Differences in mycorrhizal preferences between two tropical orchids

Orchids parasitize their mycorrhizal fungi and are dependent on them for seed germination. Controversy reigns over how specific the mycorrhizal association is in tropical species. Although there is little experimental evidence to support any viewpoint, some variation is known to exist. We compared mycorrhizal specificity and performance in two phylogenetically related epiphytic orchids from Puerto Rico, Tolumnia variegata and Ionopsis utricularioides (Oncidiinae) by integrating two techniques: phylogenetic analysis of mycorrhizal fungi based on nuclear ribosomal internal transcribed spacer (ITS) sequences, and symbiotic seed germination experiments. Most of the mycorrhizal isolates from T. variegata fell into four different clades of Ceratobasidium, while most of those from I. utricularioides were restricted to a single clade of the same genus. Seeds of T. variegata germinated equally well with fungi from both T. variegata and I. utricularioides, but seeds of I. utricularioides germinated significantly better with its own isolates. Seeds of I. utricularioides germinated and developed faster than those of T. variegata. Both the molecular phylogeny and the seed germination experiments showed that T. variegata is a generalist in its association with fungal symbionts. In contrast, I. utricularioides is more specialized and more effective at exploiting a specific fungal clade. Our data are consistent with the theoretical trade-offs between specialized and generalized interactions.