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Cytotype Distribution at a Diploid-Tetraploid Contact Zone in Chamerion (Epilobium) Angustifolium (Onagraceae)

Brian C. Husband and Douglas W. Schemske
American Journal of Botany
Vol. 85, No. 12 (Dec., 1998), pp. 1688-1694
Stable URL: http://www.jstor.org/stable/2446502
Page Count: 7
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Cytotype Distribution at a Diploid-Tetraploid Contact Zone in Chamerion (Epilobium) Angustifolium (Onagraceae)
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Abstract

In North America, the geographic distributions of diploid and tetraploid Chamerion (formerly Epilobium) angustifolium overlap in a narrow zone along the southern border of the boreal forest and along the Rocky Mountains. We examined the frequency and distribution of diploid and tetraploid cytotypes in a narrow (5 kin) zone of sympatry across an elevational gradient and in putatively uniform diploid and tetraploid reference populations on the Beartooth Pass, in the Rocky Mountains of southern Montana-northern Wyoming. All five reference populations sampled were dominated by a single cytotype, but only one was completely uniform In the zone of sympatry, 27 transects were sampled every 2 m for a total of 238 plants. Reproductive status (vegetative, flower buds, open flowers) was recorded, and the ploidy of each plant was determined by flow cytometry. Diploid and tetraploid plants predominated (36 and 55%, respectively) but were heterogeneously distributed among the transects. Six of the 27 transects were fixed for a single cytotype (four transects, diploid; two transects, tetraploid), and in seven others either diploids or tetraploids predominated (frequency >75%). Triploids represented 9% of the total sample and occurred most frequently in transects containing both diploids and tetraploids (G = 3 4, df = 2, P = 0.07). Diploids were more often reproductive (in bud, flower, or fruit) than either triploids or tetraploids (G = 12.0, df = 2, P < 0 001) and were the only cytotype to have produced open flowers. These results suggest that the zone of sympatry is best characterized as a mosaic rather than a cline, with diploid and tetraploids in close proximity and that the distribution of polyploidy is regulated by ecological sorting in a heterogeneous physical environment.

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