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Widespread mycorrhizal specificity correlates to mycorrhizal function in the neotropical, epiphytic orchid Ionopsis utricularioides (Orchidaceae)¹

Departamento de Biología, Universidad de Puerto Rico-Río Piedras, P.O. Box 23360, San Juan, Puerto Rico 00931-3360 USA; Departamento de Ciencias Biológicas, Universidad Nacional de Colombia-Palmira, Carrera 32 Chapinero, Palmira, Valle del Cauca, Colombia, AA 237; Smithsonian Tropical Research Institute, Unit 0948, APO AA 34002-0948 USA Genetics and Biotechnology, Department of Biochemistry, University College Cork, Cork, Ireland;

ABSTRACT

Tropical orchids constitute the greater part of orchid diversity, but little is known about their obligate mycorrhizal relationships. The specificity of these interactions and associated fungal distributions could influence orchid distributions and diversity. We investigated the mycorrhizal specificity of the tropical epiphytic orchid Ionopsis utricularioides across an extensive geographical range. DNA ITS sequence variation was surveyed in both plants and mycorrhizal fungi. Phylogeographic relationships were estimated for the mycorrhizal fungi. Orchid functional outcomes were determined through in vitro seed germination and seedling growth with a broad phylogenetic representation of fungi. Most fungal isolates derived from one clade of Ceratobasidium (anamorphs assignable to Ceratorhiza), with 78% within a narrower phylogenetic group, clade B. No correlation was found between the distributions of orchid and fungal genotypes. All fungal isolates significantly enhanced seed germination, while fungi in clade B significantly enhanced seedling growth. These results show that I. utricularioides associates with a phylogenetically narrow, effective fungal clade over a broad distribution. This preference for a widespread mycorrhizae may partly explain the ample distribution and abundance of I. utricularioides and contrasts with local mycorrhizal diversification seen in some nonphotosynthetic orchids. Enhanced orchid function with a particular fungal subclade suggests mycorrhizal specificity can increase orchid fitness.

Key Words: Ceratobasidium • functional outcomes • Ionopsis utricularioides • neotropics • Orchidaceae • orchid mycorrhizae • Rhizoctonia • specificity

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