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Mycoheterotrophic plants obtain organic carbon from associated mycorrhizal fungi, fully or partially. Angiosperms with this form of nutrition possess exceptionally small 'dust seeds' which after germination develop 'seedlings' that remain subterranean for several years, fully dependent on fungi for supply of carbon. Mycoheterotrophs which as adults have photosynthesis thus develop from full to partial mycoheterotrophy, or autotrophy, during ontogeny. Mycoheterotrophic plants may represent a gradient of variation in a parasitism-mutualism continuum, both among and within species. Previous studies on plant-fungal associations in mycoheterotrophs have focused on either germination or the adult life stages of the plant. Much less is known about the fungal associations during development of the subterranean seedlings. We investigated germination and seedling development and the diversity of fungi associated with germinating seeds and subterranean seedlings (juveniles) in five Monotropoideae (Ericaceae) species, the full mycoheterotroph Monotropa hypopitys and the putatively partial mycoheterotrophs Pyrola chlorantha, P. rotundifolia, Moneses uniflora and Chimaphila umbellata. Seedlings retrieved from seed sowing experiments in the field were used to examine diversity of fungal associates, using pyrosequencing analysis of ITS2 region for fungal identification. The investigated species varied with regard to germination, seedling development and diversity of associated fungi during juvenile ontogeny. Results suggest that fungal host specificity increases during juvenile ontogeny, most pronounced in the fully mycoheterotrophic species, but a narrowing of fungal associates was found also in two partially mycoheterotrophic species. We suggest that variation in specificity of associated fungi during seedling ontogeny in mycoheterotrophs represents ongoing evolution along a parasitism-mutualism continuum.

Mycoheterotrophic plants obtain organic carbon from associated mycorrhizal fungi, fully or partially. Angiosperms with this form of nutrition possess exceptionally small 'dust seeds' which after germination develop 'seedlings' that remain subterranean for several years, fully dependent on fungi for supply of carbon. Mycoheterotrophs which as adults have photosynthesis thus develop from full to partial mycoheterotrophy, or autotrophy, during ontogeny. Mycoheterotrophic plants may represent a gradient of variation in a parasitism-mutualism continuum, both among and within species. Previous studies on plant-fungal associations in mycoheterotrophs have focused on either germination or the adult life stages of the plant. Much less is known about the fungal associations during development of the subterranean seedlings. We investigated germination and seedling development and the diversity of fungi associated with germinating seeds and subterranean seedlings (juveniles) in five Monotropoideae (Ericaceae) species, the full mycoheterotroph Monotropa hypopitys and the putatively partial mycoheterotrophs Pyrola chlorantha, P. rotundifolia, Moneses uniflora and Chimaphila umbellata. Seedlings retrieved from seed sowing experiments in the field were used to examine diversity of fungal associates, using pyrosequencing analysis of ITS2 region for fungal identification. The investigated species varied with regard to germination, seedling development and diversity of associated fungi during juvenile ontogeny. Results suggest that fungal host specificity increases during juvenile ontogeny, most pronounced in the fully mycoheterotrophic species, but a narrowing of fungal associates was found also in two partially mycoheterotrophic species. We suggest that variation in specificity of associated fungi during seedling ontogeny in mycoheterotrophs represents ongoing evolution along a parasitism-mutualism continuum.

Dust seeds are the smallest existing seeds in angiosperms. This paper summarizes taxonomic distribution, phylogeny, ontogeny, morphology, and recruitment behavior of dust seeds, concluding with a general hypothesis on the evolution of dust seeds. Plants with dust seeds depend on external sources of organic carbon for seedling development and are thus parasitic during recruitment. Species with dust seeds are either mycoheterotrophic (fully or partially) or parasitic on plants. Dust seeds are a derived feature which has evolved independently in at least 12 families (Burmanniaceae, Corsiaceae, Orchidaceae, Triuridaceae, Petrosaviaceae, Ericaceae, Gentianaceae, Polygalaceae, Orobanchaceae, Rubiaceae, Buddlejaceae and Gesneriaceae). For the three latter families parasitic behavior during recruitment has not yet been described, and should be considered as a hypothesis. Many, but not all, dust seeds possess features that are likely to have been selected for increasing buoyancy in air or water. Selection for maximal fecundity at the expense of reducing maternal resources per seed is the probable driver of dust seed evolution. As endosperm was reduced, undifferentiated embryo evolved as a by-product due to endosperm mediated control of embryo development. Ultimately, seed size reduction passed a threshold where resource acquisition became dependent on external hosts. In order to embark on an evolutionary trajectory towards host dependence, facultative parasitism must have been established in ancestral lineages. Mycoheterotrophic and mixotrophic plants probably evolved along with the rise of angiosperm dominated tropical forests beginning in the Late Cretaceous. It is suggested that selection for increasing seed size associated with the expansion of modern type tropical forests spurred a competition/colonization trade-off initiating a reversed evolutionary trajectory towards smaller seeds. A different process is suggested for true parasites with dust seeds (Orobanchaceae), wh, authorCount :2