Exploring mycorrhizal diversity in sympatric mycoheterotrophic plants: a comparative study of Monotropastrum humile var. humile and M. humile var. glaberrimum

Mycoheterotrophic plants (MHPs) rely on their mycorrhizal fungus for carbon and nutrient supply, thus a shift in mycobionts may play a crucial role in speciation. This study aims to explore the mycorrhizal diversity of two closely related and sympatric fully MHPs, Monotropastrum humile var. humile (Mhh) and M. humile var. glaberrimum (Mhg), and determine their mycorrhizal associations. A total of 1,108,710 and 1,119,071 ectomycorrhizal fungal reads were obtained from 31 Mhh and 31 Mhg, and these were finally assigned to 227 and 202 operational taxonomic units, respectively. Results show that sympatric Mhh and Mhg are predominantly associated with different fungal genera in Russulaceae. Mhh is consistently associated with members of Russula, whereas Mhg is associated with members of Lactarius. Associating with different mycobionts and limited sharing of fungal partners might reduce the competition and contribute to their coexistence. The ectomycorrhizal fungal communities are significantly different among the five forests in both Mhh and Mhg. The distinct mycorrhizal specificity between Mhh and Mhg suggests the possibility of different mycobionts triggered ecological speciation between sympatric species.

Distinct orchid mycorrhizal fungal communities among co-occurring Vanilla species in Costa Rica: root substrate and population-based segregation

Despite being the second largest family of flowering plants, orchids represent community structure variation in plant-microbial associations, contributes to niche partitioning in metacommunity assemblages. Yet, mycorrhizal communities and interactions remain unknown for orchids that are highly specialized or even obligated in their associations with their mycorrhizal partners. In this study, we sought to compare orchid mycorrhizal fungal (OMF) communities of three co-occurring hemiepiphytic Vanilla species (V. hartii, V. pompona, and V. trigonocarpa) in tropical forests of Costa Rica by addressing the identity of their OMF communities across species, root types, and populations, using high-throughput sequencing. Sequencing the nuclear ribosomal internal transcribed spacer (nrITS) yielded 299 fungal Operational Taxonomic Units (OTUs) from 193 root samples. We showed distinct segregation in the putative OMF (pOMF) communities of the three coexisting Vanilla hosts. We also found that mycorrhizal communities associated with the rare V. hartii varied among populations. Furthermore, we identified Tulasnellaceae and Ceratobasidiaceae as dominant pOMF families in terrestrial roots of the three Vanilla species. In contrast, the epiphytic roots were mainly dominated by OTUs belonging to the Atractiellales and Serendipitaceae. Furthermore, the pOMF communities differed significantly across populations of the widespread V. trigonocarpa and showed patterns of distance decay in similarity. This is the first report of different pOMF communities detected in roots of wild co-occurring Vanilla species using high-throughput sequencing, which provides evidence that three coexisting Vanilla species and their root types exhibited pOMF niche partitioning, and that the rare and widespread Vanilla hosts displayed diverse mycorrhizal preferences.

Symbiotic seed germination and seedling growth of mycorrhizal fungi in Paphiopedilum hirsutissimun (Lindl.Ex Hook.) Stein from China

Similar to other orchid species, Paphiopedilum hirsutissimum (Lindl.ex Hook.) Stein, relies on nutrients provided by mycorrhizal fungus for seed germination and seedling development in the wild owing to a lack of endosperm in its seeds. Therefore, obtaining suitable and specialized fungi to enhance seed germination, seedling formation, and further development is considered a powerful tool for orchid seedling propagation, reintroduction, and species conservation. In this study, we investigated the diversity, abundance, and frequency of endophytic fungal strains in the root organs of P. hirsutissimum. One family and five genera of the fungi were isolated and identified through rDNA-ITS sequencing. The ability of isolated fungi to germinate in vitro from the seeds of this species was evaluated, and the development of P. hirsutissimum protocorm has been described. The findings showed that the treatments inoculated with endophytic fungal DYXY033 may successfully support the advanced developmental stage of seedlings up to stage 5. In addition, scanning electron microscopy (SEM) revealed that the mycelium of this strain began to invade from either end of the seeds up to the embryo, extending rapidly from the inside to the outside. Its lengthening resulted in the bursting of the seed coat to form protocorms, which developed into seedlings. The results showed that DYXY033 has a high degree of mycobiont specificity under in vitro symbiotic seed germination conditions and is a representative mycorrhizal fungus with ecological value for the species. In summary, this strain may particularly be significant for the protection of P. hirsutissimum species that are endangered in China. In the long run, it may also contribute to global efforts in reintroducing orchid species and in realizing in situ restorations of threatened orchid populations.

Soil microbial community composition and functions are disrupted by fire and land use in a Mediterranean woodland

The intersection of fire, land use transformations, and climate change is putting Mediterranean climate-type ecosystems at risk of soil degradation and loss of ecosystem services. Ondik et al. (2022b) showed that in a Mediterranean dry sclerophyll woodland of South Australia, high severity fire and clearing and grazing practices impacted both physicochemical and biological soil quality indicators. Building upon the work of Ondik et al. (2022b) this study aims to 1) identify soil physicochemical properties impacted by fire and land management that are indirect drivers of changes to soil microbial community composition and 2) determine whether the observed changes to soil microbial community composition affect soil microbial functions. Via a redundancy analysis, we identified fire and management-induced changes to pH, soil water repellency, nutrient stoichiometry, and total nutrient content as significant drivers of the composition of soil microbial communities. We then measured basal respiration, substrate induced respiration, and the carbon mineralisation quotient, and calculated functional trait distributions among microbial communities by linking 16S and 18S rRNA sequences to respiration modes and functional guilds, respectively. We found that fire reduced soil microbial respiration and the relative abundance (RA) of microbial symbionts, anaerobic bacteria, and microaerophilic bacteria, while increasing the RA of aerobic bacteria. Furthermore, management increased the RA of post-fire ectomycorrhizal fungi and may have reduced pathogenic load, microbial efficiency, and wood saprotrophs, while increasing litter, soil, and other saprotrophic species that are adapted to grasslands. This study shows that, through changes to microbial community composition, high severity wildfire and land management affected soil respiration rates, bacterial modes of respiration, prevalence of symbiotic bacteria and fungi, and microbial substrate preference. Having identified the main physicochemical drivers of changes to microbial community composition, we provide valuable insights into how fire and land management can impact soils in Mediterranean woodland.

Co-occurring epiphytic orchids have specialized mycorrhizal fungal niches that are also linked to ontogeny

Mycorrhizal symbiosis has been related to the coexistence and community assembly of coexisting orchids in few studies despite their obligate dependence on mycorrhizal partners to establish and survive. In hyper-diverse environments like tropical rain forests, coexistence of epiphytic orchids may be facilitated through mycorrhizal fungal specialization (i.e., sets of unique and dominant mycorrhizal fungi associated with a particular host species). However, information on the role of orchid mycorrhizal fungi (OMF) in niche differentiation and coexistence of epiphytic orchids is still scarce. In this study, we sought to identify the variation in fungal preferences of four co-occurring epiphytic orchids in a tropical rainforest in Costa Rica by addressing the identity and composition of their endophytic fungal and OMF communities across species and life stages. We show that the endophytic fungal communities are formed mainly of previously recognized OMF taxa, and that the four coexisting orchid species have both a set of shared mycorrhizal fungi and a group of fungi unique to an orchid species. We also found that adult plants keep the OMF of the juvenile stage while adding new mycobionts over time. This study provides evidence for the utilization of specific OMF that may be involved in niche segregation, and for an aggregation mechanism where adult orchids keep initial fungal mycobionts of the juvenile stage while adding others.

Mycorrhizal diversity and community composition in co-occurring Cypripedium species

Orchids commonly rely on mycorrhizal fungi to obtain the necessary resources for seed germination and growth. Whereas most photosynthetic orchids typically associate with so-called rhizoctonia fungi to complete their life cycle, there is increasing evidence that other fungi may be involved as well and that the mycorrhizal communities associated with orchids may be more diverse. Coexisting orchid species also tend to associate with different fungi to reduce competition for similar resources and to increase long-term population viability. However, few studies have related the mycorrhizal communities in the rhizosphere to communities found in the roots of closely related coexisting orchid species. In this study, we used high-throughput sequencing to investigate the diversity and community composition of orchid mycorrhizal fungi in the roots and the rhizosphere of four Cypripedium species growing in forests in Northeast China. The results showed that the investigated Cypripedium species associated with a wide variety of fungi including members of Tulasnellaceae, Psathyrellaceae, and Herpotrichiellaceae, whereas members of Russulaceae, Cortinariaceae, Thelephoraceae, and Herpotrichiellaceae showed high abundance in rhizosphere soils. The diversity of fungi detected in the rhizosphere soil was much higher than that in the roots. The observed variation in fungal communities in Cypripedium roots was not related to forest site or orchid species. On the other hand, variation in mycorrhizal communities of rhizosphere soil was significantly related to sampling site. These results indicate that orchid mycorrhizal communities in the rhizosphere display considerable variation among sites and that orchids use only a subset of the locally available fungi. Future studies focusing on the fine-scale spatial distribution of orchid mycorrhizal fungi and more detailed assessments of local environmental conditions will provide novel insights into the mechanisms explaining variation of fungal communities in both orchid roots and the rhizosphere.

Low Specificity but Dissimilar Mycorrhizal Communities Associating with Roots May Contribute to the Spatial Pattern of Four Co-Occurring Habenaria (Orchidaceae) Species

Fungi with orchid roots have been increasingly proven to play important roles in orchid growth, spatial distribution, and coexistence of natural communities. Here, we used 454 amplicon pyrosequencing with two different primer combinations to investigate the spatial variations in the community of OMF and endophytic fungi associates within the roots of four co-occurring Habenaria species. The results showed that all investigated Habenaria species were generalists and the different fungi communities may contribute to the spatial separation of the four Habenaria species. Firstly, the fungal OTUs identified in the roots of the four species overlapped but their presence differed amongst species and numerous distinct OMF families were unique to each species. Second, NMDS clustering showed samples clustered together based on associated species and PERMANOVA analyses indicated that fungi communities in the roots differed significantly between the Habenaria species, both for all endophytic fungi communities and for OMF communities. Third, the network structure of epiphytic fungi was highly specialized and modular but demonstrated lowly connected and anti-nested properties. However, it calls for more soil nutrition and soil fungal communities' studies to elucidate the contribution of habitat-specific adaptations in general and mycorrhizal divergence.

Pollination syndrome accurately predicts pollination by tangle-veined flies (Nemestrinidae: Prosoeca s.s.) across multiple plant families

The idea that a syndrome of floral traits predicts pollination by a particular functional group of pollinators remains simultaneously controversial and widely used because it allows plants to be rapidly assigned to pollinators. To test the idea requires demonstrating that there is an association between floral traits and pollinator type. I conducted such a test in the Cape Floristic Region of South Africa, by studying the pollination of eight plant species from six families that flower in spring and have scentless, actinomorphic, upwards-facing flowers, with orbicular petals all held in the same plane. The petals are brilliant-white with red-purple nectar guides. The tubes are short and hold small volumes of concentrated nectar, except in the rewardless Disa fasciata (Orchidaceae). Pollinators were photographed and captured, pollen loads were analysed and pollination networks were constructed. Consistent with the pollination syndrome hypothesis, the species with the defined syndrome shared a small group of pollinators. The most frequent pollinators belonged to a clade of four tangle-veined fly species with relatively short proboscises (Nemestrinidae: Prosoeca s.s.), while functionally similar Bombyliidae and Tabanidae played minor roles. Among the four Prosoeca species, only Prosoeca westermanni has been described, a result that highlights our ignorance about pollinators. The demonstration of an association between the syndrome of traits and pollination by this group of flies explains the repeated evolution of the syndrome across multiple plant families, and allows prediction of pollinators in additional species. More generally, the result validates the idea that the traits of organisms determine their ecology.

Induced polyploidy deeply influences reproductive life cycles, related phytochemical features, and phytohormonal activities in blackberry species

In some cases, polyploidy is an important phenomenon in the evolution of fruit crops. Polyploidy can be used in fruit breeding programs to develop varieties with higher yields and better fruit quality, as well as better adaptation to adverse environmental conditions. In this study, three wild species of blackberry were subjected to different degrees of induced polyploidy, and the effects of which were evaluated on morphological, physiological, and phytohormonal traits. With the aim of gaining a deep insight into the generative phase of plant growth and development, different levels of induced polyploidy were evaluated on the three blackberry species, i.e., Rubus persicus Bioss. (2x, 4x, and 8x), R. caesius L. (2x and 4x), and R. hirtus Schreb. (2x and 4x). The results showed that the polyploid plants performed significantly better than their diploid counterparts in terms of morphological traits such as flower count per spike and berry weight, as well as biochemical traits such as total soluble solids in the leaves. Induced polyploidy increased berry weight and drupe count per fruit. Microscopic examinations revealed a smaller number of viable pollen in the polyploids, compared to the diploids. Electron microscopy showed that the octaploid R. persicus had larger conical cells on the flower surface, compared to the diploid R. persicus. Correlation analysis showed that the ratio of indoleacetic acid to jasmonic acid changed synergistically with the total soluble solids in the leaves during the fruit set. The ploidy level correlated significantly with the number of pistils, leaf green index, total soluble solids in the leaves, and glucose content in floral nectar. Overall, induced polyploidy allowed Rubus to develop advantageous traits that can benefit future breeding programs and expand reproductive research in blackberries.

Do distylous syntopic plant species partition their floral morphological traits?

Morphological niche partitioning between related syntopic plants that are distylous (with short- and long-styled morphs) is complex. Owing to differences in the heights of stigmas and anthers, each floral morph must place pollen onto two distinct parts of the body of the pollinator. This led us to hypothesize that such partitioning should be more accurate among distylous syntopic species in comparison to combinations with other related plants that do not co-occur. We tested these assumptions using a set of Palicourea (Rubiaceae) species as a model system. We compared the distribution, flowering phenology, floral measurements and reciprocity of sexual organ heights of two syntopic species (Palicourea rigida and Palicourea coriacea) and one non-syntopic congener (Palicourea marcgravii). The three species overlapped in their distributions and flowering periods. The position of sexual organs was, in most cases, partitioned between syntopic populations, with low overlap in anther and stigma heights. However, we found a higher overlap involving the non-syntopic species, especially between Palicourea rigida and Palicourea marcgravii. Additionally, reciprocity of sexual organs was more accurate in intraspecific inter-morph combinations (i.e. legitimate organ correspondence) in comparison to intraspecific intra-morph, interspecific syntopic and interspecific non-syntopic combinations. The partitioning of morphological traits between syntopic species might facilitate the differential placement of pollen on the body of the pollinator and reduce the chances of interspecific interference.

Reconstructing an historical pollination syndrome: keel flowers

Background

Keel flowers are bilaterally symmetrical, pentamerous flowers with three different petal types and reproductive organs enclosed by keel petals; generally there is also connation of floral parts such as stamens and keel petals. In this study, the evolution of keel flowers within the order Fabales is explored to investigate whether the establishment of this flower type within one of the species-rich families, the Fabaceae (Leguminosae), preceded and could have influenced the evolution of keel flowers in the Polygalaceae. We conducted molecular dating, and ancestral area and ancestral state analyses for a phylogeny constructed for 678 taxa using published matK, rbcL and trnL plastid gene regions.

Results

We reveal the temporal and spatial origins of keel flowers and traits associated with pollinators, specifically floral symmetry, the presence or absence of a pentamerous corolla and three distinct petal types, the presence or absence of enclosed reproductive organs, androecium types, inflorescence types, inflorescence size, flower size, plant height and habit. Ancestral area reconstructions show that at the time keel flowers appeared in the Polygaleae, subfamily Papilionoideae of the Fabaceae was already distributed almost globally; at least eight clades of the Papilionoideae had keel flowers with a functional morphology broadly similar to the morphology of the first evolving Polygaleae flowers.

Conclusions

The multiple origins of keel flowers within angiosperms likely represent convergence due to bee specialization, and therefore pollinator pressure. In the case of the Fabales, the first evolving keel flowers of Polygaleae have a functional morphology that corresponds with keel flowers of species of the Papilionoideae already present in the environment. These findings are consistent with the keel-flowered Polygaleae exploiting pollinators of keel-flowered Papilionoideae. The current study is the first to use ancestral reconstructions of traits associated with pollination to demonstrate that the multiple evolutionary origins of the keel flower pollinator syndrome in Fabales are consistent with, though do not prove, mimicry.

The Waiting Room Hypothesis revisited by orchids: were orchid mycorrhizal fungi recruited among root endophytes?

BACKGROUND

As in most land plants, the roots of orchids (Orchidaceae) associate with soil fungi. Recent studies have highlighted the diversity of the fungal partners involved, mostly within Basidiomycotas. The association with a polyphyletic group of fungi collectively called rhizoctonias (Ceratobasidiaceae, Tulasnellaceae and Serendipitaceae) is the most frequent. Yet, several orchid species target other fungal taxa that differ from rhizoctonias by their phylogenetic position and/or ecological traits related to their nutrition out of the orchid roots (e.g. soil saprobic or ectomycorrhizal fungi). We offer an evolutionary framework for these symbiotic associations.

SCOPE

Our view is based on the 'Waiting Room Hypothesis', an evolutionary scenario stating that mycorrhizal fungi of land flora were recruited from ancestors that initially colonized roots as endophytes. Endophytes biotrophically colonize tissues in a diffuse way, contrasting with mycorrhizae by the absence of morphological differentiation and of contribution to the plant's nutrition. The association with rhizoctonias is probably the ancestral symbiosis that persists in most extant orchids, while during orchid evolution numerous secondary transitions occurred to other fungal taxa. We suggest that both the rhizoctonia partners and the secondarily acquired ones are from fungal taxa that have broad endophytic ability, as exemplified in non-orchid roots. We review evidence that endophytism in non-orchid plants is the current ecology of many rhizoctonias, which suggests that their ancestors may have been endophytic in orchid ancestors. This also applies to the non-rhizoctonia fungi that were secondarily recruited by several orchid lineages as mycorrhizal partners. Indeed, from our review of the published literature, they are often detected, probably as endophytes, in extant rhizoctonia-associated orchids.

CONCLUSION

The orchid family offers one of the best documented examples of the 'Waiting Room Hypothesis': their mycorrhizal symbioses support the idea that extant mycorrhizal fungi have been recruited among endophytic fungi that colonized orchid ancestors.

Seven new Serendipita species associated with Australian terrestrial orchids

Serendipita is one of the main fungal genera that form mutualistic associations with species of orchids (Orchidaceae). Here, seven new Serendipita species associated with various Australian orchid genera are described. These Serendipita species were originally characterized by multilocus DNA sequence species delimitation analyses (three mtDNA and four nuclear genes) and confirmed as distinct with addition of further isolates and reanalysis of nuc rDNA internal transcribed spacer region ITS1-5.8S-ITS2 (ITS) and nuc 28S rDNA (28S). Culture morphology and microscopic features are presented for each species, three of which are binucleate and four multinucleate. For the ITS region, the seven species have within-species sequence divergence between 1.07% and 4.31%, and all but one of the species pairs is separated by interspecific divergence of at least 4.35%. The newly described Serendipita species, S. australiana, S. communis, S. occidentalis, S. rarihospitum, S. secunda, S. talbotii, and S. warcupii, are shown to be separate species from S. vermifera on the basis of comparison against a sequence from the type. Isolates originally identified by Warcup as Sebacina "vermifera" from Caladenia orchids are revised and shown to belong to three of the species newly described here. Some non-Caladenia isolates identified by Warcup as S. "vermifera" are also shown to be non-conspecific with the type of S. vermifera. On the basis of ITS sequences, 346 isolates from 26 other studies, previously identified under provisional designations, are accommodated under the novel species. The species of Serendipta described here associate with the Australian orchid genera Caladenia, Cyanicula, Elythranthera, Ericksonella, Eriochilus, Glossodia, and Pheladenia. Most of the novel Serendipita species occur widely across Australia, often with widely distributed hosts, but one species, Serendipita rarihospitum, associates with narrowly distributed orchid species.

Orchid Root Associated Bacteria: Linchpins or Accessories?

Besides the plant-fungus symbiosis in arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) plants, many endorhizal and rhizosphere bacteria (Root Associated Bacteria, or RAB) also enhance plant fitness, diversity, and coexistence among plants via bi- or tripartite interactions with plant hosts and mycorrhizal fungi. Assuming that bacterial associations are just as important for the obligate mycorrhizal plant family Orchidaceae, surprisingly little is known about the RAB associated with orchids. Herein, we first present the current, underwhelming state of RAB research including their interactions with fungi and the influence of holobionts on plant fitness. We then delineate the need for novel investigations specifically in orchid RAB ecology, and sketch out questions and hypotheses which, when addressed, will advance plant-microbial ecology. We specifically discuss the potential effects of beneficial RAB on orchids as: (1) Plant Growth Promoting Rhizobacteria (PGPR), (2) Mycorrhization Helper Bacteria (MHB), and (3) constituents of an orchid holobiont. We further posit that a hologenomic view should be considered as a framework for addressing co-evolution of the plant host, their obligate Orchid Mycorrhizal Fungi (OMF), and orchid RAB. We conclude by discussing implications of the suggested research for conservation of orchids, their microbial partners, and their collective habitats.

How Mycorrhizal Associations Influence Orchid Distribution and Population Dynamics

Orchid distribution and population dynamics are influenced by a variety of ecological factors and the formation of holobionts, which play key roles in colonization and ecological community construction. Seed germination, seedling establishment, reproduction, and survival of orchid species are strongly dependent on orchid mycorrhizal fungi (OMF), with mycorrhizal cheating increasingly observed in photosynthetic orchids. Therefore, changes in the composition and abundance of OMF can have profound effects on orchid distribution and fitness. Network analysis is an important tool for the study of interactions between plants, microbes, and the environment, because of the insights that it can provide into the interactions and coexistence patterns among species. Here, we provide a comprehensive overview, systematically describing the current research status of the effects of OMF on orchid distribution and dynamics, phylogenetic signals in orchid-OMF interactions, and OMF networks. We argue that orchid-OMF associations exhibit complementary and specific effects that are highly adapted to their environment. Such specificity of associations may affect the niche breadth of orchid species and act as a stabilizing force in plant-microbe coevolution. We postulate that network analysis is required to elucidate the functions of fungal partners beyond their effects on germination and growth. Such studies may lend insight into the microbial ecology of orchids and provide a scientific basis for the protection of orchids under natural conditions in an efficient and cost-effective manner.

Progress and Prospects of Mycorrhizal Fungal Diversity in Orchids

Orchids form mycorrhizal symbioses with fungi in natural habitats that affect their seed germination, protocorm growth, and adult nutrition. An increasing number of studies indicates how orchids gain mineral nutrients and sometime even organic compounds from interactions with orchid mycorrhizal fungi (OMF). Thus, OMF exhibit a high diversity and play a key role in the life cycle of orchids. In recent years, the high-throughput molecular identification of fungi has broadly extended our understanding of OMF diversity, revealing it to be a dynamic outcome co-regulated by environmental filtering, dispersal restrictions, spatiotemporal scales, biogeographic history, as well as the distribution, selection, and phylogenetic spectrum width of host orchids. Most of the results show congruent emerging patterns. Although it is still difficult to extend them to all orchid species or geographical areas, to a certain extent they follow the "everything is everywhere, but the environment selects" rule. This review provides an extensive understanding of the diversity and ecological dynamics of orchid-fungal association. Moreover, it promotes the conservation of resources and the regeneration of rare or endangered orchids. We provide a comprehensive overview, systematically describing six fields of research on orchid-fungal diversity: the research methods of orchid-fungal interactions, the primer selection in high-throughput sequencing, the fungal diversity and specificity in orchids, the difference and adaptability of OMF in different habitats, the comparison of OMF in orchid roots and soil, and the spatiotemporal variation patterns of OMF. Further, we highlight certain shortcomings of current research methodologies and propose perspectives for future studies. This review emphasizes the need for more information on the four main ecological processes: dispersal, selection, ecological drift, and diversification, as well as their interactions, in the study of orchid-fungal interactions and OMF community structure.

Fungal symbionts may modulate nitrate inhibitory effect on orchid seed germination

Many orchid species are threatened, while some disappear from their natural habitats without obvious reasons. Eutrophication has been suggested as a possible factor and nitrate, which is able to suppress non-symbiotic orchid seed germination even at very low concentrations, and could pose a serious threat for natural orchid populations. Early ontogenesis of all orchids entirely depends on orchid mycorrhizal symbiosis, and at this initial mycoheterotrophic stage, many terrestrial green orchids associate with polyphyletic fungal symbionts (i.e., mycobionts), collectively called "rhizoctonias." We asked whether these fungi might also have some non-nutritional roles, i.e., whether they might confer resistance to eutrophication. To test this hypothesis, we co-cultivated seeds of the terrestrial orchid Dactylorhiza majalis with five rhizoctonias (two Tulasnella, two Ceratobasidium and one Serendipita isolate) at various ecologically meaningful nitrate concentrations (0 to 100 mg/L). With the exception of one Tulasnella isolate, all mycobionts supported the growth of protocorms and formed orchid mycorrhiza, i.e., intracellular hyphal pelotons, in the protocorms. Nitrate suppressed asymbiotic, as well as symbiotic, seed germination in all but one fungal treatment; the seeds co-cultivated with one of the Ceratobasidium isolates were indeed insensitive to nitrate. We conclude that nitrates also negatively affect symbiotic orchid germination, depending on the available compatible mycobionts. Thus, eutrophication with nitrate may decrease the number of orchid mycobionts capable of supporting seed germination.

The Diverse Assemblage of Fungal Endophytes from Orchids in Madagascar Linked to Abiotic Factors and Seasonality

The inselbergs of the Central Highlands of Madagascar are one of many ‘micro-hotspots’ of biodiversity on the island, particularly for Orchidaceae. In this region are several genera that have a large number of endemic species that are in serious decline or edging towards extinction. Studies relating to diversity of orchids and their fungal partners (both mycorrhizal and non-mycorrhizal root associates) deserve more attention, as climate change and human induced decline in resilience of species in the wild is at an all-time high. Identification of mycorrhizal fungi (MF) via conventional seed baited-protocorms has limitations for large scale studies and its application for time-bound conservation projects. The paper describes the value of understanding fungal diversity in the roots of orchids at different stages of maturity. The first part of the study was a preliminary investigation mainly to identify culturable Rhizoctonia endophytes, and the second part looked at all life forms of available taxa together with associated soil characteristics. We isolated and identified 19 putative MF from 18 of the 50 taxa spread over an area of 250 sq. km, covering three life forms, growth phases of the orchid taxa, and habitat types. In the rest of the taxa, we were unable to detect any putative MF, but had varying numbers of non-mycorrhizal endophytes. We also found that diversity of putative MF was higher in plants from soils with the lowest P levels recorded. Putative mycorrhizal OTUs were predominantly from the Tulasnella lineage, followed by Ceratobasidium and Serendipita. Within a small subset of samples, a difference in colonised endophytes depending on the collection season was observed. In vitro germination studies using 10 OTUs of mycorrhizal fungi in 14 orchid species showed mostly generalist associations. When orchid seed and fungal sources were studied irrespective of habitat, life form, and distance from each other (orchid seed and fungal source), compatibility for symbiotic seed germination was observed in most cases. Issues with the identification of compatible MF and symbiotic system of seed germination are discussed.

Geographic Mosaics of Fly Pollinators With Divergent Color Preferences Drive Landscape-Scale Structuring of Flower Color in Daisy Communities

The striking variation in flower color across and within Angiosperm species is often attributed to divergent selection resulting from geographic mosaics of pollinators with different color preferences. Despite the importance of pollinator mosaics in driving floral divergence, the distributions of pollinators and their color preferences are seldom quantified. The extensive mass-flowering displays of annual daisy species in Namaqualand, South Africa, are characterized by striking color convergence within communities, but also color turnover within species and genera across large geographic scales. We aimed to determine whether shifts between orange and white-flowered daisy communities are driven by the innate color preferences of different pollinators or by soil color, which can potentially affect the detectability of different colored flowers. Different bee-fly pollinators dominated in both community types so that largely non-overlapping pollinator distributions were strongly associated with different flower colors. Visual modeling demonstrated that orange and white-flowered species are distinguishable in fly vision, and choice experiments demonstrated strongly divergent color preferences. We found that the dominant pollinator in orange communities has a strong spontaneous preference for orange flowers, which was not altered by conditioning. Similarly, the dominant pollinator in white communities exhibited an innate preference for white flowers. Although detectability of white flowers varied across soil types, background contrast did not alter color preferences. These findings demonstrate that landscape-level flower color turnover across Namaqua daisy communities is likely shaped by a strong qualitative geographic mosaic of bee-fly pollinators with divergent color preferences. This is an unexpected result given the classically generalist pollination phenotype of daisies. However, because of the dominance of single fly pollinator species within communities, and the virtual absence of bees as pollinators, we suggest that Namaqua daisies function as pollination specialists despite their generalist phenotypes, thus facilitating differentiation of flower color by pollinator shifts across the fly pollinator mosaic.

Evolution of pollination syndromes and corolla symmetry in Balsaminaceae reconstructed using phylogenetic comparative analyses

BACKGROUND AND AIMS

Floral diversity as a result of plant-pollinator interactions can evolve by two distinct processes: shifts between pollination systems or divergent use of the same pollinator. Although both are pollinator driven, the mode, relative importance and interdependence of these different processes are rarely studied simultaneously. Here we apply a phylogenetic approach using the Balsaminaceae (including the species-rich genus Impatiens) to simultaneously quantify shifts in pollination syndromes (as inferred from the shape and colour of the perianth), as well as divergent use of the same pollinator (inferred from corolla symmetry).

METHODS

For 282 species we coded pollination syndromes based on associations between floral traits and known pollination systems, and assessed corolla symmetry. The evolution of these traits was reconstructed using parsimony- and model-based approaches, using phylogenetic trees derived from phylogenetic analyses of nuclear ribosomal and plastid DNA sequence data.

KEY RESULTS

A total of 71 % of studied species have a bee pollination syndrome, 22 % a bimodal syndrome (Lepidoptera and bees), 3 % a bird pollination syndrome and 5 % a syndrome of autogamy, while 19 % of species have an asymmetrical corolla. Although floral symmetry and pollination syndromes are both evolutionarily labile, the latter shifts more frequently. Shifts in floral symmetry occurred mainly in the direction towards asymmetry, but there was considerable uncertainty in the pattern of shift direction for pollination syndrome. Shifts towards asymmetrical flowers were associated with a bee pollination syndrome.

CONCLUSION

Floral evolution in Impatiens has occurred through both pollination syndrome shifts and divergent use of the same pollinator. Although the former appears more frequent, the latter is likely to be underestimated. Shifts in floral symmetry and pollination syndromes depend on each other but also partly on the region in which these shifts take place, suggesting that the occurrence of pollinator-driven evolution may be determined by the availability of pollinator species at large geographical scales.

The mycorrhizal community of the epiphytic orchid Thrixspermum japonicum is strongly biased toward a single Ceratobasidiaceae fungus, despite a wide range of fungal partners

PREMISE

Orchids depend primarily on mycorrhizal fungi to obtain nutrients throughout their life cycle. Epiphytic orchids account for 69% of orchid diversity. The unstable availability of water and nutrients in their arboreal habitats often results in severe water and nutrient stresses. Consequently, mycorrhizal associations may be important for the survival of epiphytic orchids, but our understanding thereof remains limited. Here, we investigated the mycorrhizal community in a single epiphytic orchid species, using more samples than in any previous study.

METHODS

We assessed the mycorrhizal communities of Thrixspermum japonicum, one of the most common epiphytic orchids in the temperate region of Japan. In total, 144 individuals were collected from 28 host tree species at 20 sites across 1300 km. The mycorrhizal fungi were identified based on nuclear ribosomal DNA internal transcribed spacer sequences and assigned operational taxonomic units (OTUs) based on 97% sequence similarity.

RESULTS

We obtained 24 OTUs; 9 belonged to the Ceratobasidiaceae and 15 to the Tulasnellaceae. These OTUs are widely distributed throughout the phylogenetic trees of the two fungal families. However, a single Ceratobasidiaceae OTU accounted for 49.7% of all fungal sequences and was predominant in samples from 15 host tree species and 12 sites.

CONCLUSIONS

Our results imply that despite having a broad range of mycorrhizal partners, T. japonicum was predominantly associated with a single fungal taxon at most of the sites among the host-tree species investigated. These findings contribute to elucidating mycorrhizal symbiosis in epiphytic habitats.

Fungal colonization associated with phenological stages of a photosynthetic terrestrial temperate orchid from the Southern Iberian Peninsula

Fungal endophytes, both mycorrhizal and non-mycorrhizal, are involved in the development of the life cycle of orchids, providing potential beneficial relationships. Here, we assess the succession of changes in the diversity of fungal symbionts associated with a terrestrial temperate orchid species, Anacamptis morio subsp. champagneuxii, over three phenological stages: developed leaves but no stem elongation, flowering, and fruiting. Fungi endophyte associated with roots were obtained by culture in sterile conditions. A total of 18 morphotypes-one Mortierellomycota, two Basidiomycota and 15 Ascomycota-were differentiated, and were also characterized using PCR and DNA sequencing techniques. Only three of the 18 OTUs are shared among the three phenological stages examined: Westerdykella sp., a member of Ceratobasidiaceae, and Fusarium oxysporum, representing a relative abundance of between 28% (fruiting) to 41% (flowering). Our research confirmed that fungal symbionts varied among the different phenological stages examined, the peak of endophyte diversity appearing in the flowering stage. The availability of a diverse mycobiota seems to be important for the survival of orchid plants because it may cover particular physiological needs, and knowledge concerning this mycobiota is of special relevance in the establishment of reliable conservation programmes.

Niche Perspectives on Plant-Pollinator Interactions

Ecological niches are crucial for species coexistence and diversification, but the niche concept has been underutilized in studying the roles of pollinators in plant evolution and reproduction. Pollination niches can be objectively characterized using pollinator traits, abundance, and distributions, as well as network topology. We review evidence that floral traits represent adaptations to pollination niches, where tradeoffs in trait deployment reinforce niche specialization. In turn, specialized pollination niches potentially increase speciation rates, foster species coexistence, and constrain species range limits. By linking studies of adaptation with those on speciation and coexistence, the pollination niche provides an organizing principle for research on plant reproduction, and conceptually unites these studies with fields of biology where the niche perspective is already firmly established.

Mycorrhizal Fungi Isolated from Native Terrestrial Orchids from Region of La Araucanía, Southern Chile

Mycorrhizal interactions of orchids are influenced by several environmental conditions. Hence, knowledge of mycorrhizal fungi associated with orchids inhabiting different ecosystems is essential to designing recovery strategies for threatened species. This study analyzes the mycorrhizal associations of terrestrial orchids colonizing grassland and understory in native ecosystems of the region of La Araucanía in southern Chile. Mycorrhizal fungi were isolated from peloton-containing roots and identified based on the sequence of the ITS region. Their capacities for seed germination were also investigated. We detected Tulasnella spp. and Ceratobasidium spp. in the pelotons of the analyzed orchids. Additionally, we showed that some Ceratobasidium isolates effectively induce seed germination to differing degrees, unlike Tulasnella spp., which, in most cases, fail to achieve protocorm growth. This process may underline a critical step in the life cycle of Tulasnella-associated orchids, whereas the Ceratobasidium-associated orchids were less specific for fungi and were effectively germinated with mycorrhizal fungi isolated from adult roots.

The Effect of Plant Geographical Location and Developmental Stage on Root-Associated Microbiomes of Gymnadenia conopsea

Gymnadenia conopsea (L.) R. Br. is an important perennial terrestrial photosynthetic orchid species whose microbiomes are considered to play an important role in helping its germination and growth. However, the assemblage of G. conopsea root-associated microbial communities is poorly understood. The compositions of fungal and bacterial communities from the roots and corresponding soil samples in G. conopsea across distinct biogeographical regions from two significantly different altitudes were characterized at the vegetative and reproductive growth stages. The geographical location, developmental stage and compartment were factors contributing to microbiome variation in G. conopsea. Predominant fungal taxa include Ascomycota, Basidiomycota, Mortierellomycota and Chytridiomycota, whereas Proteobacteria, Bacteroidetes, Acidobacteria, Actinobacteria, Verrucomicrobia, Chloroflexi, TM7 and Planctomycetes were predominant bacterial taxa. Using G. conopsea as a model, the structural and functional composition in G. conopsea root-associated microbiomes were comprehensive analyzed. Contrary to previous studies, biogeography was the main factor influencing the microbial community in this study. Besides, compartment and developmental stage should also be considered to analyze the variation of microbiota composition. Although the microbial composition varied greatly by location, the symbiotic microorganisms of G. conopsea still have certain specificity. This study gives an abundant information of G. conopsea root-associated microbiomes and provides new clues to better understanding the factors affecting the composition and diversity of fungal/bacterial communities associated with orchids. Our results also laying a foundation for harnessing the microbiome for sustainable G. conopsea cultivation. Moreover, these results might be generally applicable to other orchidaceae plants.

Do fungal associates of co-occurring orchids promote seed germination of the widespread orchid species Gymnadenia conopsea?

Interactions with mycorrhizal fungi have been increasingly recognized as one of the most important ecological factors determining the distribution and local abundance of orchids. While some orchid species may interact with a variety of fungal associates, others are more specific in their choice of mycorrhizal partners. Moreover, orchids that co-occur at a given site, often associate with different partners, possibly to avoid competition and to allow stable coexistence. However, whether differences in mycorrhizal partners directly affect seed germination and subsequent protocorm formation remains largely unknown. In this research, we used in vitro germination experiments to investigate to what extent seed germination and protocorm formation of Gymnadenia conopsea was affected by the origin and identity of fungal associates. Fungi were isolated from G. conopsea and three other co-occurring orchid species (Dactylorhiza viridis (Coeloglossum viride), Herminium monorchis, and Platanthera chlorantha). In total, eight fungal associates, belonging to Tulasnellaceae, Ceratobasidiaceae, and Serendipitaceae, were successfully isolated and cultured. While all eight fungal strains were able to promote early germination of G. conopsea seeds, only fungal strain GS2, a member of the Ceratobasidiaceae isolated from G. conopsea itself, was able to promote protocorm formation and subsequent growth to a seedling. Two other fungal strains isolated from G. conopsea only supported seed germination until the protocorm formation stage. The other five fungal strains isolated from the co-occurring orchid species did not support seed germination beyond the protocorm stage. We conclude that, although G. conopsea is considered a mycorrhizal generalist that associates with a wide range of fungi during its adult life, it requires specific fungi to promote protocorm formation and growth to a seedling.

Orchid seed sensitivity to nitrate reflects habitat preferences and soil nitrate content

Orchids are distributed around the world, however, the factors shaping their specific distribution and habitat preferences are largely unknown. Moreover, many orchids are at risk of becoming threatened as landscapes change, sometimes declining without apparent reason. One important factor affecting plant distribution is nutrient levels in the environment. Nitrates can inhibit not only orchid growth and persistence, but also seed germination. We used in vitro axenic cultures to exactly determine the germination sensitivity of seven orchid species to nitrates and correlated this with soil properties of the natural sites and with the species' habitat preferences. We found high variation in response to nitrate between species. Orchids from oligotrophic habitats were highly sensitive, while orchids from more eutrophic habitats were almost insensitive. Sensitivity to nitrate was also associated with soil parameters that indicated a higher nitrification rate. Our results indicate that nitrate can affect orchid distribution via direct inhibition of seed germination. Nitrate levels in soils are increasing rapidly due to intensification of agricultural processes and concurrent soil pollution, and we propose this increase could cause a decline in some orchid species.

Availability of orchid mycorrhizal fungi on roadside trees in a tropical urban landscape

Urban expansion threatens biodiversity worldwide, therefore urban spaces need to be amenable to biodiversity conservation. On trees in urban environments, natural colonisation and successful translocation of epiphytic orchids are necessary to enhance urban biodiversity, and depend on the availability of compatible orchid mycorrhizal fungi (OMF). However, the extent of OMF presence and distribution, as well as niche requirements for the OMF, remain poorly studied. To identify and quantify OMF on urban trees as well as assess their suitability for native epiphytic orchids, we conducted high-throughput sequencing on tree bark and orchid root samples. OMF were detected at 60% of the study sites on 16% of 270 bark samples (from stem, fork, and branch microsites within each tree). OMF presence and richness on bark samples were related to multiple biophysical factors; in general, humus presence and precipitation levels were positively predictive of OMF presence and richness. We found Ceratobasidiaceae- and Serendipitaceae-associated OMF both on bark and within roots. Orchid species also showed differing mycorrhizal specificity. Sites associated with fungal genera Ceratobasidium, Rhizoctonia, and Serendipita were considered suitable habitats for seven orchid species. The results suggest that urban trees support OMF and are therefore suitable for native orchid species; however, OMF availability are largely constrained by biophysical factors. To maximise the likelihood of translocation success and consequent natural establishment, we propose that (micro)sites are screened for compatible OMF prior to any intervention.

Nitrogen-fixing bacteria and Oxalis - evidence for a vertically inherited bacterial symbiosis

BACKGROUND

Plant-endophyte symbioses often revolve around nitrogen metabolism, and involve varying degrees of intimacy. Although evidence for vertical inheritance of nitrogen-fixing endophytic bacteria is increasing, it is confined mostly to crop plants, and to date no such system has been reported for geophytes.

METHODS

Bacterial endophytes associated with Oxalis, the most species-rich geophytic genus form the Cape Flora in southern Africa was studied. Culturable endophytes were isolated from surface-sterilized vegetative and reproductive plant organs for six host species at three locations. Colonies of microbes on various artificial media were morphotyped, enumerated and identified using sequence data. Filter exclusion experiments were conducted to determine if endophytes were vertically transmitted to seeds, determine if mucilage plays a role to actively attract microbes from the soil and to assess microbial richness isolated from the mucilage of Oxalis seedlings. Fluorescent microscopy was implemented in order to visualize endophytic bacteria in cryo-sectioned seeds.

RESULTS

Evidence for a novel, vertically transmitted symbiosis was reported. Communities of nitrogen-fixing and plant growth-promoting Bacillus endophytes were found to associate with selected Oxalis hosts from nitrogen-deficient environments of the Cape. Bacillus endophytes were ubiquitous and diverse across species and plant bodies, and were prominent in seeds. Three common nitrogen-fixing Bacillus have known oxalotrophic properties and appear to be housed inside specialised cavities (containing oxalates) within the plant body and seeds.

CONCLUSIONS

The discovery of vertical transmission and potential benefits to both host and endophyte suggest a particularly tight mutualism in the Oxalis-endophyte system. This discovery suggests unexpected ways in which geophytes might avoid nitrogen deficiency, and suggest that such symbioses are more common than previously expected.

A symbiotic balancing act: arbuscular mycorrhizal specificity and specialist fungus gnat pollination in the mycoheterotrophic genus Thismia (Thismiaceae)

BACKGROUND AND AIMS

Mycorrhizal associations in mycoheterotrophic plants are generally more specialized than in autotrophs. Mycoheterotrophs typically bear small, inconspicuous flowers that often self-pollinate to maximize seed set, although some have structurally complex flowers indicative of xenogamy. A trade-off has previously been proposed between specialization in these above- and below-ground symbioses, although empirical data are lacking.

METHODS

We used next-generation DNA sequencing to compare the mycorrhizal communities from the roots of a mycoheterotrophic species, Thismia tentaculata (Thismiaceae), and its neighbouring autotrophs. We furthermore conducted detailed assessments of floral phenology and pollination ecology, and performed artificial pollination experiments to determine the breeding system.

KEY RESULTS

Thismia tentaculata maintains a symbiotic association with a single arbuscular mycorrhizal Rhizophagus species. The flowers are pollinated by a single species of fungus gnats (Corynoptera, Sciaridae), which are attracted by the yellow pigments and are temporarily restrained within the perianth chamber before departing via apertures between the anthers. The plants are self-compatible but predominantly xenogamous.

CONCLUSIONS

Our findings demonstrate that T. tentaculata maintains highly specialized associations with pollinators and mycorrhizal fungi, both of which are widely distributed. We suggest that specialization in multiple symbiotic interactions is possible in mycoheterotrophs if redundant selective pressures are not exerted to further restrict an already constrained suite of life-history traits.

Environmental drivers for cheaters of arbuscular mycorrhizal symbiosis in tropical rainforests

Hundreds of nonphotosynthetic mycoheterotrophic plant species cheat the arbuscular mycorrhizal symbiosis. Their patchy local occurrence suggests constraints by biotic and abiotic factors, among which the role of soil chemistry and nutrient status has not been investigated. Here, we examine the edaphic drivers predicting the local-scale distribution of mycoheterotrophic plants in two lowland rainforests in South America. We compared soil chemistry and nutrient status in plots where mycoheterotrophic plants were present with those without these plants. Soil pH, soil nitrate, and the interaction between soil potassium and nitrate concentrations were the best predictors for the occurrence of mycoheterotrophic plants in these tropical rainforests. Mycoheterotrophic plant occurrences decreased with a rise in each of these predictors. This indicates that these plants are associated with low-fertility patches. Such low-fertility conditions coincide with conditions that potentially favour a weak mutualism between plants and arbuscular mycorrhizal fungi according to the trade balance model. Our study points out which soil properties favour the cheating of arbuscular mycorrhizal networks in tropical forests. The patchy occurrence of mycoheterotrophic plants suggests that local soil heterogeneity causes the stability of arbuscular mycorrhizal networks to vary at a very small scale.

Scent chemistry is key in the evolutionary transition between insect and mammal pollination in African pineapple lilies

Volatile emissions may play a key role in structuring pollination systems of plants with morphologically unspecialised flowers. Here we test for pollination by small mammals in Eucomis regia and investigate whether its floral scent differs markedly from fly- and wasp-pollinated congeners and attracts mammals. We measured floral traits of E. regia and made comparisons with insect-pollinated congeners. We observed floral visitors and examined fur and faeces of live-trapped mammals for pollen. We determined the contributions of different floral visitors to seed set with selective exclusion and established the breeding system with controlled pollination experiments. Using bioassays, we examined whether mammals are attracted by the floral scent and are effective agents of pollen transfer. Eucomis regia differs from closely related insect-pollinated species mainly in floral scent, with morphology, colour and nectar properties being similar. We found that mice and elephant-shrews pollinate E. regia, which is self-incompatible and reliant on vertebrates for seed production. Mammals are strongly attracted to the overall floral scent, which contains unusual sulphur compounds, including methional (which imparts the distinctive potato-like scent and which was shown to be attractive to small mammals). The results highlight the important role of scent chemistry in shifts between insect and mammal pollination systems.

Conceptual differences lead to divergent trait estimates in empirical and taxonomic approaches to plant mycorrhizal trait assignment

Empirical and taxonomic approaches are the two main methods used to assign plant mycorrhizal traits to species lists. While the empirical approach uses only available empirical information, the taxonomic approach extrapolates certain core information about plant mycorrhizal types and statuses to related species. Despite recent claims that the taxonomic approach is now almost definitive, with little benefit to be gained from further empirical data collection, it has not been thoroughly compared with the empirical approach. Using the most complete available plant mycorrhizal trait information for Europe and both assignment approaches, we calculate the proportion of species for each trait, and model environmental drivers of trait distribution across the continent. We found large degrees of mismatch between approaches, with consequences for biogeographical interpretation, among facultatively mycorrhizal (FM; 91% of species mismatched), non-mycorrhizal (NM; 45%), and to a lesser extent arbuscular mycorrhizal (AM; 16%) plant species. This can partly be attributed to the taxonomic precision of the taxonomic approach and the use of different AM, NM, and FM concepts. Our results showed that the extrapolations of the taxonomic approach do not consistently match with empirical information and indicate that more empirical data are needed, in particular for FM, NM, and AM plant species. Clarifying certain concepts underlying mycorrhizal traits and empirically describing NM, AM, and FM species within plant families can greatly improve our understanding of the biogeography of mycorrhizal symbiosis.

Small and genetically highly structured populations in a long-legged bee, Rediviva longimanus, as inferred by pooled RAD-seq

Adaptation to local host plants may impact a pollinator's population genetic structure by reducing gene flow and driving population genetic differentiation, representing an early stage of ecological speciation. South African Rediviva longimanus bees exhibit elongated forelegs, a bizarre adaptation for collecting oil from floral spurs of their Diascia hosts. Furthermore, R. longimanus foreleg length (FLL) differs significantly among populations, which has been hypothesised to result from selection imposed by inter-population variation in Diascia floral spur length. Here, we used a pooled restriction site-associated DNA sequencing (pooled RAD-seq) approach to investigate the population genetic structure of R. longimanus and to test if phenotypic differences in FLL translate into increased genetic differentiation (i) between R. longimanus populations and (ii) between phenotypes across populations. We also inferred the effects of demographic processes on population genetic structure and tested for genetic markers underpinning local adaptation. RESULTS: Populations showed marked genetic differentiation (average FST = 0.165), though differentiation was not statistically associated with differences between populations in FLL. All populations exhibited very low genetic diversity and were inferred to have gone through recent bottleneck events, suggesting extremely low effective population sizes. Genetic differentiation between samples pooled by leg length (short versus long) rather than by population of origin was even higher (FST = 0.260) than between populations, suggesting reduced interbreeding between long and short-legged individuals. Signatures of selection were detected in 1119 (3.8%) of a total of 29,721 SNP markers, CONCLUSIONS: Populations of R. longimanus appear to be small, bottlenecked and isolated. Though we could not detect the effect of local adaptation (FLL in response to floral spurs of host plants) on population genetic differentiation, short and long legged bees appeared to be partially differentiated, suggesting incipient ecological speciation. To test this hypothesis, greater resolution through the use of individual-based whole-genome analyses is now needed to quantify the degree of reproductive isolation between long and short legged bees between and even within populations.

Structural plasticity in root-fungal symbioses: diverse interactions lead to improved plant fitness

Root-fungal symbioses such as mycorrhizas and endophytes are key components of terrestrial ecosystems. Diverse in trophy habits (obligate, facultative or hemi-biotrophs) and symbiotic relations (from mutualism to parasitism), these associations also show great variability in their root colonization and nutritional strategies. Specialized interface structures such as arbuscules and Hartig nets are formed by certain associations while others are restricted to non-specialized intercellular or intracellular hyphae in roots. In either case, there are documented examples of active nutrient exchange, reinforcing the fact that specialized structures used to define specific mycorrhizal associations are not essential for reciprocal exchange of nutrients and plant growth promotion. In feremycorrhiza (with Austroboletus occidentalis and eucalypts), the fungal partner markedly enhances plant growth and nutrient acquisition without colonizing roots, emphasizing that a conventional focus on structural form of associations may have resulted in important functional components of rhizospheres being overlooked. In support of this viewpoint, mycobiome studies using the state-of-the-art DNA sequencing technologies have unearthed much more complexity in root-fungal relationships than those discovered using the traditional morphology-based approaches. In this review, we explore the existing literature and most recent findings surrounding structure, functioning, and ecology of root-fungal symbiosis, which highlight the fact that plant fitness can be altered by taxonomically/ecologically diverse fungal symbionts regardless of root colonization and interface specialization. Furthermore, transition from saprotrophy to biotrophy seems to be a common event that occurs in diverse fungal lineages (consisting of root endophytes, soil saprotrophs, wood decayers etc.), and which may be accompanied by development of specialized interface structures and/or mycorrhiza-like effects on plant growth and nutrition.

In situ Orchid Seedling-Trap Experiment Shows Few Keystone and Many Randomly Associated Mycorrhizal Fungal Species During Early Plant Colonization

Orchids are known for their vast diversity and dependency on mycorrhizal fungi. Under in situ conditions, the biotic and abiotic factors determining the composition and distribution of orchid mycorrhizal fungi (OMF) communities remain largely unexplored. Therefore in situ experiments are needed to better understand the interactions between orchids and fungi. A seedling-trap experiment was conducted in the Reserva Biológica San Francisco, a well-known biodiversity hotspot located in the Andes of southern Ecuador. The objective was to investigate the effect of orchid species, site, elevation or temporal variation on the assembly and structure of OMF associated with Cyrtochilum retusum and Epidendrum macrum. The OMF community composition was determined using the Illumina MiSeq sequencing of the internal transcribed spacer 2 (ITS2) region. The results exhibited 83 OMF operational taxonomic units belonging to Tulasnellaceae, Ceratobasidiaceae, Serendipitaceae and Atractiellales. It was observed that the composition of the OMF communities was different among orchid species and temporal variation but was not different among sites. The results further support that orchids have a core of keystone OMF that are ubiquitously distributed and stable across temporal change, whereas the majority of these fungi are randomly associated with the plants.

Fungal networks and orchid distribution: new insights from above- and below-ground analyses of fungal communities

Orchids are critically dependent on fungi for seedling establishment and growth, so the distribution and diversity of orchids might depend on the associated fungal communities. We characterised the communities associated with eight orchid species in three Mediterranean protected areas, using a combination of above-ground analyses of sporophores and below-ground molecular analyses of orchid root samples. In three years of sporophore collection in 25 plots around flowering orchid plants, 268 macrofungal species belonging to 84 genera were observed. Statistical analyses indicated a correlation between macrofungal diversity and orchid community variation, regardless of the effect of environmental and spatial factors characterizing the investigated orchid sites. Fungal ITS-DNA PCR amplification, cloning, and sequencing revealed Rhizoctonia-like fungi belonging to Ceratobasidiaceae (26 %), Tulasnellaceae (22.5 %), and Sebacinaceae (3.5 %), as well as other basidiomycetes and ascomycetes, in the roots of 99 orchid plants. Mycorrhizal specificity was low but co-occurring orchid species showed preferences for different partners. The diverse macrofungal communities found in the sites may contribute to orchid community variation without colonizing the orchid roots. Molecular analyses revealed a segregation of associated fungi, which may contribute to Mediterranean orchid coexistence in nature.

Long-legged bees make adaptive leaps: linking adaptation to coevolution in a plant-pollinator network

Adaptation is evolution in response to natural selection. Hence, an adaptation is expected to originate simultaneously with the acquisition of a particular selective environment. Here we test whether long legs evolve in oil-collecting Rediviva bees when they come under selection by long-spurred, oil-secreting flowers. To quantify the selective environment, we drew a large network of the interactions between Rediviva species and oil-secreting plant species. The selective environment of each bee species was summarized as the average spur length of the interacting plant species weighted by interaction frequency. Using phylogenetically independent contrasts, we calculated divergence in selective environment and evolutionary divergence in leg length between sister species (and sister clades) of Rediviva We found that change in the selective environment explained 80% of evolutionary change in leg length, with change in body size contributing an additional 6% of uniquely explained variance. The result is one of four proposed steps in testing for plant-pollinator coevolution.

Arbuscular mycorrhizal fungi promote coexistence and niche divergence of sympatric palm species on a remote oceanic island

Microbes can have profound effects on their hosts, driving natural selection, promoting speciation and determining species distributions. However, soil-dwelling microbes are rarely investigated as drivers of evolutionary change in plants. We used metabarcoding and experimental manipulation of soil microbiomes to investigate the impact of soil and root microbes in a well-known case of sympatric speciation, the Howea palms of Lord Howe Island (Australia). Whereas H. forsteriana can grow on both calcareous and volcanic soils, H. belmoreana is restricted to, but more successful on, volcanic soil, indicating a trade-off in adaptation to the two soil types. We suggest a novel explanation for this trade-off. Arbuscular mycorrhizal fungi (AMF) are significantly depleted in H. forsteriana on volcanic soil, relative to both H. belmoreana on volcanic soil and H. forsteriana on calcareous soil. This is mirrored by the results of survival experiments, where the sterilization of natural soil reduces Howea fitness in every soil-species combination except H. forsteriana on volcanic soil. Furthermore, AMF-associated genes exhibit evidence of divergent selection between Howea species. These results show a mechanism by which divergent adaptation can have knock-on effects on host-microbe interactions, thereby reducing interspecific competition and promoting the coexistence of plant sister species.

Mutualism between co-occurring plant species in South Africa's Mediterranean climate heathland is mediated by birds

Interactions among plant species via pollinators vary from competitive to mutualistic and can influence the probability of stable coexistence of plant species. We aimed to determine the nature of the interaction via flower visitors between Leucospermum conocarpodendron and Mimetes fimbriifolius, two shrubs in the Proteaceae that share many ecological traits and coexist on the Cape Peninsula, South Africa. To assess the extent of pollinator sharing we analysed nectar properties and recorded the pollinator fauna, their behaviour and contribution to seed set. To test for competition via interspecific pollen transfer, we recorded the movement patterns of pollinators and quantified pollen loads. To determine the effect of co-flowering on visitation rates we recorded visits in stands that varied in the density of the two species. We found that the species produce similar rewards and share pollinating Cape Sugarbirds (Promerops cafer). Interspecific pollen transfer is avoided by placing pollen on different parts of the bird. Both species are visited by nectar-thieving Orange-breasted Sunbirds (Anthobaphes violacea). Insects and autonomous self-pollination contributed little to seed set. Pollinator visits increased with conspecific density in both species, and the slope of the increase was steepest in the presence of high densities of the co-occurring plant species. Nectar thief visits also increased with conspecific density in both species, but the slope declined with increasing density of the co-occurring species. Co-occurrence enhanced pollinator visits and alleviated nectar robbing in both plant species, consistent with mutualisms. Mutualism within a trophic level is unusual, but may help to explain the stable coexistence of ecologically similar species.