Deciphering interfungal relationships in the 410 million-year-old Rhynie chert: Glomoid spores under attack

Various types of other fungi colonize glomeromycotinan (Mucoromycota) spores in the Early Devonian Rhynie chert. However, relatively few of these associations have been described and evaluated in detail. One particular type of glomoid spore located in degrading land plant axes from the Rhynie chert provides evidence of (simultaneous) interaction with three different fungi. Massive callosities occur around the intrusion filaments of a chytrid-like parasite with epibiotic sporangia, while the hyphae of a delicate mycelial fungus extend into the spore lumen without triggering a recognizable host response. Several spores show large numbers of inwardly directed projections, which are regularly distributed and consist of a short hyphal branch encased in host wall material. The projections represent the penetration sites of a distinctive, mycelial fungus that forms a mantle-like hyphal sheath around the spores. This type of fungal interaction with glomeromycotinan spores has not previously been reported, and thus expands our knowledge of the numerous interfungal relationships that existed in early continental ecosystems.     

Germination shields in Scutellospora (Glomeromycota: Diversisporales, Gigasporaceae) from the 400 million-year-old Rhynie chert

Glomeromycotan spores from the Lower Devonian Rhynie chert provide the first evidence for germination shields in fossil fungi and demonstrate that this complex mode of germination was in place in some fungi at least 400 million years ago. Moreover, they represent the first direct marker relative to the precise systematic position of an Early Devonian endomycorrhizal fungus. In extant fungi, germination shields occur exclusively in the genus Scutellospora (Glomeromycota: Diversisporales, Gigasporaceae). These structures are regarded as a derived feature within the phylum Glomeromycota, and hence their presence in the Rhynie chert suggests that major diversification within this group of fungi occurred before the Early Devonian.Taxonomical novelties Scutellosporites Dotzler, M. Krings, T.N. Taylor and Agerer Scutellosporites devonicus Dotzler, M. Krings, T.N. Taylor and AgererStürmer 1998     

Deciphering interfungal relationships in the 410-million-yr-old Rhynie chert: Morphology and development of vesicle-colonizing microfungi

The intraradical portion of arbuscular mycorrhizal (AM) fungi comprises mycelium, vesicles, and special physiological interfaces termed arbuscules; sometimes mycorrhizal fungi also produce spores within their hosts. Arbuscules are ephemeral structures that collapse after a few days, while the hyphae and vesicles appear to remain intact for some time after arbuscule senescence (post-arbuscule stage). However, little is known about the fate of mycorrhizal fungi in the post-arbuscule state. The Lower Devonian Rhynie chert yields the oldest fossil evidence of arbuscular mycorrhizas, including multiple specimens of mycorrhizal axes in the post-arbuscule stage. These fossils indicate that many older vesicles of Rhynie chert mycorrhizal fungi are colonized by other microfungi. Three types of fungal remains in vesicles are distinguished based on morphology and development: (1) spheroidal propagules up to 55 μm in diameter extending from short, distal branches of a hypha; (2) spheroidal propagules up to 23 μm in diameter produced within a tenuous mycelium; and (3) thin-walled propagules up to 10 μm in diameter within a hyphal inflation. The abundance of microfungal propagules in vesicles of Rhynie chert mycorrhizal fungi suggests that the mycorrhizal vesicles in some way positively affected the development and reproduction of the intrusive microfungi. Although the systematic affinities of the intrusive microfungi remain unresolved, this discovery is important because it broadens our understanding of the levels of organismal interactions that existed in early non-marine ecosystems.     

An Alternative Mode of Early Land Plant Colonization by Putative Endomycorrhizal Fungi

Rhizomatous axes of Nothia aphylla, a land plant from the 400-myr-old Rhynie chert, host a fungus that closely resembles Glomites rhyniensis (Glomeromycota), the endomycorrhizal fungus of the Rhynie chert plant Aglaophyton major. However, G. rhyniensis is an intercellular endophyte that becomes intracellular exclusively within a well-defined region of the cortex, while the fungus in N. aphylla initially is intracellular but later becomes intercellular in the cortex. We hypothesize that N. aphylla displays an alternative mode of colonization by endomycorrhizal fungi, perhaps related to the peculiar internal anatomy of the lower portion of the rhizomatous axis, in which the radial arrangement of cells, along with the virtual absence of intercellular spaces, provides no intercellular infection pathway into the cortex.Key Words: Aglaophyton major, endomycorrhiza, Glomeromycota, Nothia aphylla, Early Devonian, Rhynie chertThe Early Devonian (c. 400 Ma) Rhynie chert is an in situ silicified hot springs environment that has become significant in our understanding of the complexity of life in early terrestrial ecosystems because of the extraordinary preservation of plants, animals, and microorganisms.¹ Moreover, various associations and interactions between different organisms can be directly examined,² including the earliest fossil examples of arbuscular endomycorrhizae.^3,4 The Rhynie chert land plant Aglaophyton major is characterized by arching, stomatiferous prostrate axes that grow along the substrate surface, and form rhizoid-bearing bulges, usually around stomata, upon contact with the substrate. Extramatrical hyphae of the mycorrhizal fungal enter the axes through these stomata, and spread out through the intercellular system of the hypodermis and cortex, subsequently penetrating individual cells within a well-defined region of the cortex (i.e., the mycorrhizal arbuscule-zone) to form arbuscules.⁴A recently published study⁵ reports on three fungal endophytes that (co-)occur in the Rhynie chert plant Nothia aphylla. This plant consists of upright aerial axes arising from a system of non-stomatiferous, subterranean rhizomatous axes characterized by a prominent ventral rhizoidal ridge.^6,7 The rhizoidal ridge, which is unique among Rhynie chert land plants, consists of a rhizoid-bearing epidermis, a multi-layered hypodermis, files of parenchyma cells that connect to the stele, and extra-stelar conducting elements (Fig. 1A); intercellular spaces are virtually absent.Open in a separate windowFigure 1Nothia aphylla from the Lower Devonian Rhynie chert. (A) Ventral portion of a rhizomatous axis with rhizoidal ridge (cross section); bar = 250 µm. (B) Fungal hypha [arrows] entering the axis through a rhizoid; bar = 30 µm. (C) Sheathed intracellular hyphae [arrows] in hypodermal cells (transverse section); bar = 20 µm. (D) Intercellular hyphae and vesicles in the cortex (longitudinal section); bar scale = 50 µm. (E) Hyphae, vesicles and a thick-walled spore in the cortex (longitudinal section); bar = 100 µm. All images from the original paper; reproduced with permission.One of the fungal endophytes in N. aphylla closely resembles Glomites rhyniensis (Glomeromycota), the endomycorrhizal fungus of A. major.⁴ In N. aphylla, this fungus occurs as an intracellular endophyte in rhizoids and tissues of the rhizoidal ridge. Moreover, it is abundant in the intercellular system of the cortex of both prostrate and proximal portions of aerial axes. The fungus enters the axes through rhizoids (Fig. 1B). Once in the hypodermis, hyphae become sheathed by cell wall material (Fig. 1C). In the cortex, the fungus produces intercellular vesicles (Fig. 1D) and thick-walled spores (Fig. 1E). Based on the presence of vesicles that are similar to those of G. rhyniensis, and spores like those in extant Glomeromycota, we hypothesize that this fungus is an endomycorrhizal member of the Glomeromycota; however, arbuscules have not been observed to date.If this interpretation is accurate, N. aphylla displays an alternative pattern of colonization by endomycorrhizal fungi. Although the morphology of the fungus and distribution in N. aphylla correspond to that of G. rhyniensis in A. major, the infection pathway is distinctly different. While G. rhyniensis is an intercellular endophyte that penetrates individual cells exclusively within the mycorrhizal arbuscule-zone,⁴ the fungus of N. aphylla enters the plant as an intracellular endophyte, and remains intracellular until it reaches the cortex. The host plant apparently does not respond to the invading fungus because infected rhizoids are not altered morphologically. Once in the hypodermis, however, hyphae become separated from the host cell protoplast. This feature suggests a shift from (i) uncontrolled intracellular occurrence of the fungus in the rhizoids, to (ii) controlled intracellular occurrence in the rhizoidal ridge, to (iii) intercellular occurrence in the cortex.The fact that the rhizomatous axes of N. aphylla are subterranean, along with the peculiar internal anatomy of the rhizoidal ridge, may have provided the selective pressure for an alternative mode of colonization by endomycorrhizal fungi. The fungus probably enters the plant through rhizoids because the axes are non-stomatiferous. Moreover, the morphology and radial arrangement of cells in the rhizoidal ridge, along with the virtual absence of intercellular spaces, perhaps does not provide an intercellular infection pathway into the cortex. We speculate that N. aphylla tolerated intracellular penetration in the rhizoids and within the tissues of the rhizoidal ridge in order to become inoculated. Tolerating (or even facilitating) intracellular penetration within a limited area of the axis may simultaneously have provided the plant with a means of recognizing and subsequently distinguishing the endomycorrhizal fungus from potentially harmful parasites (e.g., by surface features of the hyphae or chemical signals). Once recognized, the endomycorrhizal fungi become sheathed and “guided” through the ridge without being able to extract nutrients from the host, and into the cortex where intracellular penetration is not longer possible. The parasites, once recognized, are confined in the tissues of the rhizoidal ridge by specific or unspecific host responses, e.g., secondarily thickened cell walls.⁵ Conversely, if the endomycorrhizal fungus entered the plant through surface openings, and spread out exclusively through the intercellular system, the mechanisms that might confine simultaneous parasite infections were probably much more limited.Endomycorrhizal relationships are believed to have evolved from parasitic interactions.⁸ It has been postulated that modern enodomycorrhizal fungi in some way control parasites because both compete for the same resources.⁹ It may be that, during the evolution of fungal endophytism, the initial benefits of mycorrhizae included protection of the host from pathogenic fungi.¹⁰ Nothia aphylla from the Lower Devonian Rhynie chert adds support to this hypothesis, and may demonstrate that more than a single pattern of colonization by endomycorrhizal fungi occurred during the early evolution of land plants.     

Palaeoecology and palaeophytogeography of the rhynie chert plants: evidence from integrated analysis of in situ and dispersed spores

The Rhynie cherts yield exceptionally preserved early land plants, and provide a unique insight into the nature of Lower Devonian vegetation. Hitherto they have been poorly age constrained, and the palaeoecology and palaeophytogeography of the flora are poorly understood. Well-preserved dispersed-spore assemblages have been recovered from a number of borehole cores through the stratigraphical sequence of the Rhynie outlier. They are all similar and belong with the polygonalis-emsiensis (PE) spore zone, indicating an Early Devonian age (Early (but not earliest) Pragian to earliest Emsian). Comparisons with PE spore-zone assemblages from elsewhere suggest that the flora of the Rhynie drainage basin was slightly impoverished, with certain plant taxa that occurred at other locations not represented. This probably reflects differences between the flora of an inland intermontaine basin (Rhynie) and that of the lowland flood-plains. In situ spores have been characterized for all seven Rhynie chert plants. Analysis of the distribution of these spore types in the Rhynie sequence, in addition to those of coeval deposits from elsewhere, enables interpretation of the palaeoecology and palaeophytogeography of the Rhynie chert plants. It is concluded that at least some of the plants were not highly specialized or adapted to the peculiar hot-springs environment in which they are preserved. Rather, they were components of a diverse and widespread flora, but were the only elements able to tolerate the inhospitable hot-springs environment (i.e. they were preadapted).     

Forms of nitrogen uptake, translocation, and transfer via arbuscular mycorrhizal fungi: A review

Arbuscular mycorrhizal (AM) fungi are obligate symbionts that colonize the roots of more than 80% of land plants. Experiments on the relationship between the host plant and AM in soil or in sterile root-organ culture have provided clear evidence that the extraradical mycelia of AM fungi uptake various forms of nitrogen (N) and transport the assimilated N to the roots of the host plant. However, the uptake mechanisms of various forms of N and its translocation and transfer from the fungus to the host are virtually unknown. Therefore, there is a dearth of integrated models describing the movement of N through the AM fungal hyphae. Recent studies examined Ri T-DNA-transformed carrot roots colonized with AM fungi in ¹⁵N tracer experiments. In these experiments, the activities of key enzymes were determined, and expressions of genes related to N assimilation and translocation pathways were quantified. This review summarizes and discusses the results of recent research on the forms of N uptake, transport, degradation, and transfer to the roots of the host plant and the underlying mechanisms, as well as research on the forms of N and carbon used by germinating spores and their effects on amino acid metabolism. Finally, a pathway model summarizing the entire mechanism of N metabolism in AM fungi is outlined.     

Acaulosporoid glomeromycotan spores with a germination shield from the 400-million-year-old Rhynie chert

Scutellosporites devonicus from the Early Devonian Rhynie chert is the only fossil glomeromycotan spore taxon known to produce a germination shield. This paper describes a second type of glomeromycotan spore with a germination shield from the Rhynie chert. In contrast to S. devonicus, however, these spores are acaulosporoid and develop laterally in the neck of the sporiferous saccule. Germination shield morphology varies, from plate-like with single or double lobes to tongue-shaped structures usually with infolded margins that are distally fringed or palmate. Spore walls are complex and appear to be constructed of at least three wall groups, the outermost of which includes the remains of the saccule. The complement of features displayed by the fossils suggests a relationship with the extant genera Ambispora, Otospora, Acaulospora or Archaeospora, but which of these is the closest extant relative cannot be determined. The acaulosporoid spores from the Rhynie chert document that this spore type was in existence already ∼400 mya, and thus contribute to a more complete understanding of the evolutionary history of the Glomeromycota. This discovery pushes back the evolutionary origin of all main glomeromycotan groups, revealing that they had evolved before rooted land plants had emerged.     

Perithecial ascomycetes from the 400 million year old Rhynie chert: an example of ancestral polymorphism

We describe a perithecial, pleomorphic ascomycetous fungus from the Early Devonian (400 mya) Rhynie chert; the fungus occurs in the cortex just beneath the epidermis of aerial stems and rhizomes of the vascular plant Asteroxylon. Perithecia are nearly spherical with a short, ostiolate neck that extends into a substomatal chamber of the host plant; periphyses line the inner surface of the ostiole. The ascocarp wall is multilayered and formed of septate hyphae; extending from the inner surface are elongate asci interspersed with delicate paraphyses. Asci appear to be unitunicate and contain up to 16 smooth, uniseriate-biseriate ascospores. The method of ascospore liberation is unknown; however, the tip of the ascus is characterized by a narrow, slightly elevated circular collar. Ascospores appear 1-5 celled, and germination is from one end of the spore. Also present along the stems and interspersed among the perithecia are acervuli of conidiophores that are interpreted as the anamorph of the fungus. Conidiogenesis is thallic, basipetal and probably of the holoarthric-type; arthrospores are cube-shaped. Some perithecia contain mycoparasites in the form of hyphae and thick-walled spores of various sizes. The structure and morphology of the fossil fungus is compared with modern ascomycetes that produce perithecial ascocarps, and characters that define the fungus are considered in the context of ascomycete phylogeny.     

FUNGI FROM THE LOWER DEVONIAN RHYNIE CHERT: CHYTRIDIOMYCETES

Several different chytridiomycetes are described from the Lower Devonian (Siegenian) Rhynie chert. Included are both eucarpic and apparently holocarpic forms that occur in Palaeonitella, Aglaophyton, Lyonophyton, Horneophyton, and clusters of algal cells, as well as in the surrounding chert matrix. Holocarpic types consist of endobiotic sporangia, each characterized by one discharge tube. Sporangia can be traced from the thallus stage to the discharge of zoospores. Monocentric and polycentric eucarpic chytrids are associated with the miospores of Aglaophyton and various thick-walled fungal spores. In these forms the sporangia are variable in size and shape ranging up to 30 μm. Most appear to be inoperculate and there is evidence that the sporangium ruptured on the distal surface. Some contain zoospores with flagella. One operculate eucarpic form had parasitized the cellular gametophyte emerging from the proximal surface of an Aglaophyton spore. Several of the Rhynie chert chytrids are comparable with a number of extant forms (e.g., Olpidiaceae and Spizellomycetaceae), while others possess features that encompass several groups. These fossil fungi are discussed in the context of their interactions with other organisms in this Lower Devonian freshwater paleoecosystem.     

A cyanolichen from the Lower Devonian Rhynie chert

The 400 million-year-old Rhynie chert has provided a wealth of information about various types of fungal interactions that existed in this Early Devonian paleoecosystem. In this paper we report the first unequivocal evidence of a lichen symbiosis from the Rhynie chert. Specimens of a new genus, Winfrenatia, consist of a thallus of superimposed layers of aseptate hyphae and, on the upper surface, numerous uniform depressions. Extending into the base of each depression are hyphae that form a three-dimensional netlike structure. Enclosed within each of the net spaces is a coccoid cyanobacterium, each cell of which is surrounded by a thick sheath. These photobiont cells divide in three planes, resulting in cell clusters of up to perhaps 64 individuals. The photobiont is parasitized by the fungus in the base of each net as new cyanobacterial cells are formed distally. Reproduction is by endospores and soredia. Affinities of the mycobiont appear closest to members of the Zygomycetes, while the photobiont is most similar to coccoid cyanobacteria of the Gloeocapsa and Chroococcidiopsis types. We speculate that this cyanobacterial symbiosis was well adapted to exploit and colonize new ecological niches, especially in the periodically desiccated environment postulated for the Rhynie chert paleoecosystem.     

Hassiella monospora gen. et sp. nov., a microfungus from the 400 million year old Rhynie chert

A new microfungus, Hassiella monospora gen. et sp. nov., consisting of coenocytic hyphae is associated with degraded plant material in the Early Devonian silicified Rhynie chert ecosystem. Some hyphae produce small bulb-like projections that subsequently develop into spherical, thick-walled and highly ornamented reproductive structures. Mature reproductive structures are characterized by a prominent, funnel-shaped appendage that is interpreted as an amphigynous antheridium. When combined, these features are suggestive of the oogonia/oosporangia in certain extant members of the Peronosporomycetes (Oomycota).     

Fungi from the Lower Devonian Rhynie chert: My Coparasitism

Several examples of mycoparasitism are described from the Lower Devonian (Siegenian) Rhynie chert. These fungal interactions include thick-walled chlamydospores and vesicles in which epibiotic fungi are attached to the outer surface of the spore. Other fossil spores are characterized by mycoparasites developing between the layers of the spore wall or within the lumen. The presence of callosities extending from the inner surface of some fossil spores demonstrates that the hosts were alive when parasitized. This response by the mycohost is identical to that found in certain modem mycoparasitic symbioses involving vesicular arbuscular mycorrhizae that are parasitized by various aquatic fungi. The presence of mycoparasitism in a 400-million-year-old ecosystem underscores the potential significance of the fungal genome early in the evolution of other organisms.     

宿主植物栽培密度对AM真菌生长发育的影响

温室盆栽条件下宿主植物高粱(SorghumvulgarePers.)的栽培密度对丛枝菌根(Arbuscularmycorrhizae,AM)真菌Glomusmosseae(Nicol.&Gerd.)Gerdemann&Trappe生长发育的影响试验结果表明:60株/盆密度处理的根外菌丝量及孢子数均高于其它处理。在一定栽培密度下(20~60株/盆),植株根系可溶性糖浓度与根外菌丝量呈显著负相关,与菌根侵染率呈显著正相关。植株根中磷浓度与根外菌丝量、根外菌丝量与孢子数均呈显著正相关。植株根中磷浓度与菌根侵染率呈显著负相关。结果说明:适当密植虽对植株生长有一定影响,但却促进了真菌的生长,此时菌根共生体有可能由互惠共生开始向偏利共生或弱寄生转化。密植作为一种调控手段,在菌剂生产中能获得较大数量的侵染根段、菌丝及孢子等繁殖体。     

Seven years of carbon dioxide enrichment, nitrogen fertilization and plant diversity influence arbuscular mycorrhizal fungi in a grassland ecosystem

? We tested the prediction that the abundance and diversity of arbuscular mycorrhizal (AM) fungi are influenced by resource availability and plant community composition by examining the joint effects of carbon dioxide (CO(2) ) enrichment, nitrogen (N) fertilization and plant diversity on AM fungi. ? We quantified AM fungal spores and extramatrical hyphae in 176 plots after 7 yr of treatment with all combinations of ambient or elevated CO(2) (368 or 560 ppm), with or without N fertilization (0 or 4 g Nm(-2) ), and one (monoculture) or 16 host plant species (polyculture) in the BioCON field experiment at Cedar Creek Ecosystem Science Reserve, Minnesota, USA. ? Extramatrical hyphal lengths were increased by CO(2) enrichment, whereas AM spore abundance decreased with N fertilization. Spore abundance, morphotype richness and extramatrical hyphal lengths were all greater in monoculture plots. A structural equation model showed AM fungal biovolume was most influenced by CO(2) enrichment, plant community composition and plant richness, whereas spore richness was most influenced by fungal biovolume, plant community composition and plant richness. ? Arbuscular mycorrhizal fungi responded to differences in host community and resource availability, suggesting that mycorrhizal functions, such as carbon sequestration and soil stability, will be affected by global change.     

Nutrient uptake in mycorrhizal symbiosis

The role of mycorrhizal fungi in acquisition of mineral nutrients by host plants is examined for three groups of mycorrhizas. These are; the ectomycorrhizas (ECM), the ericoid mycorrhizas (EM), and the vesicular-arbuscular mycorrhizas (VAM). Mycorrhizal infection may affect the mineral nutrition of the host plant directly by enhancing plant growth through nutrient acquisition by the fungus, or indirectly by modifying transpiration rates and the composition of rhizosphere microflora. A capacity for the external hyphae to take up and deliver nutrients to the plant has been demonstrated for the following nutrients and mycorrhizas; P (VAM, EM, ECM), NH₄ ⁺ (VAM, EM, ECM), NO₃ ^- (ECM), K (VAM, ECM), Ca (VAM, EM), SO₄ ^2- (VAM), Cu (VAM), Zn (VAM) and Fe (EM). In experimental chambers, the external hyphae of VAM can deliver up to 80% of plant P, 25% of plant N, 10% of plant K, 25% of plant Zn and 60% of plant Cu. Knowledge of the role of mycorrhiza in the uptake of nutrients other than P and N is limited because definitive studies are few, especially for the ECM. Although further quantification is required, it is feasible that the external hyphae may provide a significant delivery system for N, K, Cu and Zn in addition to P in many soils. Proposals that ECM and VAM fungi contribute substantially to the Mg, B and Fe nutrition of the host plant have not been substantiated. ECM and EM fungi produce ectoenzymes which provide host plants with the potential to access organic N and P forms that are normally unavailable to VAM fungi or to non mycorrhizal roots. The relative contribution of these nutrient sources requires quantification in the field. Further basic research, including the quantification of nutrient uptake and transport by fungal hyphae in soil and regulation at the fungal-plant interface, is essential to support the selection and utilization of mycorrhizal fungi on a commercial scale.     

培养容器容积对AM真菌生长发育的影响

研究宿主植物栽培容器对丛枝菌根(Arbuscularmycorrhizae,AM)真菌Glomusmosseae生长发育的影响。结果表明:小容积容器的根系密度相对较大,在菌根共生体建立初期,菌根真菌繁殖体与根接触的机会增大,对于菌根真菌的迅速侵染及共生体的迅速建立非常有利,同时还增大了根外菌丝二次侵染的机会,从而使菌根真菌生长发育形成了一个良性循环,最终有利于根外孢子的形成。容器对共生体的影响决不是简单的盆的体积问题,而与其面积和体积之比有关,也和种植密度有密切关系。     

Arbuscular mycorrhizal fungi can transfer substantial amounts of nitrogen to their host plant from organic material

Nitrogen (N) capture by arbuscular mycorrhizal (AM) fungi from organic material is a recently discovered phenomenon. This study investigated the ability of two Glomus species to transfer N from organic material to host plants and examined whether the ability to capture N is related to fungal hyphal growth. Experimental microcosms had two compartments; these contained either a single plant of Plantago lanceolata inoculated with Glomus hoi or Glomus intraradices, or a patch of dried shoot material labelled with (15)N and (13)carbon (C). In one treatment, hyphae, but not roots, were allowed access to the patch; in the other treatment, access by both hyphae and roots was prevented. When allowed, fungi proliferated in the patch and captured N but not C, although G. intraradices transferred more N than G. hoi to the plant. Plants colonized with G. intraradices had a higher concentration of N than controls. Up to one-third of the patch N was captured by the AM fungi and transferred to the plant, while c. 20% of plant N may have been patch derived. These findings indicate that uptake from organic N could be important in AM symbiosis for both plant and fungal partners and that some AM fungi may acquire inorganic N from organic sources.     

Infection structures of biotrophic and hemibiotrophic fungal plant pathogens

Biotrophic plant pathogenic fungi are one of the major causes of crop losses. The infection processes they exhibit are typified by infected host plant cells remaining alive for several days. This requires the development of specialized infection structures such as haustoria which are produced by obligate biotrophs, and intracellular hyphae which are produced by many hemibiotrophs. These infection hyphae are surrounded by the host plant plasma membrane, and in the case of haustoria the extrahaustorial membrane differs biochemically and structurally from the normal membrane. An interfacial matrix separates haustoria and intracellular hyphae from the invaginated membrane and this seems to be characteristic of biotrophic interactions. There is clear evidence for molecular differentiation of the haustorial plasma membrane in powdery mildews and rusts in comparison with the other fungal membranes. Relatively few pathogenicity genes related to biotrophy, and the switch from biotrophy to necrotrophy in hemibiotrophs, have been identified.     

Early Devonian fungi: a blastocladalean fungus with sexual reproduction

In this paper we describe a fossil fungus–Paleoblastocladia milleri gen. et sp. nov.–from the 400 million-year-old Early Devonian Rhynie chert that shares numerous features with modern zoosporic fungi placed in the order Blastocladiales. The fungus occurs in tufts that arise from stomata or between the cuticle and epidermis of Aglaophyton major axes. Thallus development begins from an irregular bipolar basal cell that produces a system of intramatrical rhizoids and clavate-shaped extramatrical, nonseptate hyphae. These hyphae develop into two types of mature thalli. Sporothalli are characterized by several orders of dichotomous branching and the production of terminal, globose zoosporangia, as well as thick-walled, pitted resting sporangia. On separate dichotomously branched thalli (gametothalli) are terminal chains of two or three gametangia, in which the terminal one is slightly larger. Despite the fact that all of the reproductive organs contain either zoospores or gametes, none show evidence of discharge papillae. The fossil fungus is compared with extant members of the Blastocladiales, and the presence of sexual reproduction is discussed.