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(American Journal of Botany. 2003;90:25-31.)
© 2003 Botanical Society of America, Inc.

Genetics and Molecular Biology

Spatial pattern of allozyme variation in a contact zone of Pinus Ponderosa and P. arizonica (Pinaceae)1

Bryan K. Epperson2,5, Myong Gi Chung3 and Frank W. Telewski4

2Department of Forestry, Michigan State University, East Lansing, Michigan 48824 USA; 3Department of Biology, Gyeongsang National University, Jinju 660-701, The Republic of Korea; 4W. J. Beal Botanical Garden, Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824 USA

The spatial distribution of genotypes for nine polymorphic allozyme loci was examined in a contact zone between Pinus ponderosa var. scopulorum and another tree regarded as either a separate species, Pinus arizonica, or variety, Pinus ponderosa var. arizonica, in southern Arizona. Previous work had identified a steep elevational cline for a key taxonomic trait, number of leaf-needles per fascicle, on the south slope of Mt. Lemmon. The present results indicate that the taxa are not fully interbreeding in this contact zone, because allozyme genotypes are considerably more spatially structured than expected for the dispersal characteristics of pines. The amount of spatial differentiation is also much less than that observed for needle number. It appears that this is due to the lack of differentiation for allozyme gene frequencies for the two types of trees, which is further evidenced by analysis of samples from two other populations away from the contact zone. It is likely that if the two taxa were isolated in the past, it was not for long enough nor complete enough to allow mutation-drift to create substantial differentiation between them. Another possible explanation is that introgression after recontact is so advanced that any differences have been erased throughout the Santa Catalina mountain range.

Key Words: allozymes • contact zone • genetic differentiation • geography • mutation drift • Pinus arizonica • Pinus ponderosa • spatial genetic structure






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