1999: 8 1719-1732
Population, habitat and genetic
mycorrhizal specialization in the cheating
maculata and C. mertensiana
D. Lee Taylor1
and Thomas D. Bruns2
Department of Ecology, Evolution and Marine Biology,
University of California, Santa Barbara, CA 93106, USA.
Correspondence: Dr D. Lee Taylor. Fax: +01-805-893-4724;
Department of Plant and Microbial Biology, 321 Koshland Hall,
University of California, Berkeley, California, 94720, USA
Unlike photosynthetic plants, several distantly related
nonphotosynthetic plants are highly specialized toward their
mycorrhizal fungi. It is unknown whether this specialization
varies geographically or is influenced by the environment. We
have investigated these questions in the nonphotosynthetic
orchids Corallorhiza maculata and C.
mertensiana by amplifying fungal internal transcribed
spacer (ITS) fragments from widespread mycorrhiza samples and
then discriminating putative fungal species using ITS
restriction fragment length polymorphisms (RFLPs). Three
fungal species were found across 27 plants representing seven
populations of C. mertensiana; 20 species were found
across 104 plants and 21 populations of C. maculata.
All fungi belonged to the Russulaceae, an ectomycorrhizal
family. Partitioning of Simpsons diversity showed that
48% of the variance in occurrences of fungal species
coincided with population boundaries in C.
mertensiana, vs. 68% in C. maculata. This
differentiation coincided with geography but not habitat in
C. mertensiana. In contrast, likelihood ratio tests
showed strong associations between fungal occurrence and both
habitat and phenotype in C. maculata. For example,
C. maculata populations growing under oaks had no
fungi in common with nearby populations growing under
conifers, and those above 2000 m had no fungi in common
with those below 2000 m. However, plant genetic
differentiation may underlie some of these patterns. C.
mertensiana and C. maculata never shared fungal
species, even when growing intermixed at the same site,
demonstrating genetic control that was independent of
habitat. Similarly, intermixed normal and pale-coloured
variants of C. maculata had no fungal species in
common. These results demonstrate fine-scale genetic
influences and geographical mosaicism in a mycorrhizal
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