Lecture 12. Ascomycota II

Transcription

1 Lecture 12 Ascomycota II - Saccharomycotina - Pezizomycotina 1 Lutzoni et al., 2004, American Journal of Botany

2 Saccharomycotina - the true budding yeasts (--> Saccharomycetes, or the Saccharomycetales in traditional classification systems). but now also includes filamentous forms in modern classification. - lack ascomata; - mostly clonal organisms - widespread in nature - < 800 recognized species, however, many more to be discovered - simple morphology; many species are distinguished from their physiological properties Major taxa - Saccharomyces :beer, bread etc. - Candida, Yarrowia, and the filamentous teleomorphic form Pichia, includes the widespread human pathogen Candida albicans : The human pathogen Candida albicans - dimorphic yeast; diploid - widespread, particularly in hospitals! - Oral and genital opportunistic, or anywhere after surgery; any time the immune system is weakened - yeast form is best for propagation in blood and lymphatic tissues; hyphal form is best for tissue invasion. - genome sequenced ( (NOTE: the genus Candida is polyphyletic)

3 The beetle gut: a hyperdiverse source of novel yeasts Sung-Oui SUH, Joseph V. McHUGH, David D. POLLOCK, and Meredith BLACKWELL Mycological Research 109: (2005). Abstract We isolated over 650 yeasts over a three year period from the gut of a variety of beetles and characterized them on the basis of LSU rdna sequences and morphological and metabolic traits. Of these, at least 200 were undescribed taxa, a number equivalent to almost 30% of all currently recognized yeast species. A Bayesian analysis of species discovery rates predicts further sampling of previously sampled habitats could easily produce another 100 species. The sampled habitat is, thereby, estimated to contain well over half as many more species as are currently known worldwide. The beetle gut yeasts occur in 45 independent lineages scattered across the yeast phylogenetic tree, often in clusters. The distribution suggests that the some of the yeasts diversified by a process of horizontal transmission in the habitats and subsequent specialization in association with insect hosts. Evidence of specialization comes from consistent associations over time and broad geographical ranges of certain yeast and beetle species. The discovery of high yeast diversity in a previously unexplored habitat is a first step toward investigating the basis of the interactions and their impact in relation to ecology and evolution. Geography and niche occupancy as determinants of yeast biodiversity: the yeast-insect-morning glory ecosystem of Kipuka Puaulu, Hawai'i. Lachance MA, Bowles JM, Starmer WT. FEMS Yeast Res. 4: (2003). Abstract.- Biodiversity theory proposes two types of hypotheses to account for the species composition of a given community. The first encompasses geographic and historical factors. For example, local species richness is thought to be affected by area, proximity to large landmasses, dispersal mechanisms, and climatic history, collectively known as biogeography. The second type, termed niche occupancy rules, deals with the intrinsic properties of the species as they affect their interaction with the habitat and with other members of the community. The yeast-insect-morning glory ecosystem is a good model to explore biodiversity theory in ascomycetous yeasts. Here we focus on beetles that breed or feed in morning glories and a group of ascomycetous yeasts that are associated exclusively with them. Specifically, we analyse the community found in the vicinity of Kipuka Puaulu, a small patch of disturbed but mature forest situated amidst lava flows on the island of Hawai'i. Major members of the yeast community include Metschnikowia hawaiiensis, Metschnikowia lochheadii, and the related asexual species Candida ipomoeae and Candida kipukae. These species are nearly indistinguishable from one another in terms of nutritional requirements and abilities, although their phylogenetic range is enormous. Their distribution, both global and local, is far from random. As Kipuka Puaulu is an island within an island, the principles of island biogeography may be invoked to explain some aspects of its yeast species composition. M. lochheadii, C. ipomoeae, and the rare species Candida hawaiiana are recent introductions from the American continent and therefore exotic, whereas M. hawaiiensis and C. kipukae might be regarded as endemic, as they are yet to be isolated elsewhere. Vectoring by certain nitidulid beetles explains the long-range dispersal of these species. However, niche occupancy rules may account in part for the local spatial distribution of the yeasts within the island of Hawai'i and within the kipuka itself. We have identified the maximum growth temperature as a potentially critical property of the fundamental niche of these yeasts.

4 Ascomycota: Pezizomycotina: general characteristics - filamentous ascomycetes ; - formation of ascomata ( fruiting bodies ); - cells generally -- uninucleate, -- haploid -- single septum -- Woronin bodies present (except in some Plectomycetes) - cell wall: essentially chitin and beta-glucans; - sexual reproduction: mostly heterothallic and unifactorial (bipolar); some species are homothallic - traditional taxonomy essentially based on ascomata (cleistothecium, perithecium etc.) and asci (unitunicate, bitunicate etc) types - see a previous Lecture. --- still no clear picture of evolutionary relationships between the major groups ( orders ) Pezizomycotina: some definitions Tissues and cells in ascomata: - organization of mycelium into a tissue = plectenchyma --- If such a tissue is loosely woven and the mycelial strands are more or less evident = prosenchyma. --- If the hyphae have lost their individuality and the cells are more or less isodiametric (resembling the parenchyma of plants) = pseudoparenchyma. - distinctive sterile cells and hyphae that are interspersed among asci or elsewhere in the ascocarp: --- paraphyses: elongated, cylindrical or club-shaped hyphae originating at the base of an ascocarp and growing among the asci in a hymenium; branched or unbranched; septate or not. ---Paraphyses usually remain free at their tips except in some discomycetes in which the tips fuse above the asci forming a solid layer, the epithecium; --- pseudoparaphyses: originate above the level of the asci and grow down among the developing asci. They often reach the base of the ascocarp where they may fuse with cells of the plectenchyma branched or unbranched; septate or not; often anastomosing. --- periphyses: short unbranched hyphae in the ostiolar canal of perithecia. They apparently serve to direct the asci toward the tip of the ostiole prior to ascospore discharge;

5 Pezizomycotina: typical life cycle From Alexopoulos Lutzoni et al., 2004, American Journal of Botany Basal lineages: Pezizomycetes and Leotiomycetes (= traditional Discomycetes ; paraphyletic)

6 The traditional Discomycetes Taxonomy and Classification. Discomycetes have been traditionally divided into two major groups based on type of ascus : inoperculate Discomycetes vs operculate Discomycetes. Other ascal characters used to distinguish between taxa: --- the structure of the apex --- the method of dehiscence. --- the reaction of the tip to iodine (amyloidy). Operculate Discomycetes = Pezizomycetes, possibly monophyletic (excluding lichenized forms) - ascus tip has a cap or lid-like structure, the operculum, which opens and permits the escape of the ascospores. --- less frequently, instead of an operculum, a longitudinal slit opens and releases the spores during discharge. Inoperculate Discomycetes = Leotiomycetes, probably not monophyletic - asci release their spores through an apical, circular perforation or pore ( sphincter ). From Kendrick

7 Peziza - Apothecia mostly cup- or disc-shaped may be sessile to stalked; ---- minute to very large; bright-colored to dark-brown; smooth, velvety, hairy, or bristly - The ascal apex may be intensly blue or the entire ascus wall difusely blue in iodine (e.g., Melzer solution) - Single-nucleate ascospores are relatively thin-walled - grow on the ground or on animal dung. Pezizomycetes Peziza cerea Sarcoscypha, Cookeina Differs from Peziza from: - one celled, multinucleate ascospores and cells of the paraphyses - suboperculate asci: ascus possesses a thickened apical ring capped by a plug or hinged operculum, its opening often oriented obliquely. - mostly tropical fungi; - generally on wood Sarcosccypha coccinea From Kendrick Cookeina tricholoma Pezizomycetes Urnula Similar to Sarcoscypha but lack the bright colors of the apothecia and multinucleate paraphyses Urnula craterium Very common in spring in N.America

8 Pezizomycetes Ascobolaceae: Ascobolus and Saccobolus - Small sized apothecia - Very thick-walled, uninucleate ascospores. - These species are frequent in moist chamber culture of dung, e.g. Saccobolus sp. ; phototropic asci Pezizomycetes: Morchellaceae - Large, stalked apothecia, mostly with a sponge-like or bell-shaped pileus Morchella spp. grow from subterranean sclerotia - Ascospores are always multinucleate (20-60 nuclei in each), a character in common with Sarcoscyphaceae and Sarcosomataceae. - Woronin bodies elongated rather than spherical Verpa bohemica

9 Pezizomycetes: Helvellaceae - Large, stipitate ascocarps - Ascal apex does not turn blue with iodine - ascospores quadrinucleate, contain conspicuous oil droplets - Woronin bodies elongated as in Morchellaceae Gyromitra korfii Gyromitra contains gyromitrin, a substance lethal for many persons when ingested. Helvella crispa (pic G. Barron) Pezizomycetes: truffles From Kendrick - Truffles have completely closed, hypogeous ascocarps; - within the closed ascocarps asci line internal chambers in the ascocarp tissue. - spore discharge is passive and dispersal involves mycophagy by insects and, especially, mammals. - animals attracted by odors produced by the ascocarp Evolution toward the sequestrate and hypogeous condition has occured several time among the operculate discomycetes ----> truffles are polyphyletic Tuber: - mycorrhizal; - the most expensive mushroom: at least $800/pound!

10 From Kendrick Genea sp. Caloscypha fulgens Tuber aestivum Geopora cooperi Elaphomyces the deer fungus - ca. 20 known species. - look like typical truffles (Tuber) but they have no distinct hymenium layer --- non-shooting asci, produced randomly throughout the interior of the ascoma. --- in contaminated soil, has been found to contain the highest amounts of Cs-137 in comparison to other mushrooms Phylogenetic relationships unresolved.