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Abstract
Mycoheterotrophy is a nutritional strategy in which plants obtain carbon and essential nutrients from fungal partners. Comparative studies of closely related taxa differing in mycoheterotrophic dependency offer important insights into the evolutionary transitions underlying this lifestyle. We integrated stable isotope (δ¹³C and δ15N) analyses, MIG-seq-based phylogenetics, and fungal metabarcoding to investigate the physiological ecology and evolutionary history of three Odontochilus taxa: the large-leaved O. fissus, the small-leaved O. nakaianus (including albino, chlorophyll-deficient variants), and the very small-leaved O. aff. fissus. Morphologically, O. aff. fissus differs from O. fissus in having reduced, often reddish scale leaves and coralloid rhizomes, which are traits commonly observed in fully mycoheterotrophic orchids or mixotrophic orchids with high heterotrophy. Albino individuals and protocorms of O. nakaianus exhibited isotope signatures characteristic of full mycoheterotrophy, whereas normal individuals of O. fissus, O. nakaianus, and O. aff. fissus displayed isotopic patterns indicative of partial mycoheterotrophy, with fungal dependence likely inversely correlated with leaf size. Metabarcoding revealed that all taxa consistently associated with Ceratobasidiaceae OTUs, suggesting that similar rhizoctonia fungi support varying degrees of mycoheterotrophy. MIG-seq analysis confirmed that O. aff. fissus, O. fissus, and O. nakaianus form distinct genetic clusters, while albino O. nakaianus individuals were genetically indistinguishable from their green counterparts. These findings provide evidence of both genetic and nutritional divergence between O. fissus and O. aff. fissus. The results expand our understanding of the mycoheterotrophic continuum in Odontochilus species associated with rhizoctonia fungi.
Published Version
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