Zoosporic Fungi Phylum Chytridiomycota

1. Zoosporic FungiPhylum Chytridiomycota General Mycology Pl P 421/521

2. Zoospore • Microscopic (2-14 x 2-6 micron), uninucleate, unicellular, flagellated spore lacking a cell wall • Formed in a zoosporangium by a process involving mitosis and cytoplasmic cleavage • Zoospores do not feed, and rely on endogenous energy reserves Catenaria zoospore, from George Barron’s website

3. Zoospores Olpidium zoospores/D. J. S. Barr

4. Flagella (sing. flagellum) • 0.25 microns wide, up to 50 microns long • Composed of a 9(2) + 2 arrangement of microtubules enclosed in a plasma membrane

5. Flagella (sing. flagellum) • Attached to a kinetosome (=basal body) • Flagellated centriole; highly conserved structure composed of 9 triplets of microtubules arranged in a cartwheel manner • All zoospores have two kinetosomes, but if only one flagellum is formed, the second kinetosome is non-functional

6. Prokaryote vs. Eukaryote flagellum Bacterial flagellum from www.arn.org http://fig.cox.miami.edu/Faculty/Dana/flagellum.jpg

7. Centriole • Nuclear-associated organelle (NAO) involved in nuclear division—involved in formation of spindle fibers that separate chromosomes during division • All eukaryotes with a flagellated stage in their lifecycle have a pair of centrioles that replicate during cell division

9. Flagella • One to many flagella depending on the taxonomic group • Two types of flagella: • Whiplash • Smooth, usually directed backwards, propels the zoospore • Tinsel • With tripartite hairs (=mastigonemes); directed forward, pulls the zoospore • May have only whiplash, whiplash + tinsel, or only tinsel • Flagellum may be of unequal length (= heterokont)

10. Zoospore types; from Dick, 2001. Straminipilous Fungi

11. Zoospore ultrastructure Plasma membrane Lipid globules microbodies nucleus ribosomes Endoplasmic reticulum Microtubules (cytoskeleton mitochondria Kinetosome and non-functioning kinetosome flagellum

12. Motile Phase • Requires water • Free-swimming phase is influenced by: • Endogenous energy reserves (lipids) • Environmental conditions

13. Encystment • Prior to germination, zoospores must: • shed or retract flagella • Form a cell wall

14. Germination • Direct • Formation of germ tube • Indirect • Formation of another zoospore

15. Zoosporangium • A typically multinucleate structure that produces zoospores by a process call zoosporogenesis • Zoosporogenesis involves mitosis and cleavage of zoospores from zoosporangium cytoplasm • Zoospores release : • Breakdown of zoosporangial wall • Opening of cap-like cover called operculum • Discharge papillae plugged with gelatinous material

16. Thallus Types • Holocarpic • Conversion of entire thallus into one (monocentric) or more (polycentric) zoosporangia • Eucarpic • Entire thallus not converted into zoosporangium, and other structures may be formed: • Rhizomycelium—hyphal-like structures connecting sporangia, lack nuclei • Rhizoids—root-like structures, lack nuclei • Mycelium

17. rhizoids sporangium Eucarpic thallus of Spizellomyces Photo by D. J. S. Barr

18. Thallus types relative to substrate • Endobiotic • Thallus produced inside host or substrate • Epibiotic • Thallus produced outside host or substrate; rhizoids anchor thallus to substrate

19. Zoosporic fungi • Kingdom Fungi • Phylum Chytridiomycota • Kingdom Straminipila • Phylum Oomycota • Phylum Hyphochytriomycota • Phylum Labyrinthulomycota • Protista • Phylum Plasmodiophoromycota • Phylum Myxomycota

20. Phylum Chytridiomycota • True Fungi based on: • Chitinous walls • Flattened mitochondrial cristae • Lysine synthesis by the alpha aminioadipic acid (AAA) pathway characteristic of all true Fungi and some protists • compare to diaminopimelic acid pathway found in bacteria, plants, and some protists

22. ? Phylum Blastocladiomycota Phylum Chytridiomycota ? From James et al. 2006. Mycologia 98: 860-871

23. Classification • Two phyla, Blastocladiomycota and Chytridiomycota • 123 genera, 900 species in 5 orders: • Chytridiales • Spizellomycetales • Monoblepharidales • Neocallimasticales • Blastocladiales

24. Thallus types • Chytridiales, Spizellomycetales and Neocallimasticales • Relatively simple thalli, holocarpic or eucarpic with rhizoids or rhizomycelium • Monoblepharidales • Filamentous thalli (mycelium) • Blastocladiales • Stalked thalli with rhizoids

25. Asexual Reproduction • Uniflagellate zoospores • One whiplash flagellum inserted in posterior part of zoospore • Zoospores formed in zoosporangia and are released through an operculum or discharge papilla

26. Zoospore release in Chytrium Photos by D. J. S. Barr

30. Sexual Reproduction • Plasmogamy can involve one of five different structures depending on species: • Isogamous planogametes • Anisogamous planogametes • Nonmotile female gamete and motile male gamete • Gametangial copulation • Somatogamy

31. Sexual Reproduction • Plasmogamy and karyogamy results in formation of resting sporangium • Thickened, often pigmented and/or ornamented wall • Germination of resting sporangium occurs after meiosis by cleavage of cytoplasm into zoospores

32. zoosporangium Zoospore release operculum rhizoids germination encystment germination meiosis karyogamy Resting sporangium Plasmogamy by somatogamy Young zygote

33. Order Spizellomycetales • Monocentric thalli • Blunt rhizoid tips (1-2 microns diam) • Inoperculate, multipapillate zoosporangia • Amoeboid-like zoospores • Mostly soil-inhabiting

34. Order Chytridiales • Monocentric or polycentric thalli • Slender rhizoid tips (< 0.5 micron) • Inoperculate or operculate; if inoperculate, then single or multipapillate • Regular-shaped zoospores • Mostly aquatic

35. Zoospore ultrastructure Spizellomycetales Chytridiales Barr, 1990

36. Batrachochytrium dendrobatidis • Responsible for decline of amphibian populations, particularly frogs, in six continents (Africa, South, Central and North America, Europe, Australia and Oceania) • First reported in 1993 • Only member of Chytridiomycota to parasitize vertebrates

37. How does it kill amphibians? • Sporangia restricted to keratinized skin of adult frogs and keratinized mouth parts of tadpoles • Causes widespread, fatal epidermal infection only in adults • Epidermal hyperplasia that results may seriously impair cutaneous respiration and osmoregulation • Toxin production has not been demonstrated

39. Order Neocallimasticales • Also spelled Neocallimastigales • “Rumen fungi”—first discovered in 1977 • Obligately anaerobic chytrids that live in digestive tract of herbivores (ruminants and hind-gut fermenters • Some taxa produce polyflagellated zoospores • Zoospores lack mitochondria

40. Biology of rumen fungi • Zoospores encyst on plant material in rumen and intestine • Form thallus with well-developed rhizoidal system that penetrates plant material • Passed from mother to offspring, probably through licking or feces • No known sexual stage

41. Numbers represent hours after encystment Life cycle animation

42. Thallus of rumen fungus Thallus (zoosporangium) of rumen fungus Polyflagellated zoospore

45. Cow Facts • Each day, the average cow drinks ~ 120 liters and eats ~ 44 kilos (95 lb) of feed • The rumen has a volume of 100-150 liters • A cow produces 98-190 liters of saliva each day • A cow produces an average of 25 kilos (55 lbs) of manure/day

46. Phylum Blastocladiomycota • Zoospores with tightly organized organelles and characterized by ‘nuclear cap’ • Most species are saprotrophs in soil, water, mud, plant and animal debris; exceptions: • Coelomomyces, is an obligate endoparasite of insects • Catenaria species parasitize small animals • Physoderma species are plant parasites • Separate gametophytic and sporophytic thalli in several genera, including Coelomomyces, Allomyces and Blastocladiella

47. ZOOSPORE ULTRASTRUCTURE Nuclear cap Blastocladiales Monoblepharidales

48. Coelomomyces • Alternating sporophytic and gametophytic stages in mosquito larvae and copepod (fish lice) hosts, respectively • Wall-less hyphal bodies (‘hyphagens”) formed in coelom of host

49. Life Cycle of Coelomomyces Motile zygote encysts, infects mosquito larvae Conjugation of gametes Gametophytic thallus lacking cell wall forms in copepod Sporothallus develops in host, resting sporangia formed Zoospores (meiospores) infect copepod Resting sporangia Germination of resting spore

50. Coelomomyces resting spore Photograph by CC López lastra, JJ García