, Volume 27, Issue 2, pp 187-196

First online:

Origin and distribution of polyploid Achillea (Compositae) in western North America

  • Ronald J. TyrlAffiliated withSchool of Biological Sciences, Oklahoma State University

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The tetraploid and hexaploid cytotypes ofAchillea millefolium in western North America exhibit a complex distributional pattern involving overlapping ranges, mixed populations, and occasional pentaploid, septaploid, and octoploid plants. Cytogenetic and distributional evidence support the hypothesis that there has been a repeated autoploid origin of hexaploids from tetraploid progenitors. It is suggested that chromosome number differences should not be relied on as markers for infraspecific taxa in this complex.