An in-depth investigation of cryptic taxonomic diversity in the rare endemic mustard Draba maguirei.

PREMISE: Previously published evidence suggests that Draba maguirei, a mustard endemic to a few localities in the Bear River, Wellsville, and Wasatch Mountains of northern Utah, may represent a cryptic species complex rather than a single species. Conservation concerns prompted an in-depth systematic study of this taxon and its putative relatives. METHODS: Sampling most known populations of D. maguirei s.l. (D. maguirei var. maguirei and D. maguirei var. burkei), we integrate data from geography, ecology, morphology, cytogenetics and pollen, enzyme electrophoresis, and the phylogenetic analysis of nuclear internal transcribed spacer sequences to explore potential taxonomic diversity in the species complex. RESULTS: Draba maguirei var. burkei is shown here to be a distinct species (D. burkei) most closely related to D. globosa, rather than to D. maguirei. Within D. maguirei s.s., the northern (high elevation) and southern (low elevation) population clusters are genetically isolated and morphologically distinguishable, leading to the recognition here of the southern taxon as D. maguirei subsp. stonei. CONCLUSIONS: Our study reveals that plants traditionally assigned to D. maguirei comprise three genetically divergent lineages (D. burkei and two newly recognized subspecies of D. maguirei), each exhibiting a different chromosome number and occupying a discrete portion of the geographic range. Although previously overlooked and underappreciated taxonomically, the three taxa are morphologically recognizable based on the distribution and types of trichomes present on the leaves, stems, and fruit. Our clarification of the diversity and distribution of these taxa provides an improved framework for conservation efforts.

When it only takes one to tango: assessing the impact of apomixis in the fern genus Pteris.

PREMISE: Apomixis (asexual reproduction by seed, spore, or egg) has evolved repeatedly across the tree of life. Studies of animals and angiosperms show that apomictic lineages are often evolutionarily short-lived and frequently exhibit different distributions than their sexual relatives. However, apomixis is rare in these groups. Less is known about the role of apomixis in the evolution and biogeography of ferns, in which ~10% of species are apomictic. Apomixis is especially common in the fern genus Pteris (34-39% of species); however, because of the limited taxonomic and geographic sampling of previous studies, the true frequency of apomixis and its associations with geography and phylogeny in this lineage remain unclear. METHODS: We used spore analyses of herbarium specimens to determine reproductive mode for 127 previously unsampled Pteris species. Then we leveraged biogeographic and phylogenetic analyses to estimate the global distribution and evolution of apomixis in Pteris. RESULTS: Among all Pteris species examined, we found that 21% are exclusively apomictic, 71% are exclusively sexual, and 8% have conflicting reports. Apomixis is unevenly distributed across the range of the genus, with the Paleotropics exhibiting the highest frequency, and has evolved numerous times across the Pteris phylogeny, with predominantly East Asian and South Asian clades containing the most apomictic species. CONCLUSIONS: Apomixis arises frequently in Pteris, but apomictic species do not appear to diversify. Species that encompass both apomictic and sexual populations have wider ranges than exclusively sexual or apomictic species, which suggests that sexual and apomictic ferns could occupy separate ecological niches.

A drought-driven model for the evolution of obligate apomixis in ferns: evidence from pellaeids (Pteridaceae).

PREMISE: Xeric environments impose major constraints on the fern life cycle, yet many lineages overcome these limitations by evolving apomixis. Here, we synthesize studies of apomixis in ferns and present an evidence-based model for the evolution and establishment of this reproductive strategy, focusing on genetic and environmental factors associated with its two defining traits: the production of "unreduced" spores (n = 2n) and the initiation of sporophytes from gametophyte tissue (i.e., diplospory and apogamy, respectively). METHODS: We evaluated existing literature in light of the hypothesis that abiotic characteristics of desert environments (e.g., extreme diurnal temperature fluctuations, high light intensity, and water limitation) drive the evolution of obligate apomixis. Pellaeid ferns (Cheilanthoideae: Pteridaceae) were examined in detail, as an illustrative example. We reconstructed a plastid (rbcL, trnG-trnR, atpA) phylogeny for the clade and mapped reproductive mode (sexual versus apomictic) and ploidy across the resulting tree. RESULTS: Our six-stage model for the evolution of obligate apomixis in ferns emphasizes the role played by drought and associated abiotic conditions in the establishment of this reproductive approach. Furthermore, our updated phylogeny of pellaeid ferns reveals repeated origins of obligate apomixis and shows an increase in the frequency of apomixis, and rarity of sexual reproduction, among taxa inhabiting increasingly dry North American deserts. CONCLUSIONS: Our findings reinforce aspects of other evolutionary, physiological, developmental, and omics-based studies, indicating a strong association between abiotic factors and the establishment of obligate apomixis in ferns. Water limitation, in particular, appears critical to establishment of this reproductive mode.

Fungi in the Marine Environment: Open Questions and Unsolved Problems.

Terrestrial fungi play critical roles in nutrient cycling and food webs and can shape macroorganism communities as parasites and mutualists. Although estimates for the number of fungal species on the planet range from 1.5 to over 5 million, likely fewer than 10% of fungi have been identified so far. To date, a relatively small percentage of described species are associated with marine environments, with ∼1,100 species retrieved exclusively from the marine environment. Nevertheless, fungi have been found in nearly every marine habitat explored, from the surface of the ocean to kilometers below ocean sediments. Fungi are hypothesized to contribute to phytoplankton population cycles and the biological carbon pump and are active in the chemistry of marine sediments. Many fungi have been identified as commensals or pathogens of marine animals (e.g., corals and sponges), plants, and algae. Despite their varied roles, remarkably little is known about the diversity of this major branch of eukaryotic life in marine ecosystems or their ecological functions. This perspective emerges from a Marine Fungi Workshop held in May 2018 at the Marine Biological Laboratory in Woods Hole, MA. We present the state of knowledge as well as the multitude of open questions regarding the diversity and function of fungi in the marine biosphere and geochemical cycles.

Inventory of chytrid diversity in two temporary forest ponds using a multiphasic approach.

Food webs in temporary forest ponds are driven by decomposition of terrestrial inputs. Chytrid fungi are important components of the fungal community, degrading leaf litter in streams reliant on terrestrial inputs and in lake ecosystems where they may stabilize the food web. However, little is known about chytrid fungi in temporary forest ponds. We inventoried the chytrid diversity present in two temporary forest ponds via light microscopy of baited samples and ion semiconductor (Ion Torrent) sequencing of environmental DNA. We quantified trends of chytrid alpha and beta diversity as a function of spatial and temporal factors. A total of 59 chytrid taxa were detected throughout the study. Beta diversity exhibited variation across the sampled months for both the entire fungal community as well as for chytrids alone. Shifts in community composition were also apparent, although diversity metrics and composition patterns did not meet adjusted P values. The results of this study highlight the diversity of chytrid fungi in temporary forest ponds and the need for further studies on the spatial and temporal dynamics of chytrid species.

Evidence for a facultative mutualist nutritional relationship between the green coccoid alga Bracteacoccus sp. (Chlorophyceae) and the zoosporic fungus Rhizidium phycophilum (Chytridiomycota).

Symbiotic interactions between fungi and photosynthetic partners are common among derived fungal lineages. The only fungal-phototroph interactions thus far reported from the early diverging zoosporic fungi are parasitic in nature. Rhizidium phycophilum is a terrestrial, saprotrophic chytrid, which appears to be able to enter a facultative mutualism with a coccoid green alga in the absence of refractory organic material, such as pollen and chitin. Liquid and solid culturing methods were used in a series of differential fitness experiments in conjunction with microscopic analyses to characterize the interaction between R. phycophilum and the alga. The alga in this partnership is identified as a member of the genus Bracteacoccus. Under certain culturing conditions, algal cells grown in coculture with R. phycophilum were shown to grow larger and more prolifically than when cultured axenically under the same conditions. Additionally, dialysis experiments demonstrate that R. phycophilum does not parasitize Bracteacoccus sp., and can be cultured in media infused with unknown algal exudates. Rhizidium phycophilum and Bracteacoccus sp. represent the first facultative positive interaction between a zoosporic fungus and a photoautotroph and may prove a tractable system for modelling interactions between early fungi and plants.

Zoospore ultrastructure and phylogenetic position of Phlyctochytrium aureliae Ajello is revealed (Chytridiaceae, Chytridiales, Chytridiomycota).

From forest soils in Scotland Phlyctochytrium aureliae was observed and brought into pure culture. Previously included in a molecular phylogenetic study of Chytridiales as Phlyctochytrium sp. KP 061, the organism groups with Phlyctochytrium planicorne, P. bullatum, Chytridium olla and C. lagenaria in the family Chytridiaceae. Thallus morphology and development as well as zoospore ultrastructure are detailed herein. The sporangium is epibiotic, spherical or subspherical, apophysate or non-apophysate, and ornamented with dentate enations. The overall zoospore ultrastructural features are consistent with the Group II type zoospore that characterizes family Chytridiaceae in the Chytridiales, although the zoospore also has two character states unique to this taxon: the MLC cisterna fenestrations are one-third to one-half the diameter of fenestrations in other Chytridiaceae zoospores and an accumulation of electron-dense material (a kinetosome-associated structure, or KAS) proximal to the kinetosome and non-flagellated centriole is extensive and unique. This study verifies that zoospore ultrastructure of P. aureliae zoospores places this species in the Chytridiales and Chytridiaceae, as indicated in a previous molecular phylogenetic study.

Rhizidium phycophilum, a new species in Chytridiales.

Molecular and ultrastructural investigations are revealing unrealized diversity among chytrids as the taxonomic revision of order Chytridiales (Chytridiomycota) progresses. During a biodiversity survey of soil-inhabiting chytrids in Australia an undescribed chytrid was isolated from a soil sample collected in a cool temperate rainforest of New South Wales, Australia. Combined zoospore ultrastructure analysis and molecular phylogenetic analyses of partial LSU rRNA and ITS1-5.8S-ITS2 sequences demonstrated this chytrid was a new species within Chytridiales and possessed distinctive zoospore architecture previously unknown. Herein we delineate a new Rhizidium species in Chytridiales based on molecular monophyly and unique subcellular organization of the zoospore.

Rhizophlyctidales--a new order in Chytridiomycota.

Rhizophlyctis rosea (Chytridiomycota) is an apparently ubiquitous, soil-inhabiting, cellulose-degrading chytrid that is the type for Rhizophlyctis. Previous studies have revealed multiple zoospore subtypes among morphologically indistinguishable isolates in the R. rosea complex sensu Barr. In this study we analysed zoospore ultrastructure and combined nu-rRNA gene sequences (partial LSU and complete ITS1-5.8S-ITS2) of 49 isolates from globally distributed soil samples. Based on molecular monophyly and zoospore ultrastructure, this group of Rhizophlyctis rosea-like isolates is designated as a new order, the Rhizophlyctidales. Within the Rhizophlyctidales are four new families (Rhizophlyctidaceae, Sonoraphlyctidaceae, Arizonaphlyctidaceae, and Borealophlyctidaceae) and three new genera (Sonoraphlyctis, Arizonaphlyctis, and Borealophlyctis).