• Protists V

Protists V
Heterotrophic Protists:
The Slime Mold Groups
 Slime Molds:
Where do they belong?
• Slime molds have traditionally been
studied by mycologists, and, until
recently, considered members of “the
fungi”
• Today, most are considered “protists”
but relationships are uncertain
 “Slime Molds”
• Three groups will be considered in this
class and considered as Divisions:
– Dictyosteliomycota (cellular slime molds)
– Myxomycota (plasmodial slime molds)
– Plasmodiophoromycota (endoparasitic slime
molds”
– Other “fungal-like” protist groups not discussed in this class:
Acrasiomycota, Labyrinthulomycota, Hyphochytridiomycota
 Division Dictyosteliomycota
• Characteristics
– no flagellated cells
– haploid myxamoebae
aggregate to form
pseudoplasmodia
– spore walls contain
cellulose
– no true plasmodium
formed
 Dictyosteliomycota (cont.)
– about 50 species known
– inhabitants of soil, dung, litter (especially
temperature forests)
– phagotrophic on bacteria cells
– sometime called “social amoebae”
– group is used extensively in developmental
and molecular biology
– Most commonly studied genus is
Dictyostelium
Asexual Cycle of Cellular Slime Molds
 Reproduction in Dictyostelium
• Spores are released and germinate
releasing an amoeba
• These amoebae feed on bacterial cells
and divide by mitosis
• Individual amoebae may encyst forming
microcysts (under unfavorable
conditions)
Reproduction in Dictyostelium (cont.)
• When the population reaches a minimum
size, the amoebae begin to aggregate
• Cells move to an aggregation center
where cells are secreting the hormone
acrasin (actually cyclic AMP)
• The amoebae adhere to each other
forming a pseudoplasmodium (slug or
grex)
Reproduction in Dictyostelium (cont.)
• This pseudoplasmodium the “migrates”
over the substrate
• Cells of the pseudoplasmodium become
differentiated into prestalk (anterior) and
prespore cells (posterior)
• The pseudoplasmodium then ceases
movement and begins culmination and
the formation of the sorocarp
Migration
 Reproduction in Dictyostelium (cont.)
• Prestalk cells begin
forming cellulose and
elongate
• Prespore cells migrate to
the top of the sorocarp
and are transformed into
spores
• This asexual cycle is well
demonstrated in the video
The Amoebae of Dictyostelium
 Sexual Reproduction
• Two amoebae function as gametes and
fuse and the zygote which enlarges
• Large numbers of somatic amoebae are
attracted to the zygote, and a wall is
secreted around the zygote and the
somatic amoebae. This is called a
macrocyst
 Sexual Reproduction (cont.)
• The zygote feeds on the somatic
amoeboid cells and eventually undergoes
meiosis and many mitotic divisions
• Eventually a number of uninucleate
amoebae escape from the ruptured cyst
wall and renew the growth of somatic
amoebae
 Division Myxomycota
• Characteristics
– Three types of uninucleate cells (one of
which is flagellated)
– A multinucleate somatic phase called a
plasmodium
– A resistant stage called a sclerotium
– Reproductive phase culminating in the
production of sporophores
 Division Myxomycota (cont.)
– about 500 species
known
– common on soil and
bark of various woody
plants
– frequent at the edge of
melting snowbanks and
even in city lawns
Just for Fun (Fuligo septica plasmodium)
“In the Dallas suburb of Garland, an
Unidentified Growing Object (‘big as a
platter, foamy and creamy and pale
yellow’) terrorized Mrs. Marie
Harris’s backyard for three weeks
until it died of sunstroke and nicotine
poisoning.”
Newsweek, June 11, 1973
Life Cycle of Myxomycetes
meiosis encystment
sclerotium
 Sexual Reproduction
• Haploid spores are released from
sporangia
• Spores germinate releasing myxamoebae
• Depending on conditions, myxamoebae
may develop flagella and become swarm
cells
• These cells feed on bacteria and divide by
mitosis
 Sexual Reproduction (cont.)
• Either myxamoebae or swarm cells fuse
to form zygotes
• Zygote nucleus divides and the
protoplasm mass continues to grow,
sometimes incorporating other dividing
zygotes to form the plasmodium
• The multinucleate plasmodium continues
to feed and enlarge
 Types of Plasmodia
• Phaneroplasmodium
– conspicuous, streaming, granular, slime-
sheath
• Aphanoplasmodium
– non-granular, transparent, no veins with a
slime sheath
• Protoplasmodium
– about 1 mm, granular, slime sheath but no
veins, produces only 1 sporophore
The Plasmodium
Physarum
Plasmodium on Pine Needles
 Sexual Reproduction (cont.)
• Eventually the plasmodium forms various types
of spore-producing sporophores (sporangia,
aethalia, plasmodiocarps)
• In the sporangium, meiosis produces haploid
spores (often surrounded by thin threads of the
capillitium and central stalk called the
columella)
• Spores are released upon the breakdown of the
sporangium wall (peridium)
Types of Sporophores
• Sporangia
– Example: Stemonitis
 Sporophores: the capillitium
• May or may not be encrusted with
CaCO3
 Types of Sporophores (cont.)
• Plasmodiocarp
– Example: Hemitrichia
 Types of Sporophores (cont.)
• Aethalia
– Examples: Lycogala, Fuligo
 Type of Sporophores
• Naked, External Spores on the Plasmodium
– Example: Ceratiomyxa
 Division: Plasmodiophoromycota
• Characteristics
– Known as “endoparasitic slime molds”
– Economically important plant parasites
– Cause abnormal enlargement of host cells
(hypertrophy) and disruption of vascular tissue
– Some species parasitize algae or other fungi
– Motile cells are biflagellate, heterokont
– Specialized “cruciform” cell division
Cruciform Cell Division
Plasmodiophora brassicae
Spongospora subterranea
 Spongospora subterranea
Healthy Infected
Potatoes Potatoes
 Generalized Life Cycle
• See diagram handout
– resting spores (A)
– germination to biflagellated primary
zoospores (B) with whiplash flagella
– primary zoospores encyst and penetrate host
cells (C) often through root hairs
– cells divide (cruciform cell division) to form
a primary or sporangial plasmodium (D,E)
Resting Spores
Pelomyxa
resting spores
 Generalized Life Cycle (cont.)
– Sporangial plasmodium forms zoosporangia
organized into a “sorus” (F)
– Zoosporangia release secondary zoospores
(G,H) [These do not necessarily exit the host plant.]
– Secondary zoospores penetrate host cortical
cells (I)
– Development of secondary or sporogenic
plasmodium (J-N)
– Nuclei undergo meiosis (L-N)
– Formation of mature sporosorus composed of
resting spores (O)
Secondary Zoospores
 General Life Cycle (cont.)
• Invasion of secondary zoospores and
development of secondary plasmodia
induces hyperplasia and hypertrophy (typical
infection symptoms)
• Resting spores may remain in the soil for 5-
8 years
• Exact timing of nuclear fusion is still
uncertain as are many aspects of this life
cycle
 WWW Links
• WWW Dictyostelium Server
– http://dicty.cmb.nwu.edu/Dicty/dicty.html
• Fungi in General
– http://www.mykoweb.com/
• Myxomycetes
– http://www.wvonline.com/myxo/
– http://www.ucmp.berkeley.edu/protista/slimemolds.html
– http://bic.usuf1.usuhs.mil/Mark/PhysarumPlus.html
 WWW Links (cont.)
• Plasmodiophoromycetes
– http://www.plantbio.ohiou.edu/pbc/plasmos/home.htm#intro
– http://www.pa.ipw.agrl.ethz.ch/spongospora/