TOWARD A MOLECULAR-GENETIC LINKAGE MAP FOR THE APPLE MAGGOT FLY (DIPTERA, TEPHRITIDAE) - COMPARISON OF ALTERNATIVE STRATEGIES

Apple [Malus pumila (L.)] and hawthorn (Crataegus spp.) infesting popu lations of Rhagoletis pomonella Walsh are rapidly becoming a model sys tem for the study of host plant specialization and sympatric race form ation. Unfortunately, a major impediment to further progress in the Rh agoletis system is the lack of an adequate framework of genetic inform ation. Here we report on the development of a molecular genetic linkag e map for the fly and evaluate the relative merits of alternative stra tegies that we used to build the map. The most efficient method for de tecting polymorphism in R. pomonella was based on screening a compleme ntary DNA (cDNA) library for genetic variation. Twenty-four of the 98 cDNA clones that we attempted to analyze from plaques in the library d isplayed either restriction site (Alu I, Dde I, Sau 3A, or Taq I), fra gment length, or single strand conformation polymorphism that could be mapped in R. pomonella and represented 25 different loci. Linkage gro up relationships were established for all 25 of these loci. Seven cDNA loci mapped to the 3 regions of the R. pomonella genome where the 6 a llozyme loci with significant allele frequency differences between the host races are located. Furthermore, 5 of these 7 cDNA loci were in s trong linkage disequilibrium with the allozyme loci residing within 2 of these 3 genomic regions, suggesting that the cDNA linkage map will be useful for dissecting the genetics of race formation in R. pomonell a. Another successful strategy that we employed to map the locus gluco se-6-phosphate dehydrogenase (G-6-pdh) involved the use of a set of re dundant primers that had been produced based on the conserved amino ac id sequence of this gene. Surprisingly, a scheme based on screening pl asmid libraries of randomly cloned restriction fragments was ineffecti ve at detecting polymorphism, and it had an overall success rate of on ly 5.9% (3 mapped loci/51 low copy number clones screened for variatio n). In addition, application of random amplified polymorphic DNAs was unsuccessful in producing useful markers for the linkage map.