Introduction to Ascomycota (continued); Archiascomycetes and Saccharomycetales

1. Introduction to Ascomycota (continued); Archiascomycetes and Saccharomycetales Pl P 421/521 Lecture 5

2. Nuclear condition • Monokaryon • A single type of nucleus in a cell or mycelium • Dikaryon • A pair of closely associated, sexually compatible nuclei in a cell or mycelium (n + n) • Heterokaryon (heterokaryosis) • a condition in which genetically different nuclei are associated with the same protoplast or the same mycelium

3. Parasexual Cycle(Pontecorvo, 1956) • Establishment of heterokaryon • Mutation • Hyphal fusion • Fusion of two different nuclei to form diploid • Haploidization by aneuploidy

4. From Gary Cole http://gsbs.utmb.edu/microbook/ch073.htm

5. Sexual Reproduction • Sexual spores (meiospores=ascospores) formed in sac-like structure called ascus (pl. asci) • Site of meiosis • Three main types of asci in Ascomycota: • Prototunicate ascus • Unitunicate ascus • Bitunicate ascus

6. Ascus types: Prototunicate • Thin-walled ascus • Ascospores released by breakdown of wall

7. Ascus types: Unitunicate • Inner and outer ascus walls do not separate during ascospore release • Ascospores released through specialized adaptation at ascus tip

8. Unitunicate Asci

9. Ascus tip Refractive ring, Neurospora Amyloid ring, Xylaria

10. Ascus tip • Operculum • Cap that detaches along preformed line

11. Ascus types: Bitunicate • Also called “Jack-in-the Box” ascus • Inner and outer wall layers separate during ascospore discharge: • Inner (endotunica)—thin and extensible • Outer (exotunica)—thick, inextensible • Inner wall balloons out beyond outer wall

13. Bitunicate Ascus

14. Bitunicate ascus Endotunica Exotunica Protoventuria barriae

15. Ascomatal types • Cleistothecium • Completely enclosed, no preformed opening • Perithecium • Apical pore (ostiole) through with ascospores are released • Pseudothecium • Ascocarp with asci formed in cavity (locule) within stromatic tissue • Apothecium • Ascocarp with asci exposed at maturity

16. Ascocarp types stroma Perithecium Pseudothecium

17. Ascocarp types Cleistothecium Apothecium

18. What’s inside? • Asci-scattered or in hymenium • Hamathecium—sterile elements • Paraphyses • Apical paraphyses • Epithecium • Periphyses • Pseudoparaphyses

19. Filamentous ascomycetes simple septal pores; ascus Sacccharomycetales Characterized by DNA sequence analysis Archiascomycetes Basidiomycetes Ascocarps; ascogenous hyphae; specialized ascus tip; conidia; Woronin bodies Absence of ascogenous hyphae and ascocarps; most asci without specialized tips Classification from Alexopoulos et al. 1996

20. Phylogeny of basal Ascomycota—nSSU and nLSU rDNA Sugiyama et al. 2006. Mycologia 98: 1002

21. Archiascomycetes--taxa • Neolecta • Taphrinales • Taphrina—T. deformans, peach leaf curl • Protomyces • Schizosaccharomycetales • Schizosaccharomyces—fission yeast • Saitoella—soil yeast • Pneumocystis—causal agent of virulent pneumonia (P. carinii)

22. Pneumocystis • Considered to be a protozoan until 1980s • Lacks ergosterol in cell walls • Major cause of pneumonia in immunocompromised patients • Life cycle and morphology poorly understood

23. a—e: Taphrina spp. Sugiyama et al. 2006. Mycologia 98: 1001 f, g: Protomyces i, j: Saitoella Schizosaccaromyces Pneumocystis Neolecta

24. Schizosaccharomyces

25. Archiascomycetes/Taphrinomycotina • Diverse group, includes saprotrophs and parasites (animal and plant) • Grouped together primarily by rDNA sequence data

26. Saccharomycetales/Saccharomycotina(Hemiascomycetes) • Ascomycetous yeasts • Characterized by absence of ascogenous hyphae and ascocarps • Asci mostly prototunicate • Occur in slime fluxes, nectar, fresh or decaying fruit—able to grow in high osmotic conditions (high sugar content) • Others occur in soil, dung, water, digestive tracts of animals • Many species are symbiotic with insects

27. Importance • Brewing • Baking • Distilling • Food production • Industrial applications • Model systems (S. cerevisiae) • Animal pathogens

28. Somatic structures • Yeast • A single-celled fungus that reproduces by budding (or fission) • Pseudomycelium • Series of cells adhering after budding • True mycelium • Produced through apical tip growth

29. Budding Multilateral Bipolar

30. Asexual reproduction • Conidia • Arthrospores

31. karyogamy plasmogamy budding copulation 2n somatic cells 1n somatic cells meiosis Mature ascus

32. Identification • Microscopic appearance • Unicellular or budding • Size & shape of yeast cells • Multilateral or bipolar budding • Form, structure and mode of ascus formation • Ascospore shape

33. Identification • Physiological tests—91 different tests • Ferment different sugars • Assimilation tests (carbon and nitrogen source) • Vitamin requirements • Growth at 37C • Growth in cyclohexamide • Hydrolyse urea • Form acid

34. Classification • Saccharomycetaceae • Unicellular, multilateral budding • Nadsoniaceae, Saccharomycodaceae • Unicellular, lemon-shaped, bipolar budding • Eremotheciaceae, Metschnikowiaceae • Mycelial, elongate ascus, needle-like ascospores • Cephaloascaceae,Dipodascaceae, Lipomycetaceae • Mycelial, hat-shaped ascospores, ascophores, insect association