Interrelationships of Ledum species and their rust parasites in Western Canada and Alaska

Abstract

Ledum glandulosum extends northward from Western United States into southern British Columbia in and east of the Cascade Range and into extreme SE British Columbia and SW Alberta in the Rocky Mts. Further north, within the range of L. groenlandicum but beyond that of L. glandulosum, hybrids between the two species occur, notably in and near Banff National Park, indicating that L. glandulosum formerly extended further north. The hybrids form an irregular swarm among typical L. groenlandicum rather than a cline; for this reason, and because of numerous morphological and other distinctions, the taxa are maintained as species. L. groenlandicum and L. palustre ssp. decumbens overlap in Alaska, Yukon, and Mackenzie in a belt up to 700 miles wide. Six definite hybrids were detected out of ca. 300 specimens from the sympatric zone, and ca. 30 specimens showed signs of probable minor introgression. The marked sympatry with minimal hybridization makes it necessary to maintain L. groenlandicum at specific rank. A rust, Chrysomyxa ledicola, that freely attacks L. palustre ssp. decumbens and L. groenlandicum does not infect L. glandulosum, but it attacks the hybrids in the Banff region except those closely approaching L. glandulosum. Another rust, C. ledi, has developed morphologically distinct varieties each specialized to a single Ledum. In the Banff region hybrids have been found between C. ledi var. glandulosi and C. l. var. groenlandici on L. glandulosum × groenlandicum. C. ledi var. ledi, on L. palustre (including ssp. decumbens) is morphologically distinct from C. ledi var. groenlandici, further emphasizing the sharp distinctness of L. groenlandicum from L. palustre ssp. decumbens.