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