DNA sequence data from the low-copy nuclear gene alcohol dehydrogenase (Adh) were used to test the putative hybrid origin of several diploid and tetraploid species of peonies belonging to Paeonia section Paeonia (Paeoniaceae). Two paralogous loci (Adh1 and Adh2), duplicated prior to the diversification of Paeonia species, were amplified with locus-specific primers and cloned for one or more accessions from each species. All distinct clone copies were included in a single simultaneous parsimony search for each locus. Adh1 and Adh2 support that several tetraploid species, including the "arietina" species group (P. arietina, P. humilis, P. officinalis, P. parnassica), P. peregrina, and P. banatica, are of allopolyploid origin. After the initial hybridization event, tetraploid peonies may have gone on to speciate and further hybridize, creating new species such as P. officinalis. For putative hybrid diploids, distinct sequences were found at the Adh2 locus for P. emodii, and Adh1 sequence polymorphism was detected for P. japonica and P. obovata. This may imply that heterozygosity resulting from hybridization can be maintained at a nuclear locus but homogenized at the other following genome rearrangement in a diploid hybrid. Overall, Adh2 corroborates many of the findings based on Adh1; however, the tempo of gene duplication and deletion at that locus is much more dynamic than Adh1, with many pseudogenes found in several species.

Key words: alcohol dehydrogenase (Adh), allopolyploidy, hybridization, molecular systematics, Paeonia, speciation