Specificity of fungal associations of Pyroleae and Monotropa hypopitys during germination and seedling development

Abstract

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.

Mycorrhizal symbioses and the evolution of trophic modes in plants

“…Although current advances in high‐throughput sequencing have allowed to obtain more detailed insights into the mycorrhizal communities associating with autotrophic and mycoheterotrophic plants (e.g. Jacquemyn, Waud, et al, ; Johansson et al, ; Waud et al, ), they still are not suitable to obtain reliable estimates of fungal abundances and therefore to assess the contribution of each fungal partner to the carbon and nitrogen budget of partially and fully mycoheterotrophic plants. Recent research has shown that differences of relative abundances generated by high‐throughput techniques not necessarily reflect those of the actual taxon abundances (Zhang et al, ) and that relative sequence abundances may therefore lead to biased assessments of mycorrhizal abundance.…”

“…Evidence for complementarity effects over time was given by Bidartondo and Read (), who showed that the diversity of mycorrhizal fungi associating with seedlings and adult plants in a number of orchids generally increased from the seedling stage to the adult stage, indicating that plants gradually accumulate fungi during their development. Johansson et al () investigated germination and seedling development and the diversity of fungi associated with germinating seeds and subterranean seedlings (juveniles) in partially and fully mycoheterotrophic Ericaceae species. Their results showed that fungal host specificity generally increased during juvenile ontogeny, most pronounced in the fully mycoheterotrophic species, although a narrowing of fungal associates was also observed in two partially mycoheterotrophic species.…”

Mycorrhizal divergence and selection against immigrant seeds in forest and dune populations of the partially mycoheterotrophicPyrola rotundifolia

“…The quality‐filtered data set comprised 2,364 OTUs (4,302,420 sequences), of which 45 (1,963,237 sequences, 46%) were assigned to ectomycorrhizal OTUs that belonged to fungal families that have previously been shown to associate with various members of Monotropoideae and Pyroleae (Ericaceae) (Hynson et al, ; Johansson et al, ; Tedersoo et al, ; Vincenot et al, ) (Supporting Information Table ). The identified ectomycorrhizal fungi consisted of a variety of fungal classes and families, the majority belonging to the Thelephoraceae (15 OTUs–993,101 sequences), Inocybaceae (five OTUs–575,672 OTUs), Russulaceae (four OTUs–207,351 sequences) and Sebacinaceae (six OTUs–117,391 sequences).…”

“…The species produces a large number of dust seeds that rely on fungi for germination (i.e., initial mycoheterotrophy sensu Merckx ()). After germination, the species becomes partially photosynthetic, but still relies on fungi, mainly ectomycorrhizal fungi (Johansson, Bahram, Tedersoo, Kõljalg, & Eriksson, ; Tedersoo, Pellet, Kõljalg, & Selosse, ; Vincenot et al, ), to meet its carbon demands (i.e., partial mycoheterotrophy). Germinating seeds and the first underground stages of several Pyrola species have been shown to associate with different fungi than adult plants (Hashimoto et al, ; Hynson, Weiss, Preiss, Gebauer, & Treseder, ; Johansson et al, ), indicating differences in host specificity during seedling ontogeny.…”

“…After germination, the species becomes partially photosynthetic, but still relies on fungi, mainly ectomycorrhizal fungi (Johansson, Bahram, Tedersoo, Kõljalg, & Eriksson, ; Tedersoo, Pellet, Kõljalg, & Selosse, ; Vincenot et al, ), to meet its carbon demands (i.e., partial mycoheterotrophy). Germinating seeds and the first underground stages of several Pyrola species have been shown to associate with different fungi than adult plants (Hashimoto et al, ; Hynson, Weiss, Preiss, Gebauer, & Treseder, ; Johansson et al, ), indicating differences in host specificity during seedling ontogeny. Broad host specificity may allow seeds to germinate in a wide range of habitats with contrasting mycorrhizal communities, whereas narrow host specificity and possible adaptation to local mycorrhizal communities may represent a serious bottleneck for germination when seeds are dispersed from one habitat to another.…”