Immigrant and extrinsic hybrid seed inviability contribute to reproductive isolation between forest and dune ecotypes of Epipactis helleborine (Orchidaceae)

Multi-generational fitness legacies of natural immigration: theoretical and empirical perspectives and opportunities

Natural dispersal between populations, and resulting immigration, influences population size and genetic variation and is therefore a key process driving reciprocal interactions between ecological and evolutionary dynamics. Here, population dynamic and evolutionary outcomes fundamentally depend not only on the relative fitnesses of natural immigrants and existing residents, but also on the fitness of their various descendants manifested in natural environments. Yet, the fitnesses of different sets of natural immigrants' descendants have rarely been explicitly or rigorously estimated or rationalised in the context of wild spatially structured populations. We therefore still have surprisingly limited capability to understand or predict the ultimate multi-generational impacts of natural immigration on population and evolutionary dynamics. Key theoretical frameworks that predict fitness outcomes of outcrossing between lineages have been developed and widely utilised in the contexts of agriculture and speciation research. These frameworks have also been applied in conservation genetics research to predict positive (widely termed "heterosis") and negative (widely termed "outbreeding depression") outcomes in the context of genetic rescue of highly inbred populations. However, these frameworks have rarely been utilised explicitly to guide analyses of multi-generational legacies of regular natural immigrants in the context of evolutionary ecology, precluding inferences on the basis of, and implications of, sub-population divergence. Accordingly, to facilitate translation of concepts and inspire new empirical efforts, we first review and synthesise key bodies of theory on multi-generational fitness outcomes, developed in the contexts of crosses between inbred lines and between different species. Such theory reveals how diverse fitness outcomes can be generated by common underlying mechanisms, depending on the genetic architecture of fitness, the forms of genotype-phenotype-fitness maps, and the relative roles of adaptive and non-adaptive mechanisms in population differentiation. Interestingly, such theory predicts particularly diverse fitness outcomes of crosses between weakly diverged lineages, constituting the parameter space where spatially structured populations lie. We then conduct a systematic literature review to assess the degree to which multi-generational outcomes of crosses between structured natural populations have actually been quantified. Our review shows a surprising paucity of empirical studies that quantify multi-generational fitness consequences of outcrossing resulting from natural immigration in the wild. Furthermore, studies undertaking experimental crosses among populations have used inconsistent methodologies, precluding quantitative or even qualitative overall conclusions. To initiate new progress, we outline how long-standing and recent methodological developments, including cutting-edge statistical and genomic tools, could be combined with field data sets to quantify the multi-generational fitness outcomes of crosses between residents and immigrants in nature. We thereby highlight key theoretical and empirical gaps that now need to be filled to further our understanding of dispersal-mediated drivers and constraints on eco-evolutionary dynamics arising in structured populations.

Co-occurring orchid species associated with different low-abundance mycorrhizal fungi from the soil in a high-diversity conservation area in Denmark

Plant-fungal interactions are ubiquitous across ecosystems and contribute significantly to plant ecology and evolution. All orchids form obligate symbiotic relationships with specific fungi for germination and early growth, and the distribution of terrestrial orchid species has been linked to occurrence and abundance of specific orchid mycorrhizal fungi (OMF) in the soil. The availability of OMF can therefore be a habitat requirement that is relevant to consider when establishing management and conservation strategies for threatened orchid species, but knowledge on the spatial distribution of OMF in soil is limited. We here studied the mycorrhizal associations of three terrestrial orchid species (Anacamptis pyramidalis, Orchis purpurea and Platanthera chlorantha) found in a local orchid diversity hotspot in eastern Denmark, and investigated the abundance of the identified mycorrhizal fungi in the surrounding soil. We applied ITS metabarcoding to samples of orchid roots, rhizosphere soil and bulk soil collected at three localities, supplemented with standard barcoding of root samples with OMF specific primers, and detected 22 Operational Taxonomic Units (OTUs) putatively identified as OMF. The three orchid species displayed different patterns of OMF associations, supporting the theory that association with specific fungi constitutes part of an orchid's ecological niche allowing co-occurrence of many species in orchid-rich habitats. The identified mycorrhizal partners in the basidiomycete families Tulasnellaceae and Ceratobasidiaceae (Cantharallales) were detected in low abundance in rhizosphere soil, and appeared almost absent from bulk soil at the localities. This finding highlights our limited knowledge of the ecology and trophic mode of OMF outside orchid tissues, as well as challenges in the detection of specific OMF with standard methods. Potential implications for management and conservation strategies are discussed.

Historical biogeography and local adaptation explain population genetic structure in a widespread terrestrial orchid

BACKGROUND AND AIMS

Historical changes in environmental conditions and colonization-extinction dynamics have a direct impact on the genetic structure of plant populations. However, understanding how past environmental conditions influenced the evolution of species with high gene flow is challenging when signals for genetic isolation and adaptation are swamped by gene flow. We investigated the spatial distribution and genetic structure of the widespread terrestrial orchid Epipactis helleborine to identify glacial refugia, characterize postglacial population dynamics and assess its adaptive potential.

METHODS

Ecological niche modelling was used to locate possible glacial refugia and postglacial recolonization opportunities of E. helleborine. A large single-nucleotide polymorphism (SNP) dataset obtained through genotyping by sequencing was used to define population genetic diversity and structure and to identify sources of postglacial gene flow. Outlier analyses were used to elucidate how adaptation to the local environment contributed to population divergence.

KEY RESULTS

The distribution of climatically suitable areas was restricted during the Last Glacial Maximum to the Mediterranean, south-western Europe and small areas in the Alps and Carpathians. Within-population genetic diversity was high in E. helleborine (mean expected heterozygosity, 0.373 ± 0.006; observed heterozygosity, 0.571 ± 0.012; allelic richness, 1.387 ± 0.007). Italy and central Europe are likely to have acted as important genetic sources during postglacial recolonization. Adaptive SNPs were associated with temperature, elevation and precipitation.

CONCLUSIONS

Forests in the Mediterranean and Carpathians are likely to have acted as glacial refugia for Epipactis helleborine. Postglacial migration northwards and to higher elevations resulted in the dispersal and diversification of E. helleborine in central Europe and Italy, and to geographical isolation and divergent adaptation in Greek and Italian populations. Distinguishing adaptive from neutral genetic diversity allowed us to conclude that E. helleborine has a high adaptive potential to climate change and demonstrates that signals of adaptation and historical isolation can be identified even in species with high gene flow.

Epipactis bucegensis-A Separate Autogamous Species within the E. helleborine Alliance

A new species of Epipactis from Bucegi Natural Park ROSCI0013, Southern Carpathians, Central Romania is described. Three medium-sized populations of Epipactis bucegensis (65-70 individuals in total) were discovered in the south-eastern, subalpine area of the park. To properly describe and distinguish the newly found taxon from other Romanian Epipactis, 37 morphological characters were measured directly from living plants and flowers. Moreover, a detailed taxonomic treatment and description with corresponding colour photos and line drawings illustrations of the holotype are also included. Epipactis bucegensis is an obligate autogamous species that partially resembles Epipactis muelleri, from which it differs in the basal distribution of leaves on the stem (vs. median distribution); near-erect leaf posture (vs. horizontally spread, arched downwards); lanceolate-acuminate, yellowish-green leaves (vs. oval-elongate, vivid-green leaves); bipartite labellum lacking the mesochile (vs. tripartite labellum); crimson-red, wide, ovoid-elongated, flattened hypochile (vs. dark-brown to black roundish hypochile); triangular, white epichile with a sharply tapering apex (vs. heart-shaped, greenish-yellow epichile with obtuse, roundish apex); and two wide-apart, purple, pyramidal calli (vs. two closely placed, attenuated, mildly wrinkled, greenish-yellow calli). Epipactis bucegensis is easily distinguished from all other European Epipactis taxa by the bipartite, wide labellum that lacks the mesochile. In addition, information regarding its distribution (maps), habitat, ecology, phenology and IUCN conservation assessments are provided.

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

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

Range Size and Niche Breadth as Predictors of Climate-Induced Habitat Change in Epipactis (Orchidaceae)

While there is mounting evidence that ongoing changes in the climate system are shifting species ranges poleward and to higher altitudes, responses to climate change vary considerably between species. In general, it can be expected that species responses to climate change largely depend on how broad their ecological niches are, but evidence is still scant. In this study, we investigated the effects of predicted future climate change on the availability of suitable habitat for 14 Epipactis (Orchidaceae) species, and tested whether habitat specialists would experience greater changes in the extent of their habitats than habitat generalists. We used Maxent to model the ecological niche of each species in terms of climate, soil, elevation and land-use and projected it onto climate scenarios predicted for 2061–2080. To test the hypothesis that temperate terrestrial orchid species with small ranges or small niche breadths may be at greater risk under climate change than species with wide ranges or large niche breadths, we related niche breadth in both geographic and environmental space to changes in size and location of suitable habitat. The habitat distributions of half of the species shifted northwards in future projections. The area of suitable habitat increased for eight species but decreased for the remaining six species. If expansion at the leading edge of the distribution was not possible, the area of suitable habitat decreased for 12 species. Species with wide niche breadth in geographic space experienced greater northwards expansions and higher habitat suitability scores than species with small niche breadth. Niche breadth in environmental space was not significantly related to change in habitat distribution. Overall, these results indicate that terrestrial orchid species with a wide distribution will be more capable of shifting their distributions under climate change than species with a limited distribution, but only if they are fully able to expand into habitats at the leading edge of their distributions.

Extensive pollinator sharing does not promote character displacement in two orchid congeners

Pollinator sharing between close relatives can be costly and can promote pollination niche partitioning and floral divergence. This should be reflected by a higher species divergence in sympatry than in allopatry. We tested this hypothesis in two orchid congeners with overlapping distributions and flowering times. We characterized floral traits and pollination niches and quantified pollen limitation in 15 pure and mixed populations, and we measured phenotypic selection on floral traits and performed controlled crosses in one mixed site. Most floral traits differed between species, yet pollinator sharing was extensive. Only the timing of scent emission diverged more in mixed sites than in pure sites, and this was not mirrored by the timing of pollinator visitation. We did not detect divergent selection on floral traits. Seed production was pollen limited in most populations but not more severely in mixed sites than in pure sites. Interspecific crosses produced the same or a higher proportion of viable seeds than intraspecific crosses. The two orchid species attract the same pollinator species despite showing divergent floral traits. However, this does not promote character displacement, implying a low cost of pollinator sharing. Our results highlight the importance of characterizing both traits and ecological niches in character displacement studies.

Arbuscular mycorrhizal trees influence the latitudinal beta-diversity gradient of tree communities in forests worldwide

Arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) associations are critical for host-tree performance. However, how mycorrhizal associations correlate with the latitudinal tree beta-diversity remains untested. Using a global dataset of 45 forest plots representing 2,804,270 trees across 3840 species, we test how AM and EcM trees contribute to total beta-diversity and its components (turnover and nestedness) of all trees. We find AM rather than EcM trees predominantly contribute to decreasing total beta-diversity and turnover and increasing nestedness with increasing latitude, probably because wide distributions of EcM trees do not generate strong compositional differences among localities. Environmental variables, especially temperature and precipitation, are strongly correlated with beta-diversity patterns for both AM trees and all trees rather than EcM trees. Results support our hypotheses that latitudinal beta-diversity patterns and environmental effects on these patterns are highly dependent on mycorrhizal types. Our findings highlight the importance of AM-dominated forests for conserving global forest biodiversity.

Impact of mating system on range size and niche breadth in Epipactis (Orchidaceae)

BACKGROUND AND AIMS

The geographical distribution of plant species is linked fundamentally not only to environmental variables, but also to key traits that affect the dispersal, establishment and evolutionary potential of a species. One of the key plant traits that can be expected to affect standing genetic variation, speed of adaptation and the capacity to colonize and establish in new habitats, and therefore niche breadth and range size, is the plant mating system. However, the precise role of the mating system in shaping range size and niche breadth of plant species remains unclear, and different studies have provided contrasting results. In this study, we tested the hypothesis that range size and niche breadth differed with mating system in the orchid genus Epipactis.

METHODS

We modelled the ecological niches of 14 Epipactis species in Europe using occurrence records and environmental satellite data in Maxent. Niche breadth and niche overlap in both geographic and environmental space were calculated from the resulting habitat suitability maps using ENMTools, and geographic range was estimated using α-hull range definition. Habitat suitability, environmental variable contributions and niche metrics were compared among species with different mating systems.

KEY RESULTS

We did not detect significant differences in niche breadth, occurrence probability or geographical range between autogamous and allogamous Epipactis species, although autogamous species demonstrated notably low variation in niche parameters. We also found no significant differences in niche overlap between species with the same mating system or different mating systems. For all Epipactis species, occurrence was strongly associated with land cover, particularly broad-leafed and coniferous forests, and with limestone bedrock.

CONCLUSIONS

These results suggest that the mating system does not necessarily contribute to niche breadth and differentiation, and that other factors (e.g. mycorrhizal specificity) may be more important drivers of range size and niche breadth in Epipactis and orchids in general.

Epipactis tremolsii Seed Diversity in Two Close but Extremely Different Populations: Just a Case of Intraspecific Variability?

Analysis of the seed morphology is a widely used approach in ecological and taxonomic studies. In this context, intraspecific variability with respect to seed morphology (size, weight, and density) was assessed in two close Epipactis tremolsii Pau. populations sharing the same ecological conditions, except for the soil pollution distinguishing one of them. Larger and heavier seeds were found in plants growing on the heavy metal polluted site, while no differences in seed density were detected between seeds produced by plants growing on the contaminated and the control site. Moreover, seed coats and embryos varying together in their dimensions were described in the control population, while coats varying in their size independently from embryos were described in plants growing on the polluted site. Seeds from the two studied populations significantly differed in several parameters suggesting that intraspecific seed variability occurred in the case study.

Orchid conservation: from theory to practice

BACKGROUND

Given the exceptional diversity of orchids (26 000+ species), improving strategies for the conservation of orchids will benefit a vast number of taxa. Furthermore, with rapidly increasing numbers of endangered orchids and low success rates in orchid conservation translocation programmes worldwide, it is evident that our progress in understanding the biology of orchids is not yet translating into widespread effective conservation.

SCOPE

We highlight unusual aspects of the reproductive biology of orchids that can have important consequences for conservation programmes, such as specialization of pollination systems, low fruit set but high seed production, and the potential for long-distance seed dispersal. Further, we discuss the importance of their reliance on mycorrhizal fungi for germination, including quantifying the incidence of specialized versus generalized mycorrhizal associations in orchids. In light of leading conservation theory and the biology of orchids, we provide recommendations for improving population management and translocation programmes.

CONCLUSIONS

Major gains in orchid conservation can be achieved by incorporating knowledge of ecological interactions, for both generalist and specialist species. For example, habitat management can be tailored to maintain pollinator populations and conservation translocation sites selected based on confirmed availability of pollinators. Similarly, use of efficacious mycorrhizal fungi in propagation will increase the value of ex situ collections and likely increase the success of conservation translocations. Given the low genetic differentiation between populations of many orchids, experimental genetic mixing is an option to increase fitness of small populations, although caution is needed where cytotypes or floral ecotypes are present. Combining demographic data and field experiments will provide knowledge to enhance management and translocation success. Finally, high per-fruit fecundity means that orchids offer powerful but overlooked opportunities to propagate plants for experiments aimed at improving conservation outcomes. Given the predictions of ongoing environmental change, experimental approaches also offer effective ways to build more resilient populations.

Iterative allogamy-autogamy transitions drive actual and incipient speciation during the ongoing evolutionary radiation within the orchid genus Epipactis (Orchidaceae)

BACKGROUND AND AIMS

The terrestrial orchid genus Epipactis has become a model system for the study of speciation via transitions from allogamy to autogamy, but close phylogenetic relationships have proven difficult to resolve through Sanger sequencing.

METHODS

We analysed with restriction site-associated sequencing (RAD-seq) 108 plants representing 29 named taxa that together span the genus, focusing on section Epipactis. Our filtered matrix of 12 543 single nucleotide polymorphisms was used to generate an unrooted network and a rooted, well-supported likelihood tree. We further inferred genetic structure through a co-ancestry heat map and admixture analysis, and estimated inbreeding coefficients per sample.

KEY RESULTS

The 27 named taxa of the ingroup were resolved as 11 genuine, geographically widespread species: four dominantly allogamous and seven dominantly autogamous. A single comparatively allogamous species, E. helleborine, is the direct ancestor of most of the remaining species, though one of the derived autogams has generated one further autogamous species. An assessment of shared ancestry suggested only sporadic hybridization between the re-circumscribed species. Taxa with the greatest inclination towards autogamy show less, if any, admixture, whereas the gene pools of more allogamous species contain a mixture alleles found in the autogams.

CONCLUSIONS

This clade is presently undergoing an evolutionary radiation driven by a wide spectrum of genotypic, phenotypic and environmental factors. Epipactis helleborine has also frequently generated many local variants showing inclinations toward autogamy (and occasionally cleistogamy), best viewed as incipient speciation from within the genetic background provided by E. helleborine, which thus becomes an example of a convincingly paraphyletic species. Autogams are often as widespread and ecologically successful as allogams.

Morphology and genome size of Epipactis helleborine (L.) Crantz (Orchidaceae) growing in anthropogenic and natural habitats

BACKGROUND

The process of apophytism or spreading native species to human-made habitats is one of the main elements in the creation of plant cover in anthropogenic areas. Lately, an increase of anthropogenic localities with valuable flora has been observed. Apophytes are also members of the family Orchidaceae, especially from the genus Epipactis. The aim of the study was to (i) determine and compare the phenotypic variation of E. helleborine (L.) Crantz plants in anthropogenic and natural habitats, (ii) compare the genome size of plants growing in natural and anthropogenic habitats. The results reported in this study may indicate that a habitat influences morphological characteristics of plant species.

METHODS

Field studies were conducted on four native stands and four stands in anthropogenic areas of E. helleborine in Poland in years 2011-2013. Biometrical analyses were performed on shoots and flowers. The flowers were characterised by 25 biometric features and measured using a Nikon SMZ 800 binocular, microscopic Moticam-1SP cameras and the MIPlus07 programme (Conbest Co.). The nuclear DNA content was determined in fresh and young leaves of E. helleborine, collected from four natural and four anthropogenic populations.

RESULTS

We observed that in anthropogenic populations: (i) shoots were higher than shoots from natural populations, (ii) flowers differed significantly in terms of ten biometric features between habitats, (iii) the genome size of some population differed significantly between plants growing in natural and anthropogenic habitats.

DISCUSSION

According to some researchers, the presence of phenotypic variability and the occurrence of ecotypes are adaptation strategies of plants to environmental changes. In our opinion, in the case of the studied anthropogenic habitats (roadside) in which the E. helleborine populations grew, we can talk about ecofen due to the often repeated set of characteristic features, i.e., high shoots, long inflorescence and long, broad leaves. We agree, however, that it is difficult to isolate a taxonomic unit for ecofen due to the lack of experimental research.

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

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

Female Choice Undermines the Emergence of Strong Sexual Isolation between Locally Adapted Populations of Atlantic Mollies (Poecilia mexicana)

Divergent selection between ecologically dissimilar habitats promotes local adaptation, which can lead to reproductive isolation (RI). Populations in the Poecilia mexicana species complex have independently adapted to toxic hydrogen sulfide and show varying degrees of RI. Here, we examined the variation in the mate choice component of prezygotic RI. Mate choice tests across drainages (with stimulus males from another drainage) suggest that specific features of the males coupled with a general female preference for yellow color patterns explain the observed variation. Analyses of male body coloration identified the intensity of yellow fin coloration as a strong candidate to explain this pattern, and common-garden rearing suggested heritable population differences. Male sexual ornamentation apparently evolved differently across sulfide-adapted populations, for example because of differences in natural counterselection via predation. The ubiquitous preference for yellow color ornaments in poeciliid females likely undermines the emergence of strong RI, as female discrimination in favor of own males becomes weaker when yellow fin coloration in the respective sulfide ecotype increases. Our study illustrates the complexity of the (partly non-parallel) pathways to divergence among replicated ecological gradients. We suggest that future work should identify the genomic loci involved in the pattern reported here, making use of the increasing genomic and transcriptomic datasets available for our study system.

Mycorrhizal Associations and Trophic Modes in Coexisting Orchids: An Ecological Continuum between Auto- and Mixotrophy

Two distinct nutritional syndromes have been described in temperate green orchids. Most orchids form mycorrhizas with rhizoctonia fungi and are considered autotrophic. Some orchids, however, associate with fungi that simultaneously form ectomycorrhizas with surrounding trees and derive their carbon from these fungi. This evolutionarily derived condition has been called mixotrophy or partial mycoheterotrophy and is characterized by 13C enrichment and high N content. Although it has been suggested that the two major nutritional syndromes are clearly distinct and tightly linked to the composition of mycorrhizal communities, recent studies have challenged this assumption. Here, we investigated whether mycorrhizal communities and nutritional syndromes differed between seven green orchid species that co-occur under similar ecological conditions (coastal dune slacks). Our results showed that mycorrhizal communities differed significantly between orchid species. Rhizoctonia fungi dominated in Dactylorhiza sp., Herminium monorchis, and Epipactis palustris, which were autotrophic based on 13C and N content. Conversely, Liparis loeselii and Epipactis neerlandica associated primarily with ectomycorrhizal fungi but surprisingly, 13C and N content supported mixotrophy only in E. neerlandica. This, together with the finding of some ectomycorrhizal fungi in rhizoctonia-associated orchids, suggests that there exists an ecological continuum between the two syndromes. The presence of a large number of indicator species associating with individual orchid species further confirms previous findings that mycorrhizal fungi may be important factors driving niche-partitioning in terrestrial orchids and therefore contribute to orchid coexistence.