Systematics of Agamic Complexes in Amelanchier (Shadbush)

The genus Amelanchier (shadbushes) is renowned for difficulty in species delimitation due to hybridization, multiple ploidy levels and asexual seed production (apomixis). Amelanchier diploids are sexual and form species under most species concepts. Amelanchier polyploids, however, are mostly tetraploid and 97-99% apomictic. Apomixis perpetuates successful Amelanchier genotypes into microspecies, and in some cases, widespread tetraploid species. Although North American diploid Amelanchier are generally distinct from one another, retention of 1-3% sexuality in tetraploids enables backcrossing to diploids or other polyploids. This leads to semi-crytpic ploidy variation, and local and widespread hybrid swarms. Determining species boundaries in Amelanchier requires examining data from diploid sexuals and polyploid apomicts. We investigated the relationship of diploids and polyploids in three species groups in North America; arborescent Amelanchier of eastern North America, Amelanchier in northern California and southern Oregon, and the widespread western North American species A. alnifolia and A. utahensis. We investigated diploid and allotetraploid morphological affinities using ordination and ecogeography with field observations. We inferred diploid ancestry of allotetraploids with DNA sequences from five nuclear regions and morphology.In eastern North America we recognize diploids of A. arborea, A. canadensis, and A. laevis, and the widespread tetraploid A. laevis as species. We describe Amelanchier diplolaevis, and group these four species, plus morphologically related tetraploids, including several microspecies, in the Canadensis agamic complex. In northern California and southern Oregon we recognize two narrowly endemic diploids Amelanchier diplopallida and Amelanchier minuta as species. We group each diploid plus morphologically similar polyploids into the Pallida agamic complex and the Minuta agamic complex. We recognize diploid A. utahensis as a species and provide an epitype to aid interpretation of the type material. We treat the diploid A. utahensis, diploid A. alnifolia var. alnifolia, and all morphologically similar Western North American polyploids as the Alnifolia-Utahensis agamic complex.Most polyploids in these complexes do not form well-demarcated taxa and are part of magnified hybrid swarms. The use of agamic complexes recognizes the importance of diploids, accommodates the complexity generated by apomixis and semi- cryptic ploidy variation in Amelanchier, and does not burden the formal classification with poorly defined species.