Eukaryotes Genomes - PARAMECIUM TETRAURELIA

Paramecium tetraurelia isan organism which is both unicellular and complex, it is therefore an excellent model for the genetic study of the numerous differentiated functions in multicellular organisms which are absent in simpler eukaryotes such as yeast.

It is a very large eukaryotic cell (120 micrometers) covered with vibrating cilia. It belongs to the Ciliate phylum (Ciliophora). Ciliates form a group related to the unicellular parasites named apicomplexans, which include Plasmodium falciparum, the main causative agent of malaria. Ciliates have the fascinating property of having separate germinal and somatic lineages within a single cytoplasmic unit. These cells possess two nuclei. A germinal nucleus (the micronucleus) is responsible for the transmission of genetic information via sexual processes, whereas a somatic nucleus (the macronucleus) ensures expression of this information. At each sexual generation, a new somatic nucleus is produced by programmed rearrangements of the whole genome contained in the germinal nucleus.

The nuclei of Paramecium, the micronucleus and the macronucleus, differ in both structure and function. The diploid micronucleus, which is present in two copies in the P. tetraurelia species, represents the germ line and is completely silent in terms of transcription. This is the nucleus which undergoes meiosis and fertilization during sexual events (conjugation between competent cells or autogamy in a single cell).

The macronucleus, which is highly polyploid (about 1000n), represents the somatic line and is the site of transcription. Both the macronucleus and the micronucleus are derived from copies of the zygotic nucleus. The programmed development of the macronucleus includes DNA amplification by a factor of about 250, precise elimination of short internal sequences called IES and imprecise elimination of regions which are rich in transposons and repeated sequences and are probably heterochromatic. These events causes fragmentation of chromosomes. The extremities created in this way are repaired by the addition of telomeres.

Paramecium uses a sophisticated excitable membrane to control all aspects of swimming behaviour. This is made possible by a direct coupling between membrane potential and the activity of the many thousands of locomotory cilia that cover its surface.They are able to detect bacteria, their food source and swim faster with fewer turns in the presence of bacterial metabolites. Paramecium have receptors for these chemicals which include acetate, folate and glutamate. Binding of these chemoattractants cause changes in intracellular second messengers and it is believed these second messengers activate the plasma membrane calcium ATPase (PMCA), a calcium pump that removes calcium from the cell sustaining a hyperpolarization. This protein is very similar to human PMCAs.

The size of the Paramecium genome is estimated to be 100 Mb, 85- 90% of the genome is represented in the macronuclear DNA.

References:

http://www.genoscope.cns.fr/externe/English/Projets/Projet_FN/organisme_FN.html
http://paramecium.cgm.cnrs-gif.fr/