Autopolyploidy has long been considered as maladaptative and rarer than allopolyploidy in flowering plants. This perspective is changing however, following studies showing multiple origins of autopolyploidy, even if these reports are far less frequent than those for allopolyploidy. Here, we consider the likelihood of multiple origins of autopolyploidy in triploid populations of Apios americana using molecular data. Apios americana, or groundnut, is a perennial climbing legume that is widely distributed throughout eastern North America from Florida to New-Brunswick. Diploids (2n=2x=22) tend to occur in the southern part of this range, whereas triploids (2n=3x=33) are more frequent in the north. In addition, diploids are almost completely excluded from those parts of eastern North America covered by ice during the Wisconsinian glaciation (18 000 years ago). No tetraploids have been reported to date. The gene genealogy constructed from single copy nuclear H3-D sequences (2 exons/ 3 introns) suggests that triploidy has evolved at least twice in A. americana. Triploid clones appear well structured and widespread over large geographic areas. Moreover, the clear separation of two distinct triploid clones on either side of the Appalachian Mountains suggests that triploid postglacial migration may have implicated two distinct routes. These same triploid clones are further distinguishable morphologically, the eastern and western clones having light and dark coloured flowers respectively. Finally, both triploid clones bear three H3-D alleles which suggest an origin from two genetically distinct diploid individuals. This, in conjunction with the multiple origins of triploidy suggest greater genetic variability in triploids than one could have suspected. This clear demonstration for the multiple origins of triploidy in A. americana is further evidence that the role of autopolyploidy in evolution has been underestimated.

Key words: Apios americana, autopolyploidy, Fabaceae, gene genealogy, multiple origins, polyploidy