REGIONAL,LOCAL AND MICROGEOGRAPHIC ALLELE FREQUENCY VARIATION BETWEEN APPLE AND HAWTHORN POPULATIONS OF RHAGOLETIS POMONELLA IN WESTERN MICHIGAN

In the preceding study (Feder et al., 1990), we report that paired apple and hawthorn infesting populations of Rhagoletis pomonella are genetically differentiated for six allozymes. Here, we show that patterns of intra- and inter-host allele frequency variation seen for these six loci across the eastern United States are consistent on a more fine grained spatial scale in western Michigan. Malic enzyme, Aconitase-2, Mannose phosphate isomerase, and Hydroxyacid dehydrogenase all displayed significant linear relationships with latitude among five “regional” hawthorn populations sampled along a north-south transect between the cities of Cadillac and Portage, Michigan. Clines were not as evident among “regional” apple populations in western Michigan, although allele frequencies for Malic enzyme¹⁰⁰, Mannose phosphate isomerase¹⁰⁰ and Aconitase-2⁹⁵ varied with latitude among six “local” apple populations within a 60 km² area near the town of Grant. Significant allele frequency differences were observed between hawthorn and apple populations at all “regional” and “local” collecting sites analyzed in the study (a total of 20 different apple and hawthorn populations). As was the case in the geographic survey of the eastern United States, the magnitude and pattern of inter-host frequency differences at “regional” and “local” sites were a function of latitude. Host related genetic differentiation was consistent on a “microgeographic” scale as well. Allele frequencies for Malic enzyme¹⁰⁰ and Aconitase-2⁹⁵ were significantly higher over a four-year period (1984 to 1987) for flies sampled from individual hawthorn trees (N = 6) than apple trees (N = 7) within an old field (0.09-km² area) located near Grant. The fine level of genetic subdivision between hawthorn and apple populations of R. pomonella in western Michigan substantiates the existence of host associated polymorphism in the fly and supports a sympatric mode of divergence for the “apple race”.