Biology of FUNgi - courses.k-state.edu · Phylum Ascomycota - the sac fungi Apothecium of Aleuria aurentia Naked asci of Taphrina deformans Perithecium of Daldinia sp. Cleistothecium

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Biology of FUNgi

Lecture 06Archiascomycetes and deviations to

polyphyletic yeasts

A look back...

• What are the true fungi, what are the others?

• What are the lower and higher fungi?

• What defines Chytridiomycetes as a groupand how are they related to other fungi?

• How about Zygomycetes?

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Zygomycetes - Reminder of thecharacter evolution

Ancestral traitsChitin GlycogenPosterior flagellum

Loss of phagotrophy

Loss of flagella

Dikaryotic stageSeptate mycelium

Ascospores

BasidiosporesDollipore septumClamp connections

Chytridiomycota

Basidiomycota

Ascomycota

Zygomycota

Animalia

?

?

Phylum Zygomycota - the conjugatingfungi

General characteristics

The zygomycetes can be either homo- orheterothallic (one individual can give rise tosexual structures or two different mating typesrequired for sexual reproduction).

They can also form a zygosporeparthenogenetically. The resulting azygosporeis similar to a real zygospore (does this involvemeiosis and recombination events?) but hasusually just a single suspensor cell.

Some zygospores are wrappedin an elaborate hyphalcosntruction. Absidia spinosaand Phycomyces blakesleeanusas examples.

A zygospore suspendedbetween the suspensor cells -Rhizopus sp.

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Where and how to look for the zygomycetes

As a group the Zygomycetes can be isolated from avariety of substrates including soil, dung, fruit, grain,plant tissues, and animal tissues.

Nutritionally they range from saprotrophism throughweak parasitism of plants to very specializedparasitism and predation of animals.

Some have been called “sugar fungi” because theylack a good arsenal of enzymes which would allowdegradation of complex carbohydrates.

Entomophthora is aspecialized parasite of insects,including flies.

Pilobolus crystallinus is anexample of a dung- and soil-inhabiting zygomyceteutilizing mainly simple carboncompounds



The human diseased condition involving zygomycetesis known as zygomycosis. Usually zygomycetesinvade only tissues of immunocompromizedindividuals. Alternatively, zygomycetes may beinvolved as secondary infections associated with burnsor trauma.Some of the Entomophthorales (the insect-feedingzygomycetes) may infect physiologically andimmunologically normal humans (Basidiobolusranarum is an example of such).

B. ranarum can be a causalagent of human infections.Asexual sporangia.

Absidia corymbifera is amongthose fungi which mayfacultatively colonize humanhosts.

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Finally, zygomycetes are probably the mostcommon mycorrhizal partner. It has beenestimated that 95% of plants belong to familieswhich have been recorded to be mycorrhizal.Most of these plants form so-called arbuscularmycorrhizae.

Mycorrhizal structures,arbuscules and vesicles in acolonized onion (Allium) root.

Gigaspora gigantea is anexample of a mycorrhizalzygomycete. These spores donot form through anastomosisof two mycelia; they ratherappear to be formed terminallyon a hyphae as an azygospore

Phylum Zygomycota - the conjugatingfungi - life cycle

Rhizopus stolonifer - anexample of a heterothalliczygomycete. p. 39 in Carlile.

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Phylum Zygomycota - the conjugatingfungi - life cycle Let’s look at some fancy

microscopy: Phycomyces sp.

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Fungus of the day - Taphrinadeformans

Taxonomy: Phylum (subphylum) Ascomycota Order - Taphrinales Family - Taphrinaceae

Today’s fungus is a plant pathogen causing theleaf curl of peaches. There are several speciesof Taphrinaceae which are pathogens of plants.Taphrinaceae are characterized by intercellularand subcuticular colonization of their host.Sometimes they grow within the cell walls andcan penetrate deeply into the tissues.

Taphrina deformans is thecausal agent of peach leafcurl among other things

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Fungus of the day - Taphrinadeformans

Taphrinaceae is a peculiar order since thegerminating ascospores bud to comprise thesaprobic yeast stage on the host tissues.

The yeast stage initiates a mycelial phasewhich infects and colonizes the soft, youngtissues in the spring.

Taphrina deformans is thecausal agent of peach leafcurl among other things

A look ahead...

• What are ascomycetes?

• We will look at the most primitive groupsof ascomycetes.

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The true fungi

Chytridiomycota

Basidiomycota

Ascomycota

Zygomycota

Animalia

?

?

Phylum Ascomycota -the sac fungi

In ascomycetes, sexual reproductionoccurs by means of ascosporeswhich are borne in the ascus - thesac. The asci can be contained indifferent kinds of structures -collectively known as ascomata orascocarps. Some ascomycetes donot have the ascomata but nakedasci directly on the substrate surface(prototunicate ascomycetes).

Ascus withascospores

Different ascomatalstructures

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Phylum Ascomycota -the sac fungi

Apothecium of Aleuriaaurentia

Naked asci of Taphrinadeformans

Perithecium ofDaldinia sp.

Cleistothecium ofErysiphe sp.

Phylum Ascomycota - thesac fungi

Lower fungi are aseptate, often lacking orhaving only a rudimentary pore structure.

Higher fungi are septate and have clear haploid,diploid and dikaryotic cells. RecallDikaryomycotina for higher fungi.

Primitive ascomycetes have a simple septumwith perforations (micropores)

Filamentous ascomycetes (more sophisticatedones) have a simple septum with Woroninbodies.

In contrast to ascomycetes, basidiomyceteshave more complex septum (dolipore septum).

Septal structures of higher fungi.

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Chytridiomycota

Basidiomycota

Ascomycota

Zygomycota

Animalia

?

?

Phylum Ascomycota -Archiascomycetes

• We can separate three majorgroups: Archiascomycetes,Saccharomycetales and thefilamentous ascomycetes.

• The two former groups oftenhave at least a part of their lifeas yeast, the buddingunicellular stage, but may bedimorphic. They have asci, butusually no ascocarps.

Phylum Ascomycota - Archiascomycetes

Defining characteristics forthe primitive ascomycetes:

1) absence of forcibledischarge

2) absence of Woroninbodies

3) structures involved inascocarp and its production(ascogenous hyphae)Archiascomycetes (andSaccharomycetales)

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• What are the Archiascomycetes?

• Primarily Archiascomycetes havebeen united by rDNA data.

• Morphologically and biochemicallythey are a very variable group.

• Main uniting factor may be theabsence of the ascogeneous hyphaeand the ascocarps.

Taphrina spp. are dimorphicand have a saprobic yeast stageand a parasitic mycelial stageon plant hosts

Schizosaccharomyces spp. are

soil inhabiting fission yeasts



• Ascospore release from the naked asci varies:most do not forcibly discharge spores, somemay. This may vary depending on theenvironmental conditions.

• No differentiated ascus tip structure, compareto operculate and Jack-in-the box asci. Ascibasically blow up.

• Bottom line is that Archiascomycetes aresimple and unified by unicellular yeast phase.

Taphrina spp. - no differentiated tip.

Operculate and Jack-in-the-box asci

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• Let’s look at Taphrina spp. as examples.

• Dimorphic with yeast-like saprobicphase on the host plant and mycelialparasitic phase which penetrated theintercellular spaces of the host plant.

• The intercellular penetration caused thetypical curling of the peach leaves. Orthe witch’s broom on cherries

Taphrina spp. are dimorphicand have a saprobic yeast stageand a parasitic mycelial stageon plant hosts

Taphrina deformans the peachleaf curl; Taphrina cerasi thewitch’s broom of cherries.


• Taphrina spp.

• The saprobic yeast phase can bemaintained in pure culture, but themycelium will not develop. As a resultthe ploidy of the mycelial phase is notknown. It may be dikaryotic ormultinucleate.

Taphrina spp. mycelial stagedevelops only on plant hosts

Taphrina deformans the peachleaf curl; Taphrina cerasi thewitch’s broom of cherries.

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• Taphrina spp.

• The ascospores germinate producingnew unicellular yeast-like cells. Theseeventually initiate the mycelial stage.Anastomosis at yeast-myceliumchange? Is the mycelium diploid,dikaryotic, or multinucleate?

• Cells that eventually become ascienlarge and in the cuticular regions ofthe host. Although called theascogeneous cell, these cells do notfunction like the more advancedascomycetes (no crozier).


• Taphrina spp.

• Mitotically formed diploid stalk cellprotoplasm disintegrates and becomesempty. Stalk attaches the naked ascusin the host tissue.

• The ascospores may start alreadygerminating within ascus.

• Although there may be nodifferentiated ascus tip structure, theremay be forcible discharge as a resultof pressure build up in the ascus andsimple bursting to the ascus cell.

• We are back to the yeast like saprobicphase!

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• Additional Archiascomycetes

• Pneumocystis carinii - An extracellularmammal parasite. Causes pneumonia bybinding to the epithelial cells of the in thelung alveoli. May disperse to other organsand colonize the bone marrow. Serioustrouble only to immunodeficient humans.

• P. carinii was originally classified as aprotozoan. Phylogenetic analyses aided infinding more effective treatments once thetrue nature of the beast was revealed.

Lung tissue - holes showthe cysts

The cysts aremulticellular colonies inthe alveolar epithelium

Phylum Ascomycota -Saccharomycetales

• Saccharomycetales have becomeknown as the unicellular yeasts.

• Yeast really means a unicellularfungus which reproduces bybudding.

• In other words, yeast is a termreferring to a growth form nottaxon.

Unicellular yeasts-like organisms,Rhodosporium toruloides(Basidiomycota) and Saccharomycescerevisiae (Ascomycota)

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• Remember that we had yeasts-likebudding phases in the Archiascomycetes.There are also a few basidiomycetousyeasts which include for exampleFilobasidium and Rhodosporidium.

Filobasidiella (Cryptococcus)neoformans (Basidiomycota)basidiospores germinate into yeastcells.

Saccharomyces cerevisiae (Ascomycota)


• The point: yeast is nothing but a growth formand we need to think in terms ofSaccharomycetales.

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• What characterizes Saccharomycetales?

• Like the Archiascomycetes,Saccharomycetales lack ascogenoushyphae and true ascocarps.

• Similarly, asci are usually thin-walledand deliquesce or break, there is nospecialized structure for the ascosporerelease. Prototunicate asci.


• What characterizes Saccharomycetales?

• Plasmogamy occurs by fusion of two cellswhich may function as gametangia.

• Saccharomycetales have no realdikaryophase; karyogamy followsplasmogamy almost immediately.

• The zygote, either a result ofparthenogenesis or fusion of two cells maybe transformed into an ascus directly ormay persist as diploid cell line.

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• Saccharomyces cerevisaepointers for the life cycle

• Asexual reproduction bybudding in both haplo- anddiplophase.

• Heterothallism exists even intrue yeasts.

• The zygote develops into anascus after several diploidgenerations.


• Why are Saccharomycetales important

• Bread.

• Ethanol.

• Single Cell Protein.


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Summary…

• Archiascomycetes and Saccharomycetalesare the primitive asomycetes with no trueforcible discharge, no specialized ascus tipand primitive septa.

• Both these have a unicellular, budding yeastphase in their lives.

Documents

Biology of FUNgi - courses.k-state.edu · Phylum Ascomycota - the sac fungi Apothecium of Aleuria aurentia Naked asci of Taphrina deformans Perithecium of Daldinia sp. Cleistothecium