Cytotype distribution at a diploid-tetraploid contact zone in Chamerion (Epilobium) angustifolium (Onagraceae)

In North America. the geographic distributions of diploid and tetraploid Ch amerion (formerly Epilobium) angustifolium overlap in a narrow zone along t he southern border of the boreal forest and along the Rocky Mountains. We e xamined the frequency and distribution of diploid and tetraploid cytotypes in a narrow (5 km) zone of sympatry across an elevational gradient and in p utatively uniform diploid and tetraploid reference populations on the Beart ooth Pass, in the Rocky Mountains of southern Montana-northern Wyoming. All five reference populations sampled were dominated by a single cytotype, bu t only one was completely uniform. In the zone of sympatry, 27 transects we re sampled every 2 m for a total of 238 plants. Reproductive status (vegeta tive, Bower buds, open flowers) was recorded, and the ploidy of each plant was determined by Bow 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 t ransects, diploid; two transects, tetraploid), and in seven others either d iploids or tetraploids predominated (frequency >75%). Triploids represented 9% of the total sample and occurred most frequently in transects containin g both diploids and tetraploids (G = 3.4, df = 2, P = 0.07). Diploids were more often reproductive (in bud, Bower, or fruit) than either triploids or tetraploids (G = 12.0, df = 2, P < 0.001) and were the only cytotype to hav e produced open flowers. These results suggest that the zone of sympatry is best characterized as a mosaic rather than a dine, with diploid and tetrap loids in close proximity and that the distribution of polyploidy is regulat ed by ecological sorting in a heterogeneous physical environment.