]) to a derived host species (S. gigantea). The image analysis demonstrated that host races are significantly differentiated in wing patterns at a sympatric site. We also quantified wing-pattern variation across a hybrid zone in Iowa between the named subspecies of E. solidaginis, which are distinguished taxonomically solely based on wing-pattern differences. The presence of intermediate wing patterns suggests hybridization between wing forms. These results suggest that host shifts provide barriers strong enough to allow populations AA to diverge in wing patterns, but that neither sympatric host shifts nor wing pattern differences evolved during periods of geographic isolation provide complete barriers to gene flow.