Introgressive hybridization plays an important evolutionary role in plants. However, studies of the genetic structures of natural hybrid zones are often hindered by the lack of sensitive molecular markers. We employed the highly sensitive genetic markers from microsatellte loci to 1) define the genetic structure of a broad hybrid zone in Aesculus L. involving A. pavia and A. sylvatica and 2) to test previous hypotheses derived from allozyme and ISSR data. Using PCR, eight pairs of microsatellite primers developed specifically for Aesculus were used to amplify eight microsatellite loci of populations representing pure parental species and hybrids. The genotype of each locus was determined for each individual using an ABI-377 automated DNA sequencer with GeneScan version 3.1 and the data obtained analyzed with Genotyper version 2.0. Our preliminary results indicated that these microsatellite loci are highly polymorphic. Multiple alleles that differ in one to more than 15 nucleotides were found for these loci. The loci contain unambiguous species-specific alleles that can be used as species-specific markers. These species-specific markers from the parental species were combined in the hybrid populations. The results from the microsatellite analysis are congruent with previous data from morphology, allozyme, and ISSR in suggesting the existence of introgressive hybridization in the hybrid zone, and most hybrids in the zone represent latter hybrid generations.

Key words: genetic structure, Hybrid zone, microsatellite loci