Genetic differentiation between sympatric host races of the apple maggot fly Rhagoletis pomonella

Abstract

It has been proposed that true fruit flies in the Rhagoletis pomonella species group speciate sympatrically (that is, in the absence of geographic isolation) as a consequence of shifts to previously unexploited host plants^1,2. Because Rhagoletis larvae are host-specific fruit parasites and mate selection in these flies is directly coupled to host-plant recognition^3–5, variation for larval survivorship and host preference traits can act as genetically based barriers to gene flow. This reduction in gene flow results in the sympatric divergence of fly populations adapted to alternative hosts. The shift of R. pomonella from its native host hawthorn (Crataegus spp.) to domestic apples (Mains pumila) in eastern North America in the past 200 years⁶ provides an opportunity to determine whether host specialization is sufficient to differentiate populations without earlier periods of geographic isolation. We report finding genetic differentiation between co-occurring hawthorn and apple populations of R. pomonella at a field site near Grant, Michigan. The result confirms that hawthorn and apple flies represent partially reproductively isolated 'host races' and is consistent with a sympatric mode of divergence for these flies.