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• AFLP analysis and ploidy were used to evaluate 132 accessions of Aronia. • Aronia was found to be comprised of four, or possibly five, species and seven taxonomic groups. • arbutifolia was tetraploid, and A. prunifolia was typically tetraploid, and rarely triploid. • Diploid A. melanocarpa were found in New England, but elsewhere they were tetraploid. • Formation of A. prunifolia as a hybrid of A. arbutifolia by A. melanocarpa is supported by AFLP analysis and ploidy. Speciation in Aronia is complex based on Amplified Fragment Length Polymorphism (AFLP) and ploidy analysis of 132, mostly wild, accessions. There are four species of Aronia and seven taxonomic groups comprised of species by ploidy combinations. A. melanocarpa can be diploid or tetraploid, with diploid forms occurring in New England and tetraploid forms occurring primarily outside of New England. A. arbutifolia was only found as a tetraploid and did not appear to occur throughout parts of New England as is generally accepted. It is likely that numerous misidentifications of A. arbutifolia have occurred historically. Almost all wild A. prunifolia were tetraploid, with the exception of a single triploid plant. Evidence from AFLP analysis and several accessions expressing a continuum of morphological characteristics between those of A. arbutifolia and A. melanocarpa suggest that A. prunifolia is of interspecific hybrid origin. The occurrence of a natural triploid A. prunifolia accession and our ability to easily create triploid A. prunifolia progeny from diploid A. melanocarpa by tetraploid A. arbutifolia crosses supports the hybrid formation of the A. prunifolia species. Hybrid A. prunifolia origin is also supported by the prevalence of A. prunifolia accessions in geographic areas where A. arbutifolia and diploid A. melanocarpa interface. Most likely, tetraploid A. prunifolia found in the wild result from repeated formation of triploid F 1 interspecific hybrids and the triploid bridge mechanism. AFLP analysis, along with morphology and phenology, suggest that a new species of black-fruited Aronia (melanocarpa S) exists in the southern part of the A. melanocarpa geographic range. A. melanocarpa S plants are placed on a separate branch of the AFLP dendrogram that is separate from other Aronia species. Several accessions with large, wide leaves, large fruits and non-rhizomatous growth were found to be A. mitschurinii , an intergeneric hybrid between Aronia and Sorbus. All A. mitschurinii accessions were tetraploid. [ABSTRACT FROM AUTHOR]