Endosymbiosis and Cytoplasmic Inheritance in Paramecium
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Transcript of Endosymbiosis and Cytoplasmic Inheritance in Paramecium
Endosymbiosis and Cytoplasmic Inheritance in Paramecium
Kevin SpringUniversity of Houston
Population Biology SeminarFebruary 22, 2007
Paramecium aurelia
This presentation will focus on the following:
Altenburg paper (1948)–Plasmagene hypothesis–Kappa body symbiosis
Current understandingof Kappa bodies (Preer1974)
Other cytoplasmic inheritance in Paramecium (Meyer 2002)
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 self-discipline; 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
Division of Kappa and Paramecium is independent of each other
38o C kills Kappa but not Paramecium
There is an upper limit of the # of Kappa in Paramecium
More likely a symbiont than a parasite
Paramecium with symbiont (2)
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 microscopy clearly showed that Kappa is prokaryotic
Electron micrograph of symbionts (2)
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
Many other types of symbionts found
mu
alpha
gamma
pi
lambdasigma
delta
omegaKappa is the most common
The discovery of bacterial symbionts within Paramecium allows for their taxonomic classification
Alpha bodies are in the genera Cytophaga
Kappa, mu, gamma, and nu are in genera Caedobacter
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
When genes from one organism are within another organism and are transcribed, a inactive protein may form
R body is a type of inclusion body
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
Toxicity associated only with Brights
Nonbrights produce other nonbrights, but occasionally a nonbright turns into a bright
Dividing symbiont (2)
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:
Mating type
Serotypes
Genome-wide DNA rearrangements
Paramecium has a complex cellular biology
Eukaryotic
Ciliates contain at least 2 nucleiGerm-line micronucleus (MIC)Somatic macronucleus (MAC)
MAC is generated from the MIC
Extensive genome rearrangements occur in the MAC
Diagram of Paramecium (3)
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
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
Result is 4 haploid MIC, but 2 are degraded
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
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
Conjugation of P. caudatum by Yanagi
Paramecium mating types do not follow the Mendelian segregation of alleles
A. Mendelian segregation of allelic pairsB. 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
E cell
Insert E MAC
O cell
Produces
E cell
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
MIC G gene
Functional - type O
Nonfunctional - type E
MAC G gene
excision
Mutationalretention
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 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
Summary
Paramecium has many instances of cytoplasmic and maternal inheritance
Kappa bodies are bacterial symbionts that produce a killing factor and they are inherited through the cytoplasm
IES excision and retention in the MAC is maternally inherited by the genome present in the MAC
Electron micrograph of Kappa (2)
Paramecium (6)
References1.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