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Researchers Sequence Macronuclear Genome of Model Eukaryote T. thermophila

NEW YORK (GenomeWeb News) – In an effort that is expected to provide insight into eukaryotic evolution, a team of more than 50 researchers has sequenced and analyzed the genome of the single-celled ciliate Tetrahymena thermophila.
The researchers, led by Jonathan Eisen of the University of California, Davis, published the results of the study last week in PLoS Biology.
T. thermophila contains two nuclei: a micronucleus that contains five chromosomes, and a macronucleus, which has more than 200 chromosomes. The micronucleus contains the DNA necessary for reproduction while the macronucleus controls the cell's other functions.
The researchers focused on the macronuclear genome, using shotgun sequencing to conclude that it has around 105 million base pairs and includes between 185 and 287 chromosomes. These chromosomes lack centromeres and have only a very small amount of repetitive DNA.
The macronuclear genome encodes more than 27,000 protein-coding genes.
The genome sequence enabled the authors to address an evolutionary question related to the timing of plastid acquisition in the alveolates, a group of three related phyla that includes ciliates such as Tetrahymena as well as the apicomplexans and the dinoflagellates. Plastids, such as the chloroplast, are descended from formerly free-living cyanobacteria, so many genes from such endosymbionts are typically found in the nucleus of host organisms. While this is the case in apicomplexans and dinoflagellates, the authors found that T. thermophila has no plastids and no remnants of plastid genes, which suggests that plastid acquisition occurred after the other two groups split off from the ciliates.
The authors next plan to sequence the micronuclear genome of T. thermophila.

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