Cytogeography of Achillea Millefolium in Western Oregon

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Abstract
A cytogeographic study was made of the tetraploid and hexaploid forms ofAchillea millefolium L. in western Oregon in order to clarify their distributions, to determine whether pentaploid hybrids are present at zones of contact between the two, and to correlate environmental conditions with the presence of the tetraploid in the coastal areas of Coos and Curry Counties, Oregon. The distributional pattern of the two chromosome forms was found to be more complex than previously reported, involving overlapping of ranges, the occurrence of mixed populations, and the presence of disjunct populations of one form in areas occupied by the other. A naturally occurring pentaploid hybrid with meiotic pairing of 18 II’s and 9 I’s was found, which could not be distinguished morphologically from the presumed parents. The coastal tetraploid form in southwestern Oregon is limited to the Siskiyou Mountains. Plants of this area, especially those at its geographical margins, were observed to be producing a large number of viable, unreduced pollen grains. It is suggested that the North American hexaploid form originated by autopolyploidy from earlier existing tetraploids.

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