Graduate Thesis Or Dissertation
 

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

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/dj52w8284

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  • A cytogeographical study was made of populations of tetraploid (n=18) and hexaploid (n=27) Achillea millefolium (Compositae- Anthemideae) from the Pacific Coast states. There is a complex distributional pattern, involving overlapping ranges, mixed populations, migration of one form into areas occuplied by the other, pentaploid hybrids, and septaploid and octoploid plants. The distributions of the coastal tetraploids in the areas of southwestern Oregon, northwestern California, and northwestern Washington, are correlated with the distinctive geological regions of the Klamath and the Olympic Mountains. Tetraploid plants of these areas frequently produce unreduced micros pores with a chromosome number of n=36. On the basis of these observations, several inferences are advanced. First, the distributions of the two chromosome forms are not static, but rather dynamic, with man being instrumental in dispersal and establishment of Achillea populations. Second, the North American hexaploid is indigenous, having its origins in the tetraploid through the union of unreduced or tetraploid gametes with normal diploid gametes. Evidence for this hypothesis is found in the reproductive barriers between American and Eurasian hexaploids; the meiotic configurations of the pentaploid hybrids; the existence of mixed populations in which single hexaploid plants occur among tetraploids that are producing unreduced gametes; and the morphological and ecological similarities indicative of an autoploid system of evolution. From the distributional patterns detected, it is believed that the tetraploid, established during Tertiary times in the Klamath and Olympic Mountains, gave rise to the hexaploids at the geological margins of these areas. Due to these observations and the complete intergrading morphological variation characterizing the genus, it is suggested that Achillea in western North America is adequately and meaningfully described as a single species, highly polymorphic and with two or more ploidy levels.
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