Genetic differences between host races of Rhagoletis pomonella

Abstract

Sympatric speciation by the formation of host races (parasite populations associated with different plant or animal hosts) has been the subject of great controversy. It has been difficult to demonstrate the existence of host races1,2, much less to prove that host races are evolving toward species status. Genetic polymorphism attributable to association with different resources does occur3, but the phenomenon is far from ubiquitous in parasite populations. The apple maggot fly Rhagoletis pomonella uses a variety of host plants, and Bush4,5 has argued that it is a likely candidate for speciation by a sympatric mode. So far however there has been no direct evidence of any genetic differentiation between host-associated fly populations. We report significant differences in allele frequencies between fly populations reared from sympatric apple (Mains pumila) and hawthorn (Crataegus mollis) trees at a field site in Urbana, Illinois, in the United States.