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Biology of FUNgi

Lecture 5Towards the true fungi -

Chytridiomycetes and Zygomycetes

A look back

• What were the cellular slime molds? Whatunited them? Why were they important?

• What were true slime molds? What unitedthem? Why were they important?

• What were the water molds? Why were theyimportant?

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Phylum Oomycota –oomycetes Life of an oomycete - diploid

zoospores, gametangia...

Antheridium

Oogonium

Plasmogamy

Karyogamy

Meiosis occurs inantheridia and oogonia

Oospore

Zoosporangiophore

Fungus of the day -Neocallimastix sp.

Taxonomy: Phylum (subphylum) Chytridiomycota Order - Neocallimasticales Family - Neocallimasticeae Common names: -

Ruminant endosymbiont. Collected from ruminantpoop, from the stomach fluids or saliva.

Neocallimastix atypical chytrid: anaerobic andpolyflagellate. Survives in air by resistant cysts(resting spores).

The very existence of the rumen chytrids wasunknown until 1980’s - thought to be protozoa.

Typical chytridiomycete zoospore -uniflagellate. Neocallimastix -polyflagellate

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Fungus of the day -Neocallimastix sp.

Lower fungi - coenocytic thalli (bundle of hyphae,like mycelium)

Reproduction by zoospores which are released fromzoosporangia.

Zoospore = flagellated motile cell.

Neocallimastix - polyflagellatezoospore

Neocallimastix - coenocytic (aseptate)thallus

Neocallimastix - zoosporangiumreleasing the polyflagellate zoospores

A look ahead.

• What are the true fungi?

• What are the lower fungi?

• How did the true fungi evolve?

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Non-fungi vs. true fungi

True fungi - crown eukaryotes, more related toanimals than to plants.

Non-fungi - protistan. Several differentdivergence events.

Plunge into the true fungi

Chytridiomycota

Basidiomycota

Ascomycota

Zygomycota

Animalia

?

?

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Pertinent evolutionarycharacters in the true fungi

Ancestral traitsChitin GlycogenPosterior flagellum

Loss of phagotrophy

Loss of flagella

Dikaryotic stageSeptate mycelium

Ascospores

BasidiosporesDollipore septumClamp connections

Chytridiomycota

Basidiomycota

Ascomycota

Zygomycota

Animalia

?

?

From the base to the top of the tree -Phylum Chytridiomycota

What are the chytrids?

The only members in the kingdom Fungiwith motile cells in some stage in the lifehistory.

Mainly uniflagellate - rememberNeocallimastix though - with a singleposteriorily-oriented whiplash flagellum

Zoospore of Allomyces sp.

Zoosporangia ofAllomyces arbuscula

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From the base to the top of the tree -Phylum Chytridiomycota

What are the chytrids?

Thallus is coenocytic. May be mycelial orjust an elongated single hyphae. Septa areabsent or infrequent. Present regularly at thebase of the reproductive organ - like inSaprolegnia.

Many endobiotic chytrids may haveextremely reduced thallus, i.e., none at all.

Sporangium of Spizellomycessp. Note discharge papillae.

Sporangia and dischargingzoospores of Rhizophydiumsp.

From the base to the top of the tree -Phylum Chytridiomycota

What are the chytrids?

The zygote may encyst - form a restingspore - or forms resting sporangium, orpossibly both. This also explains howNeocallimastix survives in poop.

Cell wall materials are mainly chitin.

Sporangium and hyphae ofSpizellomyces sp.

Sporangia anddischarging zoospores ofRhizophydium sp.

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From the base to the top of the tree -Phylum Chytridiomycota

Where do the chytrids occur and what dothey do?

As is evident, the zoospore dispersalrequires water.

Quite a few aquatic chytrids, but onlylimited number in marine environments.

Despite the requirement for water,common also in terrestrial environment.

Sporangium and hyphae ofSpizellomyces sp. on pinepollen

From the base to the top of the tree -Phylum Chytridiomycota

Most chytrids are saprobic - examplesinclude cellulose and keratin decomposingspeciesSome chytrids are plant pathogens:Synchytrium endobioticum – the black wilt ofpotatoes

Synchytrium endobioticumcauses the black wart diseaseof taters

Synchytrium endobioticum inpotato

Synchytrium endobioticumgerminating prosorus - sorusinitiator

Sporangium and hyphae ofSpizellomyces sp. on pinepollen - saprobic?

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From the base to the top of the tree -Phylum Chytridiomycota

Neocallimastix - anaerobic and exists inthe rumen of the herbivores, potentiallyanother example of mutualistic symbiosis.

They can be hard to detect because of thesmall size and the fact that chytrids mayescape usual isolation techniques used forthe bacteria and fungi

Neocallimastix - polyflagellatezoospore

Neocallimastix - coenocytic(aseptate) thallus

Phylum Chytridiomycota - Allomycesas an examle for a life cycle

Allomyces sp.This is very similar toprimitive plantreproduction - alterationof generations -haplothallus/diplothallus

Biflagellate zygote

Fertilization

Heterogamousgametes

Male and femalegametangia insame thallus.

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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

What unites zygomycetes is the production of athick-walled resting spore - the zygospore.

The zygospore forms as a result of fusion oftwo gametangia which will remain as thesuspensor cells once zygospore forms.

Zygomycetes are generally coenocytic. Whensepta are present they usually lack pores withthe specialized plugs like in asco- andbasidiomycetes.

Asexual reproduction usually occurs viasporangia-borne mitotic spores.

A zygospore suspendedbetween the suspensor cells -Syzygites sp. - a parasite ofmushrooms.

An asexual sporangium ofAbsidia corymbifera.

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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.

Phylum Zygomycota - the conjugatingfungi

Where and how to look for the zygomycetes

As a group the Zygomycetes can be isolatedfrom a variety of substrates including soil,dung, fruit, grain, plant tissues, and animaltissues.

Nutritionally they range from saprotrophismthrough weak parasitism of plants to veryspecialized parasitism and predation ofanimals.

Some have been called “sugar fungi” becausethey lack a good arsenal of enzymes whichwould allow degradation of complexcarbohydrates.

Entomophthora is aspecialized parasite of insects,including flies.

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

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Phylum Zygomycota - the conjugatingfungi

Where and how to look for the zygomycetes

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.

Phylum Zygomycota - the conjugatingfungi

Where and how to look for the zygomycetes

Finally, zygomycetes are probably the mostcommon mycorrhizal partner (Glomomyces?).It has been estimated that 95% of plants belongto families which have been recorded to bemycorrhizal. Most of these plants form so-called arbuscular mycorrhizae.

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

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Phylum Zygomycota - the conjugatingfungi - life cycle

Rhizopus stolonifer - anexample of a heterothalliczygomycete.

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

microscopy: Phycomyces sp.

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Summary

• Lower fungi - chytridiomycetes andzygomycets

• Chytridiomycetes - the only flagellated truefungi.

• Zygomycetes - conjugation fungi with theconjugating spore and mitotic sporangia.