Phylum Oomycota

Phylum Oomycota
4032604 Lecture 20
Stramenopilous Fungi
Tinsel flagellum
Tubular mitochondrial cristae
Cellulose in cell walls
Lysine synthesis by diaminiopimelic
acid pathway (DAP)
Phylum Oomycota
1 class (Oomycetes), 8 orders, 92 genera and 800 species
Orders to cover:
Leptomitales
• Filamentous thallus with cellulin plugs
Rhipidiales
• Inflated, branched thallus with cellulin plugs
Saprolegniales
• Holocarpic or eucarpic/filamentous
Lagenidiales
• Holocarpic
Pythiales
• Filamentous thallus
Peronosporales
• Filamentous thallus
Characters
Predominantly diploid life cycle with
gametangial meiosis
Oogamous sexual reproduction:
Gametangia = oogonia and antheridia
Thick-walled, resistant oospore formed
inside oogonium from oosphere
Asexual reproduction by heterokont
zoospores with tinsel + whiplash
flagella
Characters
Cell walls composed primarily of betaglucans with small amounts of cellulose
Filamentous species generally produce
coenocytic hyphae
Principle storage compounds are watersoluble mycolaminarins (beta-glucans)
Haustoria formed by plant parasitic
species
Zoospores
Primary zoospores
First formed
Pip-shaped
Flagella emerging from anterior
Poor swimmers, encyst readily
Secondary zoospores
Reniform or bean-shaped
Flagella emerging from lateral groove
Zoospores
Dimorphic
Primary and secondary zoospores present in
life cycle
• Primary zoospores formed in zoosporangium
• Secondary zoospores formed upon germination
of primary zoospores
Monomorphic
Secondary-type of zoospores formed inside
zoosporangium
Sexual Reproduction
Mostly heterogametic (morphologically
differentiated gametangia):
Hyphal-like antheridia
• Attracted to oogonium by hormones and develop
fertilization tubes
Large, globose oogonia
• One or more oospores in oogonium
Simultaneous meiotic divisions occur in
antheridia and oogonia prior to fertilization
Karyogamy occurs in oosphere to form
oospore that matures inside oogonium
Oospore from APSnet
Oogonia & Oospores
Oospore wall consists of 3 parts:
Exospore
Epispore
Endospore
Periplasm = residual protoplasm
Plerotic = periplasm present
Aplerotic = lacking periplasm
Oogonium & Antheridium
Hypogynous
Antheridium formed in oogonial stalk
Paragynous
Antheridium attaches laterally to oogonium
• Monoclinous—branch off oogonial stalk
• Diclinous—branch off separate hypha
Amphigynous
Oogonium grows through antheridium
Hypogynous
Oosphere
Fertilization
tube
Paragynous/
monoclinous
Oogonium
antheridium
Paragynous/
diclinous
Amphigynous
Order Leptomitales
Mostly aquatic, saprotrophic
Filamentous thallus with regular
constrictions plugged by chitinous
material called cellulin granules
Asexual reproduction by elongated,
hyphal-like zoosporangia
Some species form primary and
secondary zoospores
Apodachlya
http://plantbio.berkeley.edu/~taylor/pmb110/labpages/8leptopythi/index.htm
A, B. Leptomitus
C-K. Apodachlya
From Sparrow, 1960.
Aquatic Phycomycetes.
Order Rhipidiales
Strictly aquatic, saprotrophic, commonly in stagnant
and polluted water
Thallus:
Inflated and branched
Attached to substrate by rhizoids
Oogonia, antheridia and zoosporangia formed terminally
Branches and reproductive organs constricted at base and
plugged with cellulin granules
Only secondary zoospores formed
Strong tendency towards fermentative metabolism
Zoosporangia
C. Sapromyces
D. Araiospora
E. Rhipidium
F. Mindeniella
G.Aqualinderella
Thalli
From Emerson and Weston, American Journal of Botany 54 (1967)
Order Lagenidiales
Mostly aquatic and parasitic on fungi,
algae and marine crustacean larvae
Endobiotic, mostly monocentric
Examples:
Olpidiopsis—obligate endoparasite of
Saprolegniales
Lagenidium—endoparasites of algae and
marine crustacean larvae
Lagenidium
1-4. Infection stages in
Spirogyra
5-10. Asexual
reproduction
11-17. Sexual
reproduction
Karling, 1942. Simple Holocarpic Biflagellate Phycomycetes.
Order Saprolegniales
Saprotrophs (fresh water and soil), or parasites
(plants and animals)
Aphanomyces--sugar beets, peas, radishes, aquarium plants
Achlya, Aphanomyces and Saprolegnia—fish, crustaceans
“Water mold” applies to this group
Primary and secondary zoospores formed
Filamentous thallus, abundantly branched
Stout, coenocytic hyphae able to increase in diameter as they
age
Gemmae = modified segments of hyphae with dense
cytoplasm
Saprolegniales—sexual
reproduction
Oogonia with multiple oospores
Oospores with prominent, granular, fluid
ooplast (formed by coalescence of dense
vesicles)
Extent of lipid coalescence used as taxonomic
character
Centric—small lipid drops uniformly surrounding
ooplast
Eccentric—ooplast to one side of oospore, not
completely surrounded by lipid globules
Sexual Reproduction
Most members are self-fertile
Sexual reproduction in both self-fertile
and self-sterile forms involves
pheromones
Diffusible substances produced by both
partners
First demonstrated by John Raper (1940s)
Role of pheromones
Female produces and secretes antheridiol
(hormone A; sterol pheromone) which
induces formation of antheridial branches in
male strain
Male strain is stimulated to secrete oogonial
(hormone B) which causes formation of
oogonial initials in female
Antheridiol causes chemotropic growth of
mail structures toward female
Asexual Reproduction
Primary zoospore stage formed in most
species, but may be reduced:
Achlya and Aphanomyces
• Uninucleate protoplasts emerge from zoosporangium and
encyst at opening or immediately after release
Dictyuchus
• Cysts formed inside zoosporangium
Saprolegnia
• Motile, primary zoospores released from zoosporangium
Saprolegnia
Proliferation of
zoosporangium
Secondary
zoospore
Primary
zoospores
Achlya
Uninucleate
protoplasts encyst
in ball at mouth of
zoosporangium
Lateral proliferation
of zoosporangium
Secondary
zoospores emerge
from cysts
From Webster, 1980
Dictyuchus
Primary cysts inside
zoosporangium germinate
to release secondary
zoospores
From Webster 1980
Phylum Oomycota
Pythiales Peronosporales
Classification
Order Pythiales
Families
• Pythiaceae (9 genera, 230 species)
• Pythiogetonaceae (2 genera, 8 species)
Order Peronosporales
Families
• Peronosporaceae (8 genera, 600 species)
• Albuginaceae (1 genus, 45 species)
Advanced characters
Mostly parasitic with intracellular
haustoria
Narrow diameter hyphae
Trend towards terrestrial habitats
Loss of primary zoospore stage
Trend towards loss of secondary zoospore
stage
Inability to synthesize sterols
Sexual reproduction
One oospore/oogonium
Oospores formed in host tissue
Unifactorial mating system
A1 and A2 mating types
Selfing may occur
Bisexual strains occur
Mating controlled by alpha hormones
Asexual reproduction
Zoosporangia variable in shape and size
Sporangiophore type delimits families:
Pythiaceae—indeterminate
sporangiophores
Peronosporaceae—determinate
sporangiophores
Albuginaceae—club-shaped
sporangiophores, catenate sporangia
http://helios.bto.ed.ac.uk/bto/microbes/zoospore.htm#Germination%20of%20sporangia
Zoospore
ultrastructure
http://helios.bto.ed.ac.uk/bto/microbes/zoospore.htm
Zoospore taxis
Zoospore settles and
orients on host surface
Adhesion and
encystment with release
of adhesins and cell wall
formation
Germination with a
fixed orientation from
predetermined point
adhesins
Modified from: http://helios.bto.ed.ac.uk/bto/microbes/zoospore.htm
Left: zoospores of Phytophthora palmivora accumulating at the mouth of a capillary filled
with a fluorescently tagged amino acid (dansyl-asparagine) [supplied by MJ Carlile, from
JN Cameron & MJ Carlile, unpubl.]. Right: zoospores of Pythium aphanidermatum
accumulated and encysted at the mouth of a capillary containing malt-extract agar.
http://helios.bto.ed.ac.uk/bto/microbes/zoospore.htm
Pythium Pringsheim 1858
Type species P. monospermum
127 species
First invaders of plant tissue, not good competitors
with other organisms
Major components of damping-off disease of
seedlings, collar rot of apples, foliar and stem blights,
snow rot of winter wheat
Sporangia of various shapes, sizes
Zoospores cleaved out and released from vesicle
Oogonia fertilized by one to many antheridia
Oogonial wall smooth or with spiny or blunt
projections
Chlamydospores present in some species
Pythium
Zoosporangia (above)
Oogonium/oospore (right)
Photos by D. J. S. Barr
vesicles
zoosporangia
From Webster, 1980.
Zoospore cleavage
and release in
Pythium
From Webster, 1980.
Phytophthora de Bary 1892
Type species P. infestans (Montagne) de
Bary
65 species
Wide range of diseases
Sporangia ovoid, obpyriform to lemonshaped
Sporangia papillate, semipapillate or
nonpapillate
Zoospores cleaved out inside
zoosporangium
Diseases caused by
Phytophthora species
P. infestans—late blight of potato,
tomato
P. cinnamomi—root rot of avocado,
eucalyptus and > 1000 hosts
P. cactorum—fruit rots (>154 host
genera)
P. ramorum—Sudden Oak Death
Identification—Waterhouse’s
groups
I.
II.
III.
IV.
V.
Papillate, narrow exit pore, not
proliferating, deciduous, homothallic,
paragynous
Zoosporangia as in I; heterothallic,
amphigynous
Semipapillate, narrow exit pore, not
proliferating; deciduous or not,
homothallic, mostly paragynous
Zoosporangia as in III, usually deciduous;
heterothallic, amphigynous
Nonpapillate, broad exit pore, not
deciduous, homothallic, paragynous
Sporangia,
sporangiophores
and zoospores in
Phytophthora
From Webster, 1980.
From Webster, 1980.
Family Peronosporaceae
Obligate biotrophs of vascular plants
Called “downy mildews” due to
appearance of sporangiophores on
surface of infected leaves
Sporangiophores aerial, determinate in
growth, up to 750 microns tall
Sporangia deciduous, released by drying
Sporangia germinate directly or
indirectly
Sexual reproduction
Formed inside host tissue
Oogonia larger, antheridia broader
relative to Pythiaceae
Antheridia paragynous
Periplasm forms thick, sculptured wall
on oospore
Basidiophora
Short, stout
sporangiophores
No branching
Sterigmata at apex
Sporangial
germination indirect
Sclerospora
Bulbous
sporangiophores
with short terminal
branches
Most species with
indirect germination
Restricted to
monocots
Forms digitate
haustoria
Plasmopara
Random branching
of sporangiophores,
branches at ~ 90
degrees
Sporangiophores
stiff and erect, with
short terminal
branches
Sporangial
germination indirect
Peronospora
Elongated, curved,
dichotomously
branched
sporangiophores
Sporangia germinate
directly
From Webster, 1980
Peronospora
Sporangiophores and
direct germination of
sporangia
From BioImages
Pseudoperonospora
Sporangiophores similar to Peronospora
except branching more delicate and not
dichotomous
Sporangia germinate indirectly
Bremia
Dichotomously
branched
sporangiophores
with enlarged tips
bearing ring of
sterimata
Sporangia germinate
indirectly
From Webster, 1980
Family Albuginaceae
One genus, Albugo
Called ‘white rusts’ due to white blisters
formed on stems, leaves and fruits of
host
Sporangia formed in basipetal
succession, dispersed by wind
Germinate indirectly to form 8
zoospores
Oogonia develop in host, oospores
develop thin vesicle in which zoospores
are cleaves and released
Albugo
haustoria
Release of zoospores
Club-shaped
sporangiophores and
chains of sporangia
From Webster, 1980
Albugo oospores
From Webster, 1980
White rust