Fungi are composed of five major groups!
Chytridiomycetes (pic from Kendrick)!
Zygomycetes (pic from Kendrick)!
Ascomycota!
Basidiomycota!
Lecture 2"
The fungal cell" (I)!
Glomeromycota (pic from Kendrick)!
Fungal cells have various growth forms!
(a) hyphal growth!
= formation of a mycelium.!
Most Zygo-, Asco-, and!
Basidiomycota!
!
(a)
(b)
!
The macromorphological !
aspect of the mycelium!
in both Zygo and Asco is!
commonly called “mold”!
Yeast growth!
(b) budding yeast!
(c) fission yeast!
- occurs in in the Zygo-,!
Asco-, and Basidiomycota!
(d)chytridiaceous growth, !
with rhizoids.!
Chytridiomycetes!
!
From Deacon
Hyphal growth!
(a) spore germination!
(b), (c), (d) apical growth and
branching!
(e) Anastomoses resulting!
in a typical interconnected
mycelial network (Asco and
Basidio)!
From Deacon!
Mushroom Fairy Rings results from apical mycelial growth and translocation of resources!
toward hyphal tips. Older hyphae die. The different
basidiocarps (fructifications) of a fairy ring can therefore
all be genetically identical and if so, should be viewed as
disconnected parts of the same individual. !
http://www.angliangardener.co.uk/green_up_your_fingers/!
pest_diseases/fairy_rings.htm!
Symptoms on a golf course!
http://flrec.ifas.ufl.edu/Hort/Plant_pathology/Fairy_rings/fairyfigure1.htm!
Hyphal structure/ultrastructure and hyphal growth!
A typical Ascomycota cell/mycelium is shown below!
!
From Deacon
- A mycelium has an apical growth => terminal cells are younger; older cells generally die by autolysis!
- Apical growth ==> the life span of the mycelium of a single individual is potentially infinite - but fungal!
senescence exists!
Hyphal structure/ultrastucture and hyphal growth!
!
From Deacon
This figure depicts a typical !
Ascomycota mycelium!
- The septum (S) corresponds to “a hole” that connects cells and permits cytoplasmic movements. Typically:!
- Septa with a single perforation and cells with (generally) a single haploid nucleus correspond to an Ascomycota;!
- Septa with a single perforation and cells with (generally) two haploid nuclei correspond to a Basidiomycota; !
- A mycelium with no or very few septa which can be multiperforate or not, and multinucleate (coenocytic) cells
correspond to a Zygomycota !
- The Woronin body (Wo) is typical of Ascomycota; its function is still poorly known (septal plug (SP)?).!
Hyphal structure/ultrastucture and hyphal growth!
A typical Basidimoycota mycelium differs from both Ascomycota and Zygomycetes mycelia from:!
- The presence of a dolipore at the hyphal septum(rather than a Woronin body, which is typical of Ascomycota)!
!
From Deacon
P = membraneous parenthosome!
G = narrow central pore flanges of
predominantly Glucan!
- Hyphal cells typically contains two haploid nuclei (= dikaryon); in some rare cases (e.g., in Armillaria) the two !
nuclei fuse; then, the cell nucleus correspond to that of a typical diploid Eukaryote !
- A clamp connection may form between two cells (not always!), as a result of cell division and hyphal growth!
Clamp connection formation in Basidiomycota !
a. Terminal cell of hypha. Growth only takes place at
hyphal tips; !
b. Hyphal tip elongating. !
c. Synchronous division of nuclei (mitosis) and the
beginning of hyphal branch that will for the clamp.
One nucleus migrates into the new clamp. !
d. Septum forms at base of the clamp trapping nucleus
b. Nuclei a' and b' migrate to the hyphal tip, while
nucleus a migrates away from the tip. !
e. Septum forms below clamp forming new cell at
hyphal tip. Fusion of the clamp to the adjacent cell
releases nucleus b to the adjacent cell. Now both the
terminal and subterminal cells are binucleate, each
with a compatible pair of haploid nuclei (dikaryotic
cell).!
From Wong!
http://www.botany.hawaii.edu/faculty/wong/Bot201/Basidiomycota/Clamp_connection_formation.htm!
There are many ways to differentiate between Basidiomycota, Ascomycota, and
Zygomycetes based on hyphal characteristics !
Light microscope:!
- clamp connection observed: definitively a Basidiomycota.!
- long hyphae rarely septate, not densely branched, lack of hyphal anastomoses: probably a
Zygomycetes.!
-! 2 nuclei per cell: likely a Basidiomycota!
-! 1nucleus per cell: likely an Ascomycota!
- multiple nuclei per cell: likely a Zygomycetes (some Ascomycota also have multiple nuclei/cell)!
Electron microscope:!
- Woronin body seen: definitively an Ascomycota!
- single septum between cells, presence of a dolipore: definitively a Basidiomycota!
Etc.!
Cell wall composition!
Major polysaccharides components of cell walls in Fungi and Oomycota!
(from Deacon)!
Note that the presence of Chitin is unique to fungi and some insects.!
Oomycota have been traditionally considered to be fungi, but molecular phylogenies do not
indicate a close relationships between Oomycota and the ‘True Fungi’. This view is also
supported by both the absence of chitin in their cell wall, and the presence of cellulose.!
Hyphal growth!
Anastomoses (= fusion between cells) !
- are not infrequent in Asco and Basidio and result in the formation of a mycelial network;!
- rare / do not occur in Zygomycetes!
- biological consequences? .!
!
From Deacon!
Example from Rhizoctonia solanii - a pathogen of potatoes
Somatogamy (vegetative compatibility) !
!= cell fusion of mycelia via anastomoses !
!==> protoplasmic continuity: redistribution of resources, faster translocation of nutrients !
Self-fusion: fusion between hyphal segments that originated from the same spore (genetic identity)!
Non-self fusion: fusion between hyphal segments that originated from different spores!
•! Behavior of nuclei (nucDNA):!
!- mixing and migration, resulting in a genetically heterogeneous individual mycelium;!
!- invasion/replacement of one genetic system by another, resulting in the maintenance of an!
! homogenous individual mycelium. !
•! Behavior of mitochondria (mtDNA) poorly known.!
Non-self somatic compatibility (fusion) is a trade off between two mycelia being cooperative but may result in
the "extinction" of one genotype; selection for fitness? !
Self-or non-self fusion <==> complete genetic compatibility = a genet!
!Different genets of the same species will not form anastomoses!
!==> Does a genet corresponds to a fungus individual ?!
One of the major challenges continually facing a mycelium is encountering other mycelia of the same or
similar species. !
Mechanisms that prevent anastomosis (somatic incompatibility) !
A) pre- fusion rejection = mycelial incompatibility !
Demarcation zones between colonies of the same species; often observed in nature in crustose lichens and
and wood rotters.!
http://
www.plantmanagementnetwork.
org/
http://www.visualsunlimited.com/!
B) post- fusion rejection: cytoplasmic incompatibility; results in the death of the fused compartments and
associated hyphae;!
Vegetative mycelial interaction. Recognition events between hyphae at three
physiological stages : pre-contact, pre fusion, and post-fusion (from Glass et al., 2000)!
Life cycle of a budding yeast!
From Deacon!
Yeast is a growth form !
During their growth, some Fungi can switch between yeast and mycelium form
(dimorphic Fungi):!
- ph!
- Temperature!
- Other environmental factors (more later)!
Yeasts are polyphyletic!
This contrasts with old taxonomic concepts which were based solely on
observations of gross morphology (no ultrastructure, no experimentation; no
evolutionary concepts)!
Control of the yeast-mycelium dimorphism switch!
From Deacon!
External factors:!
- cell wall receptors/transporters !
- pH!
- temperature!
- osmotic pressure!
- CO2 level!
Intracellular factors:!
- Ca binding proteins!
- cyclic adenosine monophosphate (cAMP) !
--> modification of signal transduction pathway!
--> alteration of metabolism or gene expression !
(can be detected by change in cellular mRNAs)!
Yeast cells: modes of division !
Saccharomyces!
blastic - acropetal!
Candida!
blastic - sympodial!
Cryptococcus!
BLASTIC!
Schizosaccharomyces!
blastic - percurrent!
Saccaromycodes!
Rhodotorula!
Cryptococcus!
blastic - phialidic!
Sporobolomyces!
THALLIC!
thallic - arthric!
!
From Kendrick
Geotrichum!
!
From Deacon