Cytogeography of Achillea millefolium L. in western Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qj72pb434

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  • A cytogeographic study, the relationship between chromosome number and geographical and ecological distribution, was made of the tetraploid and hexaploid chromosome forms of Achillea millefolium L. in western Oregon. Principal objectives of the investigation were three. First was a determination of the exact distributions of the two levels, as previous studies had indicated only the general distributional patterns. Second was a search for pentaploid hybrids at any contact zones between the two levels; and third was a determination of the factors enabling the interior tetraploid form to invade the coastal areas of Coos and Curry Counties, Oregon. Distributional studies were carried out by chromosome counts of population samples from transects in western Oregon and northwestern California, the counts being from squashed mounts of microspores undergoing the first post-meiotic division. Internal pollen diameters were utilized as indicators of ploidy level. Results disclosed that the distributions of the two chromosome forms are not as simple as previously reported, but involve overlapping distributions, mixed populations, disjunct populations, and penetration of one form into areas occupied by the other. Pentaploid hybrids were found to occur in nature, morphologically resembling their tetraploid and hexaploid progenitors, and showing meiotic pairing of 18 II's and 9 I's with random distribution of the univalents. The maximum internal diameter of good binucleate pollen grains can be utilized as an indicator of ploidy level, although with some reservations. The distribution of the coastal tetraploid form in southwestern Oregon was correlated with the Klamath Mountains geological area. Plants of this area, especially those at its distributional limits, were observed to be producing a large number of tetraploid pollen grains that were apparently viable. Several hypotheses are advanced as to the nature of the distributional pattern in western Oregon, the origins of the North American hexaploid chromosome form, and the cytogeographical patterns exhibited by the two chromosomal levels.
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