On the origins of extreme mycorrhizal specificity in the Monotropoideae (Ericaceae): performance trade-offs during seed germination and seedling development

Monotropastrum kirishimense (Ericaceae), a new mycoheterotrophic plant from Japan based on multifaceted evidence

Due to their reduced morphology, non-photosynthetic plants have been one of the most challenging groups to delimit to species level. The mycoheterotrophic genus Monotropastrum, with the monotypic species M. humile, has been a particularly taxonomically challenging group, owing to its highly reduced vegetative and root morphology. Using integrative species delimitation, we have focused on Japanese Monotropastrum, with a special focus on an unknown taxon with rosy pink petals and sepals. We investigated its flowering phenology, morphology, molecular identity, and associated fungi. Detailed morphological investigation has indicated that it can be distinguished from M. humile by its rosy pink tepals and sepals that are generally more numerous, elliptic, and constantly appressed to the petals throughout its flowering period, and by its obscure root balls that are unified with the surrounding soil, with root tips that hardly protrude. Based on genome-wide single-nucleotide polymorphisms, molecular data has provided clear genetic differentiation between this unknown taxon and M. humile. Monotropastrum humile and this taxon are associated with different Russula lineages, even when they are sympatric. Based on this multifaceted evidence, we describe this unknown taxon as the new species M. kirishimense. Assortative mating resulting from phenological differences has likely contributed to the persistent sympatry between these two species, with distinct mycorrhizal specificity.

Weak population spatial genetic structure and low infraspecific specificity for fungal partners in the rare mycoheterotrophic orchid Epipogium aphyllum

Some plants abandoned photosynthesis and developed full dependency on fungi for nutrition. Most of the so-called mycoheterotrophic plants exhibit high specificity towards their fungal partners. We tested whether natural rarity of mycoheterotrophic plants and usual small and fluctuating population size make their populations more prone to genetic differentiation caused by restricted gene flow and/or genetic drift. We also tested whether these genetic characteristics might in turn shape divergent fungal preferences. We studied the mycoheterotrophic orchid Epipogium aphyllum, addressing the joint issues of genetic structure of its populations over Europe and possible consequences for mycorrhizal specificity within the associated fungal taxa. Out of 27 sampled E. aphyllum populations, nine were included for genetic diversity assessment using nine nuclear microsatellites and plastid DNA. Population genetic structure was inferred based on the total number of populations. Individuals from 17 locations were included into analysis of genetic identity of mycorrhizal fungi of E. aphyllum based on barcoding by nuclear ribosomal DNA. Epipogium aphyllum populations revealed high genetic diversity (uHe = 0.562) and low genetic differentiation over vast distances (F_ST = 0.106 for nuclear microsatellites and F_ST = 0.156 for plastid DNA). Bayesian clustering analyses identified only two genetic clusters, with a high degree of admixture. Epipogium aphyllum genets arise from panmixia and display locally variable, but relatively high production of ramets, as shown by a low value of rarefied genotypic richness (R_r = 0.265). Epipogium aphyllum genotype control over partner selection was negligible as (1) we found ramets from a single genetic individual associated with up to 68% of the known Inocybe spp. associating with the plant species, (2) and partner identity did not show any geographic structure. The absence of mosaicism in the mycorrhizal specificity over Europe may be linked to preferential allogamous habit of E. aphyllum and significant gene flow, which tend to promote host generalism.

Of mutualism and migration: will interactions with novel ericoid mycorrhizal communities help or hinder northward Rhododendron range shifts?

Rapid climate change imperils many small-ranged endemic species as the climate envelopes of their native ranges shift poleward. In addition to abiotic changes, biotic interactions are expected to play a critical role in plant species' responses. Below-ground interactions are of particular interest given increasing evidence of microbial effects on plant performance and the prevalence of mycorrhizal mutualisms. We used greenhouse mesocosm experiments to investigate how natural northward migration/assisted colonization of Rhododendron catawbiense, a small-ranged endemic eastern U.S. shrub, might be influenced by novel below-ground biotic interactions from soils north of its native range, particularly with ericoid mycorrhizal fungi (ERM). We compared germination, leaf size, survival, and ERM colonization rates of endemic R. catawbiense and widespread R. maximum when sown on different soil inoculum treatments: a sterilized control; a non-ERM biotic control; ERM communities from northern R. maximum populations; and ERM communities collected from the native range of R. catawbiense. Germination rates for both species when inoculated with congeners' novel soils were significantly higher than when inoculated with conspecific soils, or non-mycorrhizal controls. Mortality rates were unaffected by treatment, suggesting that the unexpected reciprocal effect of each species' increased establishment in association with heterospecific ERM could have lasting demographic effects. Our results suggest that seedling establishment of R. catawbiense in northern regions outside its native range could be facilitated by the presence of extant congeners like R. maximum and their associated soil microbiota. These findings have direct relevance to the potential for successful poleward migration or future assisted colonization efforts.

Symbiotic germination and development of fully mycoheterotrophic plants convergently targeting similar Glomeraceae taxa

Plants producing dust seeds often meet their carbon demands by exploiting fungi at the seedling stage. This germination strategy (i.e. mycoheterotrophic germination) has been investigated among orchidaceous and ericaceous plants exploiting Ascomycota or Basidiomycota. Although several other angiosperm lineages have evolved fully mycoheterotrophic relationships with Glomeromycota, the fungal identities involved in mycoheterotrophic germination remain largely unknown. Here, we conducted in situ seed baiting and high-throughput DNA barcoding to identify mycobionts associated with seedlings of Burmannia championii (Burmanniaceae: Dioscoreales) and Sciaphila megastyla (Triuridaceae: Pandanales), which have independently evolved full mycoheterotrophy. Subsequently, we revealed that both seedlings and adults in B. championii and S. megastyla predominantly associate with Glomeraceae. However, mycorrhizal communities are somewhat distinct between seedling and adult stages, particularly in S. megastyla. Notably, the dissimilarity of mycorrhizal communities between S. megastyla adult samples and S. megastyla seedling samples is significantly higher than that between B. championi adult samples and S. megastyla adult samples, based on some indices. This pattern is possibly due to both mycorrhizal shifts during ontogenetic development and convergent recruitment of cheating-susceptible fungi. The extensive fungal overlap in two unrelated mycoheterotrophic plants indicates that both species convergently exploit specific AM fungal phylotypes.

Root-Associated Fungal Communities in Two Populations of the Fully Mycoheterotrophic Plant Arachnitis uniflora Phil. (Corsiaceae) in Southern Chile

The microbiological interactions of the roots of non-photosynthetic plants in South America have been scarcely explored. This study analyzes culturable fungal diversity associated with the mycoheterotrophic plant Arachnitis uniflora Phil. (Corsiaceae) in southern Chile, growing in two different understoreys of native (Nothofagus-dominated) and mixed forest (native, Cupressus sempervirens, and Pinus radiata). Rhizospheric and endophytic fungi were isolated, cultured, and purified to identify microorganisms associated with A. uniflora roots. We showed the different fungi associated with the plant, and that these distributions are influenced by the sampling site. We isolated 410 fungal strains (144 endophytic and 266 from the rhizosphere). We identified 13 operative taxonomical units from plants sampled in the mixed forest, while 15 were from the native forest. Rhizospheric microorganisms were mainly related to Penicillium spp., whereas some pathogenic and saprophytic strains were more frequent inside the roots. Our results have also shown that the fungal strains are weak for phosphate solubilization, but other pathways such as organic acid exudation and indole acetic acid production can be considered as major mechanisms to stimulate plant growth. Our results point to new fungal associates of A. uniflora plants reported in Andean ecosystems, identifying new beneficial endophytic fungi associated with roots of this fully mycoheterotrophic plant.

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

Plant populations occupying different habitats may diverge from each other over time and gradually accumulate genetic and morphological differences, ultimately resulting in ecotype or even species formation. In plant species that critically rely on mycorrhizal fungi, differences in mycorrhizal communities can contribute to ecological isolation by reducing or even inhibiting germination of immigrant seeds. In this study, we investigated whether the mycorrhizal communities available in the soil and associating with the roots of seedlings and adult plants of the partially mycoheterotrophic Pyrola rotundifolia differed between populations growing in sand dunes and forests. In addition, reciprocal germination experiments were performed to test whether native seeds showed higher germination than immigrant seeds. Our results showed that the mycorrhizal communities differed significantly between forest and dune populations, and that within populations seedlings and adults also associated with different mycorrhizal communities. In both forest and dune populations, mycorrhizal communities were dominated by members of the Thelephoraceae, but dune populations showed a higher incidence of members of the Inocybaceae, whereas forest populations showed a high abundance of members of the Russulaceae. Reciprocal germination experiments showed that native seeds showed a higher germination success than immigrant seeds and this effect was most pronounced in dune populations. Overall, these results demonstrate that plants of P. rotundifolia growing in dune and forest habitats associate with different mycorrhizal communities and that reduced germination of non-native seeds may contribute to reproductive isolation. We conclude that selection against immigrants may constitute an important reproductive barrier at early stages of the speciation process.

Pinewood nematode infection alters root mycoflora of Pinus tabulaeformis Carr

AIMS

This study investigates pinewood nematode's impacts on root mycoflora of Pinus tabulaeformis.

METHODS AND RESULTS

The biomass, colonization rate, community structure and diversity of root-associated fungi were investigated in pinewood nematode-infected and nematode-noninfected P. tabulaeformis. The results indicated that the roots of P. tabulaeformis were colonized highly by root-associated fungi, mainly ectomycorrhizal fungi (ECMF) and dark septate endophytes. Infection of pinewood nematode was associated with a significant (P < 0·05) decrease in root colonization rates by ECMF, dark septate endophytes and total hyphae, as well as in fungal biomass in the roots. Illumina MiSeq sequences of tagged amplicons of 18S rDNA region revealed Basidiomycota (65·70%) and Ascomycota (34·14%) as the dominant root-associated fungi in roots of P. tabulaeformis. Among the detected operational taxonomic units (OTUs), ECMF and dark septate endophytes exhibited a higher relative abundance in trees infected by pinewood nematode compared with noninfected ones.

CONCLUSIONS

The infection of pinewood nematode altered the composition and OTU abundance of root-associated fungi community in P. tabulaeformis roots with a decrease in the biomass, species richness and diversity of root-associated fungi, as well as in the colonization rates and abundance of ECMF and dark septate endophytes.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study is an important contribution for better understanding the interaction between pine wilt disease and root-associated fungi.

Russulaceae: a new genomic dataset to study ecosystem function and evolutionary diversification of ectomycorrhizal fungi with their tree associates

The family Russulaceae is considered an iconic lineage of mostly mushroom-forming basidiomycetes due to their importance as edible mushrooms in many parts of the world, and their ubiquity as ectomycorrhizal symbionts in both temperate and tropical forested biomes. Although much research has been focused on this group, a comprehensive or cohesive synthesis by which to understand the functional diversity of the group has yet to develop. Interest in ectomycorrhizal fungi, of which Russulaceae is a key lineage, is prodigious due to the important roles they play as plant root mutualists in ecosystem functioning, global carbon sequestration, and a potential role in technology development toward environmental sustainability. As one of the most species-diverse ectomycorrhizal lineages, the Russulaceae has recently been the focus of a dense sampling and genome sequencing initiative with the Joint Genome Institute aimed at untangling their functional roles and testing whether functional niche specialization exists for independent lineages of ectomycorrhizal fungi. Here we present a review of important studies on this group to contextualize what we know about its members' evolutionary history and ecosystem functions, as well as to generate hypotheses establishing the Russulaceae as a valuable experimental system.

Fungal-host diversity among mycoheterotrophic plants increases proportionally to their fungal-host overlap

The vast majority of plants obtain an important proportion of vital resources from soil through mycorrhizal fungi. Generally, this happens in exchange of photosynthetically fixed carbon, but occasionally the interaction is mycoheterotrophic, and plants obtain carbon from mycorrhizal fungi. This process results in an antagonistic interaction between mycoheterotrophic plants and their fungal hosts. Importantly, the fungal-host diversity available for plants is restricted as mycoheterotrophic interactions often involve narrow lineages of fungal hosts. Unfortunately, little is known whether fungal-host diversity may be additionally modulated by plant-plant interactions through shared hosts. Yet, this may have important implications for plant competition and coexistence. Here, we use DNA sequencing data to investigate the interaction patterns between mycoheterotrophic plants and arbuscular mycorrhizal fungi. We find no phylogenetic signal on the number of fungal hosts nor on the fungal hosts shared among mycoheterotrophic plants. However, we observe a potential trend toward increased phylogenetic diversity of fungal hosts among mycoheterotrophic plants with increasing overlap in their fungal hosts. While these patterns remain for groups of plants regardless of location, we do find higher levels of overlap and diversity among plants from the same location. These findings suggest that species coexistence cannot be fully understood without attention to the two sides of ecological interactions.

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

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.

Increased phylogenetic resolution within the ecologically important Rhizopogon subgenus Amylopogon using 10 anonymous nuclear loci

Rhizopogon species are ecologically significant ectomycorrhizal fungi in conifer ecosystems. The importance of this system merits the development and utilization of a more robust set of molecular markers specifically designed to evaluate their evolutionary ecology. Anonymous nuclear loci (ANL) were developed for R. subgenus Amylopogon. Members of this subgenus occur throughout the United States and are exclusive fungal symbionts associated with Pterospora andromedea, a threatened mycoheterotrophic plant endemic to disjunct eastern and western regions of North America. Candidate ANL were developed from 454 shotgun pyrosequencing and assessed for positive amplification across targeted species, sequencing success, and recovery of phylogenetically informative sites. Ten ANL were successfully developed and were subsequently used to sequence representative taxa, herbaria holotype and paratype specimens in R. subgenus Amylopogon. Phylogenetic reconstructions were performed on individual and concatenated data sets by Bayesian inference and maximum likelihood methods. Phylogenetic analyses of these 10 ANL were compared with a phylogeny traditionally constructed using the universal fungal barcode nuc rDNA ITS1-5.8S-ITS2 region (ITS). The resulting ANL phylogeny was consistent with most of the species designations delineated by ITS. However, the ANL phylogeny provided much greater phylogenetic resolution, yielding new evidence for cryptic species within previously defined species of R. subgenus Amylopogon. Additionally, the rooted ANL phylogeny provided an alternate topology to the ITS phylogeny, which inferred a novel set of evolutionary relationships not identified in prior phylogenetic studies.

Age-dependent mycorrhizal specificity in an invasive orchid, Oeceoclades maculata

PREMISE OF THE STUDY

Oeceoclades maculata is a naturalized, invasive, terrestrial orchid in Puerto Rico and elsewhere in the neotropics. We asked whether its success might be partly explained by its mycorrhizal associations, hypothesizing a relationship with many fungal partners or with one widely distributed partner.

METHODS

Oeceoclades maculata roots were collected throughout Puerto Rico, and the degree of mycorrhizal colonization was measured. For identification of fungi, the ITS region was sequenced from pure cultures and directly from roots. Representative fungi were used for symbiotic seed germination experiments.

KEY RESULTS

Colonization of O. maculata roots was very variable. The most common fungus identified by BLAST searches was Psathyrella cf. candolleana, but typical orchid mycorrhizal fungi (Ceratobasidium and Tulasnella) were also found, as were a range of saprotrophs. Seeds germinated in vitro only in the presence of Psathyrella.

CONCLUSIONS

These results are surprising in two respects. First, O. maculata appears to be highly specific for fungi during seed germination, but unusually promiscuous as adult plants. Second, mycorrhizal associations with Psathyrella and with other saprotrophic fungi have been previously reported, but only from mycoheterotrophic (i.e., nonphotosynthetic) orchids, not from green orchids like Oeceoclades. This combination may partly explain the success of Oeceoclades.

Morphological and molecular analyses of fungal endophytes of achlorophyllous gametophytes of Diphasiastrum alpinum (Lycopodiaceae)

PREMISE OF THE STUDY

To understand the early evolution of mycorrhizal symbioses, it is important to know the fungal partners of gametophytes and sporophytes for basal lineages of vascular plants. Subterranean mycotrophic gametophytes of the clubmoss Diphasiastrum alpinum found at three localities gave an opportunity to study their morphology and anatomy and to identify and describe their hitherto unknown fungal endophytes. In addition, sporophytes were screened for fungal partners.

METHODS

Gametophytes with attached young sporophytes were excavated, and their anatomy and their associated fungi were studied by light microscopy. DNA was isolated and amplified with both universal and group-specific fungal primers for the ITS region, the large subunit and small subunit of the nuclear rDNA, respectively, to identify the fungal partner.

KEY RESULTS

Gametophytes were uniformly colonized by a fungus with septate hyphae forming coils and vesicles. Its morphology resembles that of the sebacinoid genus Piriformospora. Both ITS and LSU sequences were identified as Sebacinales group B, a basal clade of the Agaricomycetes (Basidiomycota). This fungus was detected in 11 gametophytes from two localities and in rootlets of adjacent Calluna vulgaris (Ericaceae) plants, but was absent in roots of sporophytes. In addition, several ascomycetes and glomeromycetes were found by DNA sequencing.

CONCLUSIONS

Our study suggests a fungus belonging to Sebacinales group B as the main fungal host of the D. alpinum gametophytes. However, Sebacinales group B fungi occur as well in adjacent Ericaceae plants; therefore, we assume the mycoheterotrophic gametophyte to be epiparasitic on Ericaceae, which would explain the steady association of these plants.

Ploidy-specific symbiotic interactions: divergence of mycorrhizal fungi between cytotypes of the Gymnadenia conopsea group (Orchidaceae)

Polyploidy is widely recognized as a major mechanism of sympatric speciation in plants, yet little is known about its effects on interactions with other organisms. Mycorrhizal fungi are among the most common plant symbionts and play an important role in plant nutrient supply. It remains to be understood whether mycorrhizal associations of ploidy-variable plants can be ploidy-specific. We examined mycorrhizal associations in three cytotypes (2x, 3x, 4x) of the Gymnadenia conopsea group (Orchidaceae), involving G. conopsea s.s. and G. densiflora, at different spatial scales and during different ontogenetic stages. We analysed: adults from mixed- and single-ploidy populations at a regional scale; closely spaced adults within a mixed-ploidy site; and mycorrhizal seedlings. All Gymnadenia cytotypes associated mainly with saprotrophic Tulasnellaceae (Basidiomycota). Nonetheless, both adults and seedlings of diploids and their autotetraploid derivatives significantly differed in the identity of their mycorrhizal symbionts. Interploidy segregation of mycorrhizal symbionts was most pronounced within a site with closely spaced adults. This study provides the first evidence that polyploidization of a plant species can be associated with a shift in mycorrhizal symbionts. This divergence may contribute to niche partitioning and facilitate establishment and co-existence of different cytotypes.

Morpho-anatomical and molecular characterization of the mycorrhizas of European Polygala species

The mycorrhizas of 12 species of Polygala (Polygalaceae), including herbs, subshrubs and one shrub, collected from Germany, Mallorca (Spain) and Malta, were investigated by morpho-anatomical and molecular methods. Aseptate hyphae, arbuscules and vesicles indicate an arbuscular mycorrhiza in all species examined. Hyphal spread in Polygala is predominantly, but not exclusively, intracellular and comprises three characteristic stages of colonization: (i) intracellular, linear hyphal growth in a cascading manner after penetration towards the penultimate parenchyma layer (layer 2), (ii) initially linear hyphal growth in the cells of layer 2 from where hyphal branches repeatedly penetrate the anatomically distinct innermost parenchyma layer (layer 1), forming arbuscule-like structures therein which are subject to degeneration, (iii) more branches from the linear hyphae in layer 2 develop, but coil and make contact to the layer outside layer 2 (layer 3) in which arbuscule-like structures similar to those in layer 1 form and degenerate. This general colonization pattern differs in details between the species, and critical comparisons, in particular between the woody P. myrtifolia, the herbaceous Polygala spp. and the mycoheterotrophic Epirixanthes spp. (Polygalaceae) suggest an evolutionary shift of mycorrhizal features within the family towards an optimization of plant benefit through the fungus. Based on the molecular marker 18S rDNA mycorrhizal fungi detected in roots of Polygala spp. are largely restricted to five clades of Glomeraceae 1 (Glomus Group A). This result rejects the hypothesis of a strict symbiotic specificity in Polygalaceae but may stimulate a discussion on functionally compatible groups of fungi.

Mycoheterotrophic germination of Pyrola asarifolia dust seeds reveals convergences with germination in orchids

Dust seeds that germinate by obtaining nutrients from symbiotic fungi have evolved independently in orchids and 11 other plant lineages. The fungi involved in this 'mycoheterotrophic' germination have been identified in some orchids and non-photosynthetic Ericaceae, and proved identical to mycorrhizal fungi of adult plants. We investigated a third lineage, the Pyroleae, chlorophyllous Ericaceae species whose partial mycoheterotrophy at adulthood has recently attracted much attention. We observed experimental Pyrola asarifolia germination at four Japanese sites and investigated the germination pattern and symbiotic fungi, which we compared to mycorrhizal fungi of adult plants. Adult P. asarifolia, like other Pyroleae, associated with diverse fungal species that were a subset of those mycorrhizal on surrounding trees. Conversely, seedlings specifically associated with a lineage of Sebacinales clade B (endophytic Basidiomycetes) revealed an intriguing evolutionary convergence with orchids, some of which also germinate with Sebacinales clade B. Congruently, seedlings clustered spatially together, but not with adults. This unexpected transition in specificity and ecology of partners could support the developmental transition from full to partial mycoheterotrophy, but probably challenges survival and distribution during development. We discuss the physiological and ecological traits that predisposed to the repeated recruitment of Sebacinales clade B for dust seed germination.

Is rarity of pinedrops (Pterospora andromedea) in eastern North America linked to rarity of its unique fungal symbiont?

Like other myco-heterotrophic plants, Pterospora andromedea (pinedrops) is dependent upon its specific fungal symbionts for survival. The rarity of pinedrops fungal symbiont was investigated in the eastern United States where pinedrops are rare. Wild populations of eastern pinedrops were sampled, and the plant haplotypes and fungal symbionts were characterized with molecular techniques; these data were compared to those from the West with phylogenetic analyses. The frequency of the fungal symbiont in eastern white pine forests was assessed using a laboratory soil bioassay and in situ pinedrops seed baiting. Only one plant haplotype and fungal symbiont was detected. The plant haplotype was not unique to the East. The fungal symbiont appears to be a new species within the genus Rhizopogon, closely related to the western symbionts. This fungal species was not frequent in soils with or without pinedrops, but was less frequent in the latter and in comparison to the fungal symbionts in western forests. Seed baiting resulted in few germinants, suggesting that mycelial networks produced by the eastern fungal symbiont were rare. Results suggest that eastern pinedrops rarity is influenced by the distribution and rarity of its fungal symbiont.

Mycorrhizal fungi associated with Monotropastrum humile (Ericaceae) in central Japan

We explored the diversity of mycorrhizal fungi associated with Monotropastrum humile in the central part of Japan's main island. We collected 103 M. humile individuals from 12 sites with various forest types. We analyzed the DNA sequences of the internal transcribed spacer region from fungal and plant nuclear ribosomal DNAs to assess the genetic diversity of the fungi associated with M. humile roots and to position the plant with respect to known Monotropoideae groups, respectively. The plants formed a monophyletic clade with other members of M. humile but were separated from M. humile var. glaberrimum and other monotropes (97% bootstrap support). Of the 50 fungal phylotypes, 49 had best matches with the Russulales, and the other had highest similarity with the Thelephoraceae. Our phylogenetic analysis suggests that M. humile roots have a highly specialized association with fungal partners in the Russulaceae. Moreover, a few fungal phylotypes from the M. humile roots had positions neighboring those from Monotropa uniflora roots. These results indicated that the genetic diversity of mycorrhizal fungi of M. humile was highly specific to the Russulaceae, but with high diversity within that family, and that the fungi associated with M. humile differ from those associated with M. uniflora.

Mycorrhizal specificity in the fully mycoheterotrophic Hexalectris Raf. (Orchidaceae: Epidendroideae)

Mycoheterotrophic species have abandoned an autotrophic lifestyle and obtain carbon exclusively from mycorrhizal fungi. Although these species have evolved independently in many plant families, such events have occurred most often in the Orchidaceae, resulting in the highest concentration of these species in the tracheophytes. Studies of mycoheterotrophic species' mycobionts have generally revealed extreme levels of mycorrhizal specialization, suggesting that this system is ideal for studying the evolution of mycorrhizal associations. However, these studies have often investigated single or few, often unrelated, species without consideration of their phylogenetic relationships. Herein, we present the first investigation of the mycorrhizal associates of all species of a well-characterized orchid genus comprised exclusively of mycoheterotrophic species. With the employment of molecular phylogenetic methods, we identify the fungal associates of each of nine Hexalectris species from 134 individuals and 42 populations. We report that Hexalectris warnockii associates exclusively with members of the Thelephoraceae, H. brevicaulis and H. grandiflora associate with members of the Russulaceae and Sebacinaceae subgroup A, while each member of the H. spicata species complex associates primarily with unique sets of Sebacinaceae subgroup A clades. These results are consistent with other studies of mycorrhizal specificity within mycoheterotrophic plants in that they suggest strong selection within divergent lineages for unique associations with narrow clades of mycorrhizal fungi. Our results also suggest that mycorrhizal associations are a rapidly evolving characteristic in the H. spicata complex.

Are fungi important for breaking seed dormancy in desert species? Experimental evidence in Opuntia streptacantha (Cactaceae)

Seeds of Opuntia spp. have physiological dormancy; they need a period of after-ripening to break dormancy, and the embryos have low growth potential. We evaluated the combined effects of seed age and presence of fungi on the testa on germination of Opuntia streptacantha, an abundant species in the Chihuahuan Desert (Mexico), assuming that older seeds have broken seed dormancy and fungi can reduce mechanical resistance to germination. In a preliminary experiment, we found no germination of 9-year-old (1998) and freshly collected (2007) seeds. However, we obtained 67% and 27% germination from 9-year-old and fresh non-sterilized seeds, respectively, and found fungi growing on the testa of all germinated seeds. Two fungal strains were isolated and identified using ribosomal internal transcribed spacer (ITS) sequence analysis: Penicillium chrysogenum and Phoma sp. In a second experiment, we inoculated seeds with strains of P. chrysogenum and Phoma sp., as well as Trichoderma koningii and binucleate Rhizoctonia (Gto17S2), to evaluate their ability to break seed dormancy. Seeds inoculated with P. chrysogenum, Phoma sp. and T. koningii had higher germination than controls for both seed ages, but germination was higher in older seeds. Scanning electron microscopy showed that these fungi eroded the funiculus, reducing its resistance. Binucleate Rhizoctonia did not lead to germination and controls had almost no germination. Our results strongly indicate that fungi are involved in breaking seed dormancy of O. streptacantha, and that the effect of fungi on seeds is species-specific.

Structural characteristics of root-fungus associations in two mycoheterotrophic species, Allotropa virgata and Pleuricospora fimbriolata (Monotropoideae), from southwest Oregon, USA

All members of the Monotropoideae (Ericaceae), including the species, Allotropa virgata and Pleuricospora fimbriolata, are mycoheterotrophs dependent on associated symbiotic fungi and autotrophic plants for their carbon needs. Although the fungal symbionts have been identified for A. virgata and P. fimbriolata, structural details of the fungal-root interactions are lacking. The objective of this study was, therefore, to determine the structural features of these plant root-fungus associations. Root systems of these two species did not develop dense clusters of mycorrhizal roots typical of some monotropoid species, but rather, the underground system was composed of elongated rhizomes with first- and second-order mycorrhizal adventitious roots. Both species developed mantle features typical of monotropoid mycorrhizas, although for A. virgata, mantle development was intermittent along the length of each root. Hartig net hyphae were restricted to the host epidermal cell layer, and fungal pegs formed either along the tangential walls (P. fimbriolata) or radial walls (A. virgata) of epidermal cells. Plant-derived wall ingrowths were associated with each fungal peg, and these resembled transfer cells found in other systems. Although the diffuse nature of the roots of these two plants differs from some members in the Monotropoideae, the structural features place them along with other members of the Monotropoideae in the "monotropoid" category of mycorrhizas.

Rangewide analysis of fungal associations in the fully mycoheterotrophic Corallorhiza striata complex (Orchidaceae) reveals extreme specificity on ectomycorrhizal Tomentella (Thelephoraceae) across North America

Fully mycoheterotrophic plants offer a fascinating system for studying phylogenetic associations and dynamics of symbiotic specificity between hosts and parasites. These plants frequently parasitize mutualistic mycorrhizal symbioses between fungi and trees. Corallorhiza striata is a fully mycoheterotrophic, North American orchid distributed from Mexico to Canada, but the full extent of its fungal associations and specificity is unknown. Plastid DNA (orchids) and ITS (fungi) were sequenced for 107 individuals from 42 populations across North America to identify C. striata mycobionts and test hypotheses on fungal host specificity. Four largely allopatric orchid plastid clades were recovered, and all fungal sequences were most similar to ectomycorrhizal Tomentella (Thelephoraceae), nearly all to T. fuscocinerea. Orchid-fungal gene trees were incongruent but nonindependent; orchid clades associated with divergent sets of fungi, with a clade of Californian orchids subspecialized toward a narrow Tomentella fuscocinerea clade. Both geography and orchid clades were important determinants of fungal association, following a geographic mosaic model of specificity on Tomentella fungi. These findings corroborate patterns described in other fully mycoheterotrophic orchids and monotropes, represent one of the most extensive plant-fungal genetic investigations of fully mycoheterotrophic plants, and have conservation implications for the >400 plant species engaging in this trophic strategy worldwide.

Evidence of a myco-heterotroph in the plant family Ericaceae that lacks mycorrhizal specificity

Myco-heterotrophy is one of the longest-studied aspects of the mycorrhizal symbiosis, but there remain many critical, unanswered questions regarding the ecology and physiology of myco-heterotrophic plants and their associated fungi. The vast majority of all myco-heterotrophs studied to date have exhibited specificity towards narrow lineages of fungi, but it is unclear whether the loss of photosynthesis in these plants is contingent upon fungal specialization. Here, we examine the fungal associates of the myco-heterotroph Pyrola aphylla (Ericaceae) and its closest green relative Pyrola picta to determine the pattern of mycorrhizal specialization. Our findings show that both plant species associate with a range of root-inhabiting fungi, the majority of which are ectomycorrhizal taxa. This study provides the first example of a eudicotyledonous myco-heterotroph that is a mycorrhizal generalist, indicating that the loss of photosynthesis in myco-heterotrophs is not contingent upon fungal specialization.

Pityopus californicus: structural characteristics of seed and seedling development in a myco-heterotrophic species

Pityopus californicus (Eastw.) H. F. Copel., a monotypic member of the Monotropoideae in the family Ericaceae, is a myco-heterotrophic species with distribution limited to the Pacific Northwest of the USA. Young embryos of P. californicus developed mycorrhizal associations in seed packets that had been buried for up to 681 days, suggesting that seeds of P. californicus may require the presence of a fungus to achieve germination. Samples of nongerminated seeds and early stages in embryo and root development were subsequently processed for light microscopy, histochemistry, and transmission electron microscopy (TEM). Nongerminated seeds possessed a thick testa, lacked a shoot and root meristem, and consisted of an embryo with large parenchymatous cells containing protein bodies and starch grains as storage reserves. In the earliest developmental stage (seed coat still attached), fungal hyphae were present on the testa surface and between the testa and embryo. This stage was followed by embryo elongation, the organization of a root apical meristem, and the development of a well-developed fungal mantle surrounding the elongated embryo. At least two morphotypes were identified based on structural characteristics of the mantle. One of these, with ascomycetous septa, had Cenococcum-like features. Late-stage embryo/early root development revealed a typical mantle and Hartig net, with fungal pegs penetrating the outer tangential walls of epidermal cells. Transfer cell-like deposits of wall material, similar to those described in Monotropa spp., enclosed fungal pegs. The development of a Hartig net and fungal pegs suggests that nutrient exchange interfaces are required for seedling development.

The evolutionary ecology of myco-heterotrophy

Nonphotosynthetic mycorrhizal plants have long attracted the curiosity of botanists and mycologists, and they have been a target for unabated controversy and speculation. In fact, these puzzling plants dominated the very beginnings of the field of mycorrhizal biology. However, only recently has the mycorrhizal biology of this diverse group of plants begun to be systematically unravelled, largely following a landmark Tansley review a decade ago and crucial developments in the field of molecular ecology. Here I explore our knowledge of these evolutionarily and ecologically diverse plant-fungal symbioses, highlighting areas where there has been significant progress. The focus is on what is arguably the best understood example, the monotropoid mycorrhizal symbiosis, and the overarching goal is to lay out the questions that remain to be answered about the biology of myco-heterotrophy and epiparasitism.