Ciliates have as many genes as humans
The ciliated animal Tetrahymena thermophila, a relative of the paramecium, has over 27,000 genetic instructions for building proteins in its genome. The 105 million base pairs are distributed over 200 chromosomes, but the exact number is still unclear. The unicellular freshwater dweller is one of the model organisms in cell biology and is also used as a pollutant indicator.
An international research team led by Jonathan Eisen from the Institute for Genomic Research in Rockville has sequenced the genome of the macronucleus of T. thermophila. The ciliate has two nuclei in each cell: a micronucleus with five chromosomes, which carry the genes for reproduction and which is only activated for this purpose, and the macronucleus, which contains all other necessary information. It arises from amplified fragments of micronucleus chromosomes that form during reproduction.
When analyzing the gene sequences, the researchers found hardly any junk DNA sections that did not code for proteins and are therefore often referred to as "gene junk". While in T. thermophila just two percent of the genome harbors such sections that are apparently empty of content, it is over fifty percent in humans. These repetitive sequences are apparently excised during macronucleus formation, along with other genetic elements derived from foreign organisms.
The scientists also discovered great diversity for some protein families. For example, the ciliated animal has over 300 genes for voltage-gated ion channels that control membrane transport - a key function for the free-living, unicellular organism. The building instructions were created through gene duplication, in which copies of existing genes develop new functions and thus enable adaptation to changing environmental conditions, for example.