Clinal genetic variation and the 'rare allele phenomenon' in random mating populations of Urophora cardui (Diptera: Tephritidae)

In the present study we investigate a contact zone between two population groups of the tephritid fly Urophora cardui. We investigate scenarios that may have produced the genetic differentiation of the two groups, and we describe the 'rare allele phenomenon' from the contact zone. The rare allele phenomenon refers to alleles that are found at high frequency in contact zones but are rare or lacking outside the contact zone. The phenomenon is often observed in hybrid zones between subspecies of limited reproductive compatibility, but seldom in populations with random mating. Clinal genetic variation was observed at three loci in the contact zone. Three alleles at the locus Aat showed steep clines, between 20--70 km wide. A rare Aat-A allele occurred at high frequency in the centre of the contact zone. Two further loci, Hk and Pgd, showed less steep clinal genetic variation, the transition being in and slightly south of the centre of the Aat cline. Populations showed Hardy--Weinberg proportions and there was no evidence for linkage dis-equlibrium. These findings suggest random mating and gradual introgression between the population systems, which may originate from at least two range expansions. Aat's steep clines and rare allele may indicate selection on Aat alleles, although we presently can not quantify any agents. Because U. cardui experiences random mating in the contact zone with no apparent 'hybrid' incompatibility, mating experiments offer the possibility for future enquiries about the genetic basis of the rare allele phenomenon.