Joseph B. Morton

Host-Dependent Sporulation and Species Diversity of Arbuscular Mycorrhizal Fungi in a Mown Grassland

1 In laboratory microcosm experiments, co-occurring plant species were found to support very different rates of sporulation of arbuscular mycorrhizal (AM) fungi. These differences were not affected by the time of harvest, suggesting that they reflect host-dependent differences in fungal growth rates, rather than host-dependent timing of sporulation. 2 Spore counts in field soil and estimates from sorghum trap cultures showed that the association of AM fungi with particular host plants in the field was positively correlated with the sporulation rates observed on those hosts in the microcosm experiments. 3 The AM fungal species richness observed at the field site was high relative to estimates made in previous studies. 23 distinct species of AM fungi were found, seven of which have not been previously described. 4 The host-dependence of the relative growth rates of fungal populations may play an important role in the maintenance of fungal species diversity.

Time-course study and partial characterization of a protein on hyphae of arbuscular mycorrhizal fungi during active colonization of roots

Material on the surface of hyphal walls of arbuscular mycorrhizal fungi (AMF) during active colonization of plant roots was detected by a monoclonal antibody. Pot-cultured isolates of Glomus, Acaulospora, Gigaspora, Scutellospora, and Entrophospora had immunofluorescent material (IM) on younger, thinner, intact hyphae, but IM was scant to absent on thicker, melanized or lysing hyphae. Colonization of corn (Zea mays L.), Sudangrass (Sorghum sudanense (Piper) Staph.) or red clover (Trifolium pratense L.) was examined during 5 months of plant growth by removing cores and performing an indirect immunoassay on roots with attached hyphae. Fresh spores of some Glomus spp. had IM on the outer layer of the spore wall. Abundant IM was seen on root hairs of plants colonized by some isolates, and some IM was detected on root surfaces of all plants examined even during early colonization. After cultures were dried, hyphae, roots and spores had little to no IM. Uninoculated control roots had very rare, small patches of IM. An immunoreactive protein was extracted from hyphae of Gigaspora and Glomus isolates by using 20mM citrate (pH 7.0) at 121°C for 90 min. Gel electrophoresis profiles indicated that all isolates tested had the same banding patterns. Lectin-binding of extracted protein is suggestive of a glycoprotein. The immunofluorescence assay can be used to examine root sections for active colonization by AMF, and the potential use of the protein to quantify AMF activity in soil is discussed.

An evidence-based consensus for the classification of arbuscular mycorrhizal fungi (Glomeromycota)

The publication of a large number of taxon names at all levels within the arbuscular mycorrhizal fungi (Glomeromycota) has resulted in conflicting systematic schemes and generated considerable confusion among biologists working with these important plant symbionts. A group of biologists with more than a century of collective experience in the systematics of Glomeromycota examined all available molecular–phylogenetic evidence within the framework of phylogenetic hypotheses, incorporating morphological characters when they were congruent. This study is the outcome, wherein the classification of Glomeromycota is revised by rejecting some new names on the grounds that they are founded in error and by synonymizing others that, while validly published, are not evidence-based. The proposed “consensus” will provide a framework for additional original research aimed at clarifying the evolutionary history of this important group of symbiotic fungi.

Lack of global population genetic differentiation in the arbuscular mycorrhizal fungus Glomus mosseae suggests a recent range expansion which may have coincided with the spread of agriculture.

The arbuscular mycorrhizal fungus Glomus mosseae is commonly found in agricultural fields. The cosmopolitan species is found in Africa, Europe, America, Asia and Australia. Three hypotheses may explain this worldwide distribution: First, speciation occurred before the continents separated 120 Ma; second, the distribution is a result of human‐mediated dispersal related to agriculture and finally, the morphologically defined species may encompass several local endemic species. To test these hypotheses, three genes were sequenced from 82 isolates of G. mosseae originating from six continents and the resulting sequences analysed for geographical subdivision and estimation of migration between continents. Coalescent analyses estimated divergence and age of mutations. Bayesian coalescent modelling was used to reveal important past population changes in the global population. The sequence data showed no geographical structure, with identical genotypes found on different continents. Coalescence analyses indicated a recent diversification in the species, and the data could be explained by a recent population expansion in G. mosseae. The results of this study suggest that speciation and the range expansion happened much later than continental spread and that human activity may have had a major impact on the dispersal and the population structure of the fungus.

Levels of diversity in endomycorrhizal fungi (Glomales, Zygomycetes) and their role in defining taxonomic and non-taxonomic groups

Diversity in glomalean fungi is manifested at the molecular, morphological, and ecological levels. Characters at any of these levels can be ordered into hierarchical patterns defining taxonomic groups if they are conserved enough to be heritable through geologic time in all descendants of a common ancestor. At present, only morphological characters associated with mode of spore formation and in subcellular structure of spores are sufficiently stable and diverse to recognize at least 150 species. Ontogenetic comparisons indicate that species integrity, despite asexual reproduction, is the result of rigid internal constraints imposed on variation during the process of spore subcellular differentiation. Epigenetic factors dominate because the differentiation sequence is linear and each new stage is causally linked to preceding stages. Some morphological characters of the fungal mycelium also exist, but they define more inclusive groups at the family level and above. Most diversity in the mycorrhizae consists of life-history traits associated with abundance and architecture of fungal components, their rate of formation and longevity, and their cost in the symbiosis. These characters participate in processes at the molecular and ecological levels, so they are autonomous from morphological determinants. They often are labile or affected by external environmental conditions, so fewer stable taxonomic characters are likely to be discovered. Instead, molecular and ecological diversity has greater potential to define; (a) niche specificity of organisms/populations and (b) causal processes linked to host-fungus compatibility and mycorrhizal efficiency. Any taxonomic characters that relate to mycorrhizal functions will come only from comparative studies involving organisms from shared habitats rather than those having shared spore morphologies.

Genetic diversity of the arbuscular mycorrhizal fungus Glomus intraradices as determined by mitochondrial large subunit rRNA gene sequences is considerably higher than previously expected

Glomus intraradices is a widespread arbuscular mycorrhizal fungus (AMF), which has been found in an extremely broad range of habitats, indicating a high tolerance for environmental factors and a generalist life history strategy. Despite this ecological versatility, not much is known about the genetic diversity of this fungal species across different habitats or over large geographic scales. A nested polymerase chain reaction (PCR) approach for the mitochondrial rRNA large subunit gene (mtLSU), distinguished different haplotypes among cultivated isolates of G. intraradices and within mycorrhizal root samples from the field. From analysis of 16 isolates of this species originating from five continents, 12 mitochondrial haplotypes were distinguished. Five additional mtLSU haplotypes were detected in field-collected mycorrhizal roots. Some introns in the mtLSU region appear to be stable over years of cultivation and are ancestral to the G. intraradices clade. Genetic diversity within G. intraradices is substantially higher than previously thought, although some mtLSU haplotypes are widespread. A restriction fragment length polymorphism approach also was developed to distinguish mtLSU haplotypes without sequencing. Using this molecular tool, intraspecific genetic variation of an AMF species can be studied directly in field plants.

Three new species of Acaulospora (Endogonaceae) from high aluminum, low pH soils in West Virginia

Three new species of vesicular-arbuscular mycorrhizal fungi are described: Acaulospora dilatata, Acaulospora lacunosa, and Acaulospora rugosa. All three species were most commonly associated with Andropogon virginicus growing in low pH, high aluminum soils. Each fungus differed markedly in spore wall surface characteristics, but all possessed an elastic inner wall that was amorphous in lactophenolbased mountants and stained dark reddish purple in Melzers reagent.

EVOLUTIONARY RELATIONSHIPS AMONG ARBUSCULAR MYCORRHIZAL FUNGI IN THE ENDOGONACEAE

A cladistic analysis of 57 species in six genera ofthe Endogonaceae was performed using 27 mor? phological characters of asexual spores and the endophytic phase of mycorrhizal development. These species comprised a monophyletic group defined by two synapomorphies: mutualistic symbiosis with terrestrial plants and production of specialized dichotomously branched intraradical arbuscules interfacing host and fungal symbiont. Members of Endogone, another genus in the Endogonaceae, did not share these characters and thus were a polyphyletic group related to some arbuscular species in Glomus and Sclerocystis only by convergence. Two main branches were hypothesized to have evolved from a common arbuscular ancestor. One branch consisted of Gigaspora and Scutellospora, as defined by extraradical auxiliary cells and spores formed within a thin unit wall on a sporogenous cell. The other branch consisted of Glomus, Sclerocystis, Acaulospora, and Entrophospora, as defined by intraradical vesicles in mycorrhizal roots. Parallelisms were numerous within all descendant monophyletic fungal groups, lowering resolution of branching patterns. The cladogram and phylogenetic tree reconstructed from this analysis establish a theoretical framework for future studies.

Infectivity of vesicular-arbuscular mycorrhizal fungi: influence of host-soil diluent combinations on MPN estimates and percentage colonization

Abstract Infectivity of vesicular-arbuscular mycorrhizal fungi, native to soils from four dissimilar habitats, was assessed by the “most probable number” (MPN) dilution method and by percentage mycorrhizal colonization. Inoculum from each site was tested in various host-soil diluent combinations. MPN estimates, percentage mycorrhizal colonization, intensity of colonization in infected root segments, sporulation, and the extent of extramatrical hyphae formation were greatest when host species and soil diluent most closely matched conditions at the site from which each inoculum was collected. Hence, the MPN method estimated infectivity of propagules best when the host and soil diluent properties permitted expression of all multiple factors conditioning infection processes. MPN estimates were directly proportional to percentage mycorrhizal colonization when the number of infectious propagules were low. regardless of host or soil diluent combination.

A monograph of the genus Gigaspora, incorporating developmental patterns of morphological characters

Species concepts within the order Glomales, Zygomycetes, still are in their formative stages and require input from different character sets. Morpho? logical and developmental characters in the genus Gi? gaspora were compared among nine isolates of five species. To test plasticity of these characters, replicate cultures of the fungi were grown on Sorghum bicolor and Asparagus officinalis under controlled conditions. Sequences of spore wall development were divided into three discrete stages that were identical in all Gigaspora isolates. Neither development nor adult spore morphology of isolates varied significantly be? tween the two host plant species, suggesting strong constraints on plasticity. Gross morphological char? acteristics of mycorrhizae and extramatrical auxiliary cells did not differ among the various isolates. How? ever, spore size and color varied significantly among species, validating current morphological species con? cepts within Gigaspora. Comparisons of spore wall de? velopment in this study provide a biological rationale for character concepts because characters now can be rooted in a process (development) rather than from static observation. These developmental characters provide the basis for a model of diversity in Gigaspora and its sister genus Scutellospora. In this monograph, the genus Gigaspora is redescribed, along with each species. Three species are rejected as synonyms of other species and a dichotomous key to the remaining species of Gigaspora is presented.