Transcript Endosymbiosis and Cytoplasmic Inheritance of symbionts and

Endosymbiosis and Cytoplasmic
Inheritance in Paramecium
Paramecium aurelia
Kevin Spring
University of Houston
Population Biology Seminar
February 22, 2007
This presentation will focus on the following:
Altenburg paper (1948)
–Plasmagene hypothesis
–Kappa body symbiosis
Other cytoplasmic inheritance
in Paramecium (Meyer 2002)
Current understanding
of Kappa bodies (Preer
1974)
Paramecium biology
–Cell biology
–Life cycle
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
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 selfdiscipline; we are not born with it.”
Altenburg’s 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
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)
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
sigma
gamma
lambda
Many other types of symbionts found
delta
pi
Kappa is the most common
alpha
omega
mu
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
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)
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
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
Other types of cytoplasmic inheritance discovered in
Paramecium and other ciliates is:
Genome-wide DNA rearrangements
Mating type
Serotypes
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
Diagram of Paramecium (3)
Extensive genome rearrangements occur in the MAC
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
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
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?
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
Paramecium mating types do not follow the
Mendelian segregation of alleles
A.
B.
Mendelian segregation of allelic pairs
Maternal inheritance of mating types (4)
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
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
MIC G gene
Mutational
retention
Nonfunctional - type E
MAC G gene
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 cell’s 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
Microinjection of IES+ plasmid retains the IES in
the MAC genome after autogamy
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
Mochizuki (2004) explained the Scanning Model, a
synthesis of Meyer’s model 1 and 2
Entire MIC genome is transcribed bidirectionally 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
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
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/biology/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 Todar’s Microbial World. Introduction to the Microbial World.
http://www.bact.wisc.edu/themicrobialworld/paramecium.jpg.
7. 7. Preer JR (2006) Perspectives: anecdotal, historical and critical commentaries on genetics.
Genetics, 172: 1373-1377