Endosymbiosis and Cytoplasmic Inheritance in Paramecium

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Endosymbiosis and Cytoplasmic Inheritance in
Paramecium
Paramecium aurelia

• Kevin Spring
• University of Houston
• Population Biology Seminar
• February 22, 2007

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This presentation will focus on the following

• Altenburg paper (1948)
• Plasmagene hypothesis
• Kappa body symbiosis

Current understanding of Kappa bodies
(Preer 1974)
Other cytoplasmic inheritance in Paramecium
(Meyer 2002)

• Paramecium biology
• Cell biology
• Life cycle

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Altenburg paper (1948) investigates the evidence
that Kappa bodies are a symbiont
Kappa bodies are elements within Paramecium that
cause them to be killers
Killer Paramecium kill other Paramecium in the
immediate environment
Kappa particles, thought to be plasmagenes by
Sonneborn, but Altenburg suggest they may be
symbionts
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The plasmagene theory suggested kappa bodies were
genes within the cytoplasm
Plasmagenes defined as self-replicating structure
capable of producing traits that exist in the
cytoplasm and are independent of chromosomal
genes.
The trait that Kappa bodies produce is the
killing factor
Kappa bodies are inherited through the cytoplasm
and not through chromosomes
Sonneborn wrote in 1976, It was awful of me to
be so attached to a pet idea. That was an ordeal
between my mind and my heart and it took a while
for the mind to win and the heart to accept.
Impersonal scientific objectivity is a goal to be
sought by hard self-discipline we are not born
with it.
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Altenburgs evidence that Kappa bodies are
symbionts is strongly supported by evidence
Preer (1948) showed Kappa is large enough to see
under a light microscope
38o C kills Kappa but not Paramecium
Division of Kappa and Paramecium is independent
of each other
Paramecium with symbiont (2)
There is an upper limit of the of Kappa in
Paramecium
More likely a symbiont than a parasite
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Preer (1974) reviewed the overwhelming evidence
that Kappa bodies are symbionts
Kappa contains DNA, RNA, protein, and lipids in
proportions expected in bacteria
Kappa contains electron transport system with
cytochromes similar to bacteria and not eukaryotes
Electron micrograph of symbionts (2)
Electron microscopy clearly showed that Kappa is
prokaryotic
Electron micrograph of flagellated Kappa (2)
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Current information has shown why Kappa induces
killing and the different types of bacteria
symbiosis
Kappa bodies kill other Paramecium by releasing
toxins into the environment
The presence of the symbiont makes the host
resistant to the toxin
Kappa bodies are transmitted by the cytoplasm
during asexual division
gamma
sigma
alpha
lambda
Many other types of symbionts found
delta
pi
Kappa is the most common
omega
mu
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The discovery of bacterial symbionts within
Paramecium allows for their taxonomic
classification
Kappa, mu, gamma, and nu are in genera Caedobacter
Alpha bodies are in the genera Cytophaga
Lambda and sigma are in genera Lyticum
Delta bodies are in genera Tectobacter
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Differences have been found between Kappa bodies
in the same host
Some Kappa bodies contain refractile ( R ) bodies
R body is a type of inclusion body
When genes from one organism are within another
organism and are transcribed, a inactive protein
may form
Magnified image of coiled R body (2)
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Kappa bodies may contain R bodies and it affects
their reproductive capability
Nonbright Kappa bodies do not contain R bodies
but can reproduce
Bright Kappa bodies do contain R bodies but
cannot reproduce
Dividing symbiont (2)
Nonbrights produce other nonbrights, but
occasionally a nonbright turns into a bright
Toxicity associated only with Brights
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There is still unsolved questions regarding Kappa
body symbiosis
What benefit does Paramecium get from the
symbiosis?
How does the presence of a Kappa body induce
resistance to the toxin?
Resistance can be overcome with large toxin dose
The presence of Kappa with or without R bodies
induces resistance to the toxin
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Other types of cytoplasmic inheritance discovered
in Paramecium and other ciliates is
Genome-wide DNA rearrangements
Mating type
Serotypes
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Paramecium has a complex cellular biology

• Eukaryotic
• Ciliates contain at least 2 nuclei
• Germ-line micronucleus (MIC)
• Somatic macronucleus (MAC)
• MAC is generated from the MIC
• Extensive genome rearrangements occur in the MAC

Diagram of Paramecium (3)
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The two nuclei make the life cycle of Paramecium
more complicated than other eukaryotes
MIC goes through meiosis and the haploid MIC goes
through mitosis
Result is 4 haploid MIC, but 2 are degraded
Paramecium exchange 1 haploid MIC
MIC fuse and form diploid MIC and duplicate via
mitosis
Old MAC degrades and duplicated MIC is processed
into new MAC
In asexual reproduction, the MIC goes through
mitosis and the MAC goes through amitosis
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Genome-wide rearrangements of the MAC genome
consists of deletion of DNA sequences and
chromosome amplification
The developing new MAC loses 10 - 95 of the
genome depending on the ciliate
MAC chromosomes are amplified to a high ploidy
level
Deletion occurs after an initial amplification of
the MIC genome but before the ploidy level is
reached
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The deletion of DNA is located at specific
sequences called internal excised sequences (IES)
IES are located in coding and noncoding regions
of the MIC genome
These sequences are not present in the MAC genome
At some point in MAC development, the IES
sequences are deleted
How is IES deletion maternally inherited?
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The mating type of Paramecium shows maternal
inheritance
Conjugation of P. caudatum by Yanagi
Paramecium has 2 mating types - O and E
Both are not determined by genetic differences as
they are both produced in homozygous wild-type
strains
Mating type is the same through asexual
reproduction but can change after sexual
conjugation and MAC formation
After conjugation O cells mostly produce other O
cells and E cells produce other E cells
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Paramecium mating types do not follow the
Mendelian segregation of alleles

1. Mendelian segregation of allelic pairs
2. Maternal inheritance of mating types (4)

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Mating types O and E depends on different states
of MAC genome
Transferring E maternal MAC into O cell causes
the progeny to become E
Transferring O MAC does not change E cells
O is the default mating type
O cell
E cell
E cell
Produces
Insert E MAC
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This differential state of MAC is dependent on
the presence of IES in the MAC
The mutation mTFE causes O cells to become E
This mutation affects the excision of an IES on
the G gene
The G gene is a surface antigen and the failure
of excision causes a nonfunctional protein to be
translated
Functional - type O
excision
Mutational retention
Nonfunctional - type E
MIC G gene
MAC G gene
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Microinjection studies have shown that the
presence of an IES sequence in the MAC inhibits
the excision of its homologous IES in the MIC
O cells contain G gene in the MAC without its IES
(IES-)
E cells contain the G gene in the MAC with its
IES (IES)
Injecting a plasmid of IES G gene into O cells
MAC created the retention of the IES in the MAC
of daughter cells
Injection of IES- plasmid did not induce excision
The presence of IES in the MAC causes the
retention of the IES in subsequent generations
after sexual conjugation
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Microinjection of IES plasmid retains the IES in
the MAC genome after autogamy
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Meyer (2002) asked, How can a sequence
introduced in one nucleus affect the excision of
the homologous sequence in another nucleus?
Two models developed
Model 1 Sequence-specific protein factors are
required for the excision of the IES in the
developing MAC
The problem with this model is the large number
of protein factors needed, about 50,000
Model 2 Sequence specificity is achieved by
homologous nucleic acid (most likely RNA) that is
transported from the maternal MAC to the
developing MAC
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Mochizuki (2004) explained the Scanning Model, a
synthesis of Meyers model 1 and 2
Entire MIC genome is transcribed bi-directionally
and forms dsRNA
dsRNA is cut up into smaller RNA called scnRNA
scnRNA move to the old MAC and any matching
homologous sequences are degraded
scnRNA that were not degraded move to the
developing MAC
These scnRNAs target homologous sequences which
are deleted in an RNAi-like mechanism
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Summary
Paramecium has many instances of cytoplasmic and
maternal inheritance
Paramecium (6)
Kappa bodies are bacterial symbionts that produce
a killing factor and they are inherited through
the cytoplasm
Electron micrograph of Kappa (2)
IES excision and retention in the MAC is
maternally inherited by the genome present in the
MAC
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References
1.Altenburg E (1948) The role of symbionts and
autocatalysts in the genetics of the ciliate.
The American Naturalist, 82 252-264. 2.Preer
JR, Preer LB and Jurand A (1974) Kappa and other
endosymbionts in Paramecium aurelia.
Bacteriological Reviews, 38 113-163. 3.Spark
Notes. Protist. http//www.sparknotes.com/biolog
y/microorganisms/protista/section2.rhtml. 4.Meyer
E and Garnier O (2002) Non-Mendelian inheritance
and homology-dependent effects in ciliates.
Advances in Genetics, 46 305-337. 5.Mochizuki K
and Gorovsky MA (2004) Small RNAs in genome
rearrangements in Tetrahymena. Current Opinions
in Genetics and Development, 14 181-187. 6.Ken
Todars Microbial World. Introduction to the
Microbial World. http//www.bact.wisc.edu/themicro
bialworld/paramecium.jpg. 7. 7. Preer JR (2006)
Perspectives anecdotal, historical and critical
commentaries on genetics. Genetics, 172 1373-1377