Phylogenetic affinity of arbuscular mycorrhizal symbionts in <Emphasis Type="Italic">Psilotum nudum</Emphasis>

Many lineages of land plants (from lycopsids to angiosperms) have non-photosynthetic life cycle phases that involve obligate mycoheterotrophic arbuscular mycorrhizal (AM) associations where the plant host gains organic carbon through glomalean symbionts. Our goal was to isolate and phylogenetically identify the AM fungi associated with both the autotrophic and underground mycoheterotrophic life cycle phases of Psilotum nudum. Phylogenetic analyses recovered 11 fungal phylotypes in four diverse clades of Glomus A that form AM associations with P. nudum mycoheterotrophic gametophytes and autotrophic sporophytes, and angiosperm roots found in the same greenhouse pots. The correspondence of identities of AM symbionts in P. nudum sporophytes, gametophytes and neighboring angiosperms provides compelling evidence that photosynthetic heterospecific and conspecific plants can serve as the ultimate sources of fixed carbon for mycoheterotrophic gametophytes of P. nudum, and that the transfer of carbon occurs via shared fungal networks. Moreover, broader phylogenetic analyses suggest greenhouse Psilotum populations, like field-surveyed populations of mycoheterotrophic plants, form AM associations with restricted clades of Glomus A. The phylogenetic affinities and distribution of Glomus A symbionts indicate that P. nudum greenhouse populations have the potential to be exploited as an experimental system to further study the physiology, ecology and evolution of mycoheterotrophic AM associations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Arbuscular mycorrhizal symbionts in Botrychium (Ophioglossaceae)

Many plant species are characterized by a life cycle with a long-lived, subterranean phase that is completely dependent on mycorrhizal fungal symbionts for fixed carbon. This type of life cycle is both phylogenetically and ecologically widespread and is found in diverse vascular plant lineages from the tropics to subalpine meadows. Here we report on the molecular identities of the arbuscular mycorrhizal fungi associated with the autotrophic and underground mycoheterotrophic life cycle phases of the ferns Botrychium crenulatum and B. lanceolatum. We show that the Glomus taxa found in the mycoheterotrophic life cycle phases of B. crenulatum and B. lanceolatum are also found in conspecific and heterospecific photosynthetic neighboring plants. From our DNA sequence data, we infer carbon flow from photosynthetic plants to mycoheterotrophic plants through shared glomalean fungal networks. Finally, our phylogenetic analyses identify a major Glomus clade that forms associations with mycoheterotrophic life cycle stages of B. crenulatum and B. lanceolatum.     

Comparative analysis of the reproductive ecology of Monotropa and Monotropsis: Two mycoheterotrophic genera in the Monotropoideae (Ericaceae)

Studies of mycoheterotrophs, defined as plants that obtain carbon resources from associated mycorrhizal fungi, have fundamentally contributed to our understanding of the importance and complexity of symbiotic ecological interactions. However, to date, the reproductive ecology of these organisms remains empirically understudied, with existing literature presenting hypotheses about traits including a generalist pollination syndrome and autogamous self-pollination. To address this gap in our knowledge of the reproductive ecology of mycoheterotrophic plants, we comparatively analyzed three species of two monotropoid genera, Monotropa and Monotropsis. During three consecutive years of field observations and manipulations of four populations of Monotropa uniflora, seven of M. hypopitys (both red and yellow color forms), and two of Monotropsis odorata, we investigated flowering phenology, pollination ecology, breeding system, floral herbivory, and reproductive effort and output. Contrary to previous predictions, our results revealed that taxa are largely outcross-pollinated and specialized toward Bombus pollinators. Additionally, species differ in breeding system, timing and duration of reproductive development, fluctuations in reproductive effort and output, and fitness impacts of herbivory. This study is the first thorough investigation of the reproductive ecology of mycoheterotrophic species and provides insight into possible limitations in reproductive traits imposed by a mycoheterotrophic life history.     

Saprotrophic fungal mycorrhizal symbionts in achlorophyllous orchids: Finding treasures among the ‘molecular scraps’?

Mycoheterotrophic plants are achlorophyllous plants that obtain carbon from their mycorrhizal fungi. They are usually considered to associate with fungi that are (1) specific of each mycoheterotrophic species and (2) mycorrhizal on surrounding green plants, which are the ultimate carbon source of the entire system. Here we review recent works revealing that some mycoheterotrophic plants are not fungal-specific, and that some mycoheterotrophic orchids associate with saprophytic fungi. A re-examination of earlier data suggests that lower specificity may be less rare than supposed in mycoheterotrophic plants. Association between mycoheterotrophic orchids and saprophytic fungi arose several times in the evolution of the two partners. We speculate that this indirectly illustrates why transition from saprotrophy to mycorrhizal status is common in fungal evolution. Moreover, some unexpected fungi occasionally encountered in plant roots should not be discounted as ‘molecular scraps’, since these facultatively biotrophic encounters may evolve into mycorrhizal symbionts in some other plants.Key words: endophytic fungi, evolution of mycorrhizae, mycoheterophy, mycorrhizae, saprophytic fungi, specificityConsiderable advances were recently made in the ecology of achlorophyllous, heterotrophic plants that obtain carbon from their mycorrhizal fungi (Fig. 1). Most plants have contact with soil through mycorrhizal symbioses, in which roots associate with a suitable fungal partner. Fungi utilize soil mineral nutrients, and while sharing them with host plants, they generally receive carbon as a reward. In contrast, some achlorophyllous plants living in the shaded forest understorey have reversed the process. They receive carbon from their mycorrhizal fungi exclusively, hence the designation ‘mycoheterotrophic’ (MH) plants.¹ Mycoheterotrophy has appeared several times during the evolution of land plants, and more than 20 times among orchids that encompass half of all MH species.² In the last decade, the development of molecular tools has enabled researchers to identify many fungal symbionts, which are often uncultivable. The fungi occurring in the densely colonized roots of MH species often produce a stronger PCR signal than any fungal contaminant, making molecular tools very effective for this field of study.Open in a separate windowFigure 1Wullschlaegelia aphylla, a mycoheterotrophic orchid unspecifically associated with saprotrophic Mycena and Gymnopus species. (A) Whole plant at flowering time, with reduced, tuberoid root system at that period. (B) Section of mycorrhizal root showing intracellular hyphal pelotons at early stage (p), or late stage (undergoing lysis, lp); among orchids, the colonization of dead cortical cell (cc) is a unique feature to some saprotrophic fungi (picture by A. Faccio, University of Torino).     

Comparison of green and albino individuals of the partially mycoheterotrophic orchid Epipactis helleborine on molecular identities of mycorrhizal fungi,nutritional modes and gene expression in mycorrhizal roots

Some green orchids obtain carbon from their mycorrhizal fungi, as well as from photosynthesis. These partially mycoheterotrophic orchids sometimes produce fully achlorophyllous, leaf‐bearing (albino) variants. Comparing green and albino individuals of these orchids will help to uncover the molecular mechanisms associated with mycoheterotrophy. We compared green and albino Epipactis helleborine by molecular barcoding of mycorrhizal fungi, nutrient sources based on ¹⁵N and ¹³C abundances and gene expression in their mycorrhizae by RNA‐seq and cDNA de novo assembly. Molecular identification of mycorrhizal fungi showed that green and albino E. helleborine harboured similar mycobionts, mainly Wilcoxina. Stable isotope analyses indicated that albino E. helleborine plants were fully mycoheterotrophic, whereas green individuals were partially mycoheterotrophic. Gene expression analyses showed that genes involved in antioxidant metabolism were upregulated in the albino variants, which indicates that these plants experience greater oxidative stress than the green variants, possibly due to a more frequent lysis of intracellular pelotons. It was also found that some genes involved in the transport of some metabolites, including carbon sources from plant to fungus, are higher in albino than in green variants. This result may indicate a bidirectional carbon flow even in the mycoheterotrophic symbiosis. The genes related to mycorrhizal symbiosis in autotrophic orchids and arbuscular mycorrhizal plants were also upregulated in the albino variants, indicating the existence of common molecular mechanisms among the different mycorrhizal types.     

Two widespread green Neottia species (Orchidaceae) show mycorrhizal preference for Sebacinales in various habitats and ontogenetic stages

Plant dependence on fungal carbon (mycoheterotrophy) evolved repeatedly. In orchids, it is connected with a mycorrhizal shift from rhizoctonia to ectomycorrhizal fungi and a high natural ¹³C and ¹⁵N abundance. Some green relatives of mycoheterotrophic species show identical trends, but most of these remain unstudied, blurring our understanding of evolution to mycoheterotrophy. We analysed mycorrhizal associations and ¹³C and ¹⁵N biomass content in two green species, Neottia ovata and N. cordata (tribe Neottieae), from a genus comprising green and nongreen (mycoheterotrophic) species. Our study covered 41 European sites, including different meadow and forest habitats and orchid developmental stages. Fungal ITS barcoding and electron microscopy showed that both Neottia species associated mainly with nonectomycorrhizal Sebacinales Clade B, a group of rhizoctonia symbionts of green orchids, regardless of the habitat or growth stage. Few additional rhizoctonias from Ceratobasidiaceae and Tulasnellaceae, and ectomycorrhizal fungi were detected. Isotope abundances did not detect carbon gain from the ectomycorrhizal fungi, suggesting a usual nutrition of rhizoctonia‐associated green orchids. Considering associations of related partially or fully mycoheterotrophic species such as Neottia camtschatea or N. nidus‐avis with ectomycorrhizal Sebacinales Clade A, we propose that the genus Neottia displays a mycorrhizal preference for Sebacinales and that the association with nonectomycorrhizal Sebacinales Clade B is likely ancestral. Such a change in preference for mycorrhizal associates differing in ecology within the same fungal taxon is rare among orchids. Moreover, the existence of rhizoctonia‐associated Neottia spp. challenges the shift to ectomycorrhizal fungi as an ancestral pre‐adaptation to mycoheterotrophy in the whole Neottieae.     

High mycorrhizal specificity in a widespread mycoheterotrophic plant, Eulophia zollingeri (Orchidaceae)

Because mycoheterotrophic plants fully depend on their mycorrhizal partner for their carbon supply, the major limiting factor for the geographic distribution of these plants may be the presence of their mycorrhizal partner. Although this factor may seem to be a disadvantage for increasing geographic distribution, widespread mycoheterotrophic species nonetheless exist. The mechanism causing the wide distribution of some mycoheterotrophic species is, however, seldom discussed. We identified the mycorrhizal partner of a widespread mycoheterotrophic orchid, Eulophia zollingeri, using 12 individuals from seven populations in Japan, Myanmar, and Taiwan by DNA-based methods. All fungal ITS sequences from the roots closely related to those of Psathyrella candolleana (Coprinaceae) from GenBank accessions and herbarium specimens. These results indicate that E. zollingeri is exclusively associated with the P. candolleana species group. Further, the molecular data support the wide distribution and wide-ranging habitat of this fungal partner. Our data provide evidence that a mycoheterotrophic plant can achieve a wide distribution, even though it has a high mycorrhizal specificity, if its fungal partner is widely distributed.     

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.     

Arbuscular mycorrhizal associations in Lycopodiaceae

This study characterizes the molecular and phylogenetic identity of fungi involved in arbuscular mycorrhizal (AM) associations in extant Huperzia and Lycopodium (Lycopodiaceae). Huperzia and Lycopodium are characterized by a life cycle with long-lived autotrophic sporophytes and long-lived mycoheterotrophic (obtain all organic carbon from fungal symbionts) gametophytes. 18S ribosomal DNA was isolated and sequenced from Glomus symbionts in autotrophic sporophytes of seven species of Huperzia and Lycopodium and mycoheterotrophic Huperzia gametophytes collected from the Páramos of Ecuador. Phylogenetic analyses recovered four Glomus A phylotypes in a single clade (MH3) that form AM associations with Huperzia and Lycopodium. In addition, phylogenetic analyses of Glomus symbionts from other nonphotosynthetic plants demonstrate that most AM fungi that form mycoheterotrophic associations belong to at least four specific clades of Glomus A. These results suggest that most mycoheterotrophic plants that form AM associations do so with restricted clades of Glomus A. Moreover, the correspondence of identity of AM symbionts in Huperzia sporophytes and gametophytes raises the possibility that photosynthetic sporophytes are a source of carbon to conspecific mycoheterotrophic gametophytes via shared fungal networks.     

First flowering hybrid between autotrophic and mycoheterotrophic plant species: breakthrough in molecular biology of mycoheterotrophy

Among land plants, which generally exhibit autotrophy through photosynthesis, about 880 species are mycoheterotrophs, dependent on mycorrhizal fungi for their carbon supply. Shifts in nutritional mode from autotrophy to mycoheterotrophy are usually accompanied by evolution of various combinations of characters related to structure and physiology, e.g., loss of foliage leaves and roots, reduction in seed size, degradation of plastid genome, and changes in mycorrhizal association and pollination strategy. However, the patterns and processes involved in such alterations are generally unknown. Hybrids between autotrophic and mycoheterotrophic plants may provide a breakthrough in molecular studies on the evolution of mycoheterotrophy. We have produced the first hybrid between autotrophic and mycoheterotrophic plant species using the orchid group Cymbidium. The autotrophic Cymbidium ensifolium subsp. haematodes and mycoheterotrophic C. macrorhizon were artificially pollinated, and aseptic germination of the hybrid seeds obtained was promoted by sonication. In vitro flowering was observed five years after seed sowing. Development of foliage leaves, an important character for photosynthesis, segregated in the first generation; that is, some individuals only developed scale leaves on the rhizome and flowering stems. However, all of the flowering plants formed roots, which is identical to the maternal parent.     

Carbon and nitrogen metabolism in mycorrhizal networks and mycoheterotrophic plants of tropical forests: a stable isotope analysis

Most achlorophyllous mycoheterotrophic (MH) plants obtain carbon (C) from mycorrhizal networks and indirectly exploit nearby autotrophic plants. We compared overlooked tropical rainforest MH plants associating with arbuscular mycorrhizal fungi (AMF) to well-reported temperate MH plants associating with ectomycorrhizal basidiomycetes. We investigated (13)C and (15)N abundances of MH plants, green plants, and AMF spores in Caribbean rainforests. Whereas temperate MH plants and fungi have higher δ(13)C than canopy trees, these organisms displayed similar δ(13)C values in rainforests, suggesting differences in C exchanges. Although temperate green and MH plants differ in δ(15)N, they display similar (15)N abundances, and likely nitrogen (N) sources, in rainforests. Contrasting with the high N concentrations shared by temperate MH plants and their fungi, rainforest MH plants had lower N concentrations than AMF, suggesting differences in C/N of exchanged nutrients. We provide a framework for isotopic studies on AMF networks and suggest that MH plants in tropical and temperate regions evolved different physiologies to adapt in diverging environments.     

Molecular evolution of rbcL in the mycoheterotrophic coralroot orchids (Corallorhiza Gagnebin, Orchidaceae)

The RuBisCO large subunit gene (rbcL) has been the focus of numerous plant phylogenetic studies and studies on molecular evolution in parasitic plants. However, there has been a lack of investigation of photosynthesis gene molecular evolution in fully mycoheterotrophic plants. These plants invade pre-existing mutualistic associations between ectomycorrhizal trees and fungi, from which they obtain fixed carbon and nutrients. The mycoheterotrophic orchid Corallorhiza contains both green (photosynthetic) and non-green (putatively nonphotosynthetic) species. We sequenced rbcL from 31 accessions of eight species of Corallorhiza and hypothesized that some lineages would have pseudogenes resulting from relaxation of purifying selection on RuBisCO's carboxylase function. Phylogenetic analysis of rbcL+ITS gave high jackknife support for relationships among species. We found evidence of pseudogene formation in all lineages of the Corallorhiza striata complex and in some lineages of the C. maculata complex. Evidence includes: stop codons, frameshifts, decreased d(S)/d(N) ratios, replacements not observed in photosynthetic species, rate heterogeneity, and high likelihood of neutral evolution. The evolution of rbcL in Corallorhiza may serve as an exemplary system in which to study the effects of relaxed evolutionary constraints on photosynthesis genes for >400 documented fully mycoheterotrophic plant species.     

同位素示踪技术在丛枝菌根真菌生态学研究中的应用

丛枝菌根(arbuscular mycorrhizal,AM)真菌是生态系统中重要的土壤微生物之一。AM真菌菌丝体网络是由AM真菌菌丝体在土壤生态系统中连接两株或两株以上植物根系所形成的菌丝体网络。随着菌根学研究的深入,如何直观的揭示AM真菌的生态学功能已经成为相关领域关注的热点问题。研究发现,利用同位素示踪技术可以开展AM真菌与宿主植物对土壤矿质营养的吸收、转运等方面的研究,以及菌丝体网络对不同宿主植物之间营养物质的分配研究和AM真菌在生态系统生态学中的功能研究。基于此,为了阐明同位素示踪技术在AM真菌研究中的价值,围绕菌根学最新研究进展,系统回顾了利用同位素示踪技术探究AM共生体对不同元素吸收和转运的机制、同位素示踪技术在AM真菌菌丝体网络研究中的价值和利用同位素示踪技术研究AM真菌在生态系统中的功能,为AM真菌生态学功能的研究提供理论基础,并对本领域未来的研究方向和应用前景进行展望。     

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.     

Evolutionary histories and mycorrhizal associations of mycoheterotrophic plants dependent on saprotrophic fungi

Mycoheterotrophic plants (MHPs) are leafless, achlorophyllous, and completely dependent on mycorrhizal fungi for their carbon supply. Mycorrhizal symbiosis is a mutualistic association with fungi that is undertaken by the majority of land plants, but mycoheterotrophy represents a breakdown of this mutualism in that plants parasitize fungi. Most MHPs are associated with fungi that are mycorrhizal with autotrophic plants, such as arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi. Although these MHPs gain carbon via the common mycorrhizal network that links the surrounding autotrophic plants, some mycoheterotrophic lineages are associated with saprotrophic (SAP) fungi, which are free-living and decompose leaf litter and wood materials. Such MHPs are dependent on the forest carbon cycle, which involves the decomposition of wood debris and leaf litter, and have a unique biology and evolutionary history. MHPs associated with SAP fungi (SAP-MHPs) have to date been found only in the Orchidaceae and likely evolved independently at least nine times within that family. Phylogenetically divergent SAP Basidiomycota, mostly Agaricales but also Hymenochaetales, Polyporales, and others, are involved in mycoheterotrophy. The fungal specificity of SAP-MHPs varies from a highly specific association with a single fungal species to a broad range of interactions with multiple fungal orders. Establishment of symbiotic culture systems is indispensable for understanding the mechanisms underlying plant–fungus interactions and the conservation of MHPs. Symbiotic culture systems have been established for many SAP-MHP species as a pure culture of free-living SAP fungi is easier than that of biotrophic AM or ECM fungi. Culturable SAP-MHPs are useful research materials and will contribute to the advancement of plant science.

     

Mixotrophy in orchids: insights from a comparative study of green individuals and nonphotosynthetic individuals of Cephalanthera damasonium

Some green orchids obtain carbon (C) from their mycorrhizal fungi and photosynthesis. This mixotrophy may represent an evolutionary step towards mycoheterotrophic plants fully feeding on fungal C. Here, we report on nonphotosynthetic individuals (albinos) of the green Cephalanthera damasonium that likely represent another evolutionary step. Albino and green individuals from a French population were compared for morphology and fertility, photosynthetic abilities, fungal partners (using microscopy and molecular tools), and nutrient sources (as characterized by 15N and 13C abundances). Albinos did not differ significantly from green individuals in morphology and fertility, but tended to be smaller. They harboured similar fungi, with Thelephoraceae and Cortinariaceae as mycorrhizal partners and few rhizoctonias. Albinos were nonphotosynthetic, fully mycoheterotrophic. Green individuals carried out photosynthesis at compensation point and received almost 50% of their C from fungi. Orchid fungi also colonized surrounding tree roots, likely to be the ultimate C source. Transition to mycoheterotrophy may require several simultaneous adaptations; albinos, by lacking some of them, may have reduced ecological success. This may limit the appearance of cheaters in mycorrhizal networks.     

Seasonal and environmental changes of mycorrhizal associations and heterotrophy levels in mixotrophic Pyrola japonica (Ericaceae) growing under different light environments

? Premise of the study: Mixotrophy is a strategy whereby plants acquire carbon both through photosynthesis and heterotrophic exploitation of mycorrhizal fungi. In Euro-American Pyroleae species studied hitherto, heterotrophy levels vary according to species, sites of study, and possibly light conditions. We investigated mycorrhizal association and mixotrophy in the Asiatic forest species Pyrola japonica, and their plasticity under different light conditions. ? Methods: Pyrola japonica was sampled bimonthly in sunny and shaded conditions from a deciduous broadleaf forest. We microscopically assessed the rate of fungal colonization and sequenced the ITS to identify the mycorrhizal fungi. We measured (13)C and (15)N isotopic abundances in P. japonica as compared with neighboring autotrophic and mycoheterotrophic plants, to evaluate P. japonica's heterotrophy level. ? Key results: Pyrola japonica formed arbutoid mycorrhizas devoid of fungal mantles, with intracellular hyphal coils and a Hartig net. It tended to be more colonized by mycorrhizal fungi in spring and summer. Most associated fungi belonged to ectomycorrhizal taxa, and 84% of identified fungi were Russula spp. Rate of mycorrhizal colonization and Russula frequency tended to be higher in shaded conditions. Both δ(13)C and δ(15)N values of P. japonica were significantly higher in autotrophic plants, showing that about half of the carbon on average was received from mycorrhizal fungi. Both isotopic values negatively correlated with light availability, suggesting higher heterotrophy levels in shaded conditions. ? Conclusions: The mixotrophic P. japonica undergoes changes in mycorrhizal symbionts and carbon nutrition according to light availability. Our results suggest that during Pyroleae evolution, a tendency to increased heterotrophy emerged in the Pyrola/Orthilia clade.     

Community genetics: what have we accomplished and where should we be going?

Research in community genetics seeks to understand how the dynamic interplay between ecology and evolution shapes simple and complex communities and ecosystems. A community genetics perspective, however, may not be necessary or informative for all studies and systems. To better understand when and how intraspecific genetic variation and microevolution are important in community and ecosystem ecology, we suggest future research should focus on three areas: (i) determining the relative importance of intraspecific genetic variation compared with other ecological factors in mediating community and ecosystem properties; (ii) understanding the importance of microevolution in shaping ecological dynamics in multi-trophic communities; and (iii) deciphering the phenotypic and associated genetic mechanisms that drive community and ecosystem processes. Here, we identify key areas of research that will increase our understanding of the ecology and evolution of complex communities but that are currently missing in community genetics. We then suggest experiments designed to meet these current gaps.     

The magnitude and control of carbon transfer between plants linked by a common mycorrhizal network

Various claims have been made about the ecological significance of plant-to-plant carbon movement through common mycorrhizal networks (CMNs). Most suggest that resource competition among interconnected plants should be less important than previously thought. If true, that would profoundly alter our perception of how plants interact among themselves and with their environment. However, there are difficulties in quantifying the amounts of resource transferred via CMNs, ensuring that transfer is genuinely through hyphae, not soil, and understanding its control. Carbon movement has not been quantified in many of the published studies. Where it has, its likely functional role has not been clarified. Some recent, well-publicized research suggests that carbon transferred to trees via an ectomycorrhizal (EcM) network may be physiologically and ecologically important. Our view, however, is that the evidence for this remains equivocal. Appropriate controls for the possibility of carbon transfer via soil were not used under field conditions. In laboratory experiments, controls failed to clarify the role of EcM links in carbon transfer. To resolve some areas of uncertainty, abundances of ¹³C have been measured to estimate carbon transfers via an arbuscular mycorrhizal (AM) network connecting grasses and forbs of the same or different species. Permeable barriers to roots and hyphae allowed any direct carbon transfer via soil to be detected. Large amounts of carbon (typically 10% of that in roots) were transferred between linked plants via the CMN. Transferred carbon was never transported into shoots of 'receiver' plants. It remained in roots, probably inside fungal structures and, therefore, unavailable to the plants into which it was apparently transferred. Carbon transfer via an AM network does not allow 'resource sharing' among linked plants. It is probably irrelevant to the botanical components of a community, but it may be fundamental for fungal members. The 'mycocentric' view is that fungal structures within roots are parts of extended mycelia through which fungi move carbon according to their own carbon demands, not those of their autotrophic hosts.     

Ericoid fungal diversity: Challenges and opportunities for mycorrhizal research

Ericoid mycorrhiza occur only within the plant family Ericaceae, yet are globally widespread and contribute to carbon and nutrient cycling in many habitats where harsh conditions limit decomposition and plant nutrient uptake. An increasingly diverse range of fungi are recognized as ericoid symbionts and patterns in the distribution of ericoid taxa are beginning to emerge across scales. However, the true diversity of ericoid mycorrhizal fungi remains unresolved due to limited sampling from some regions and challenges associated with delineating mycorrhizal taxa from the broader fungal community associated with ericoid plants. Interpreting patterns in the diversity and distributions of ericoid mycorrhizal fungi will ultimately require improved understanding of their functional ecology and functional diversity, which is currently limited to a few well studied species. Fortunately, many ericoid taxa are amenable to experimental manipulation and continued ericoid mycorrhizal research promises to improve general understanding of the ecology and evolution of mycorrhizal symbioses.