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The miniature genome of Oikopleura dioica
By Birgit Reinert


Only five millimeters long, the marine animal Oikopleura dioica has the smallest genome ever found in a chordate, or animal with a spine, according to a new study. Researchers from Norway, Germany and France have been collaborating to sequence the genome. They report that O. dioica has a tiny and very compact genome, with genes tightly packed together. The small genome size may be related to the organism's very short life cycle.

Oikopleura in its highly transparent house.

Daniel Chourrout, of the Sars Centre for Marine Molecular Biology in Bergen, Norway and colleagues report that progress has been made in sequencing and annotating some larger genomic fragments. The O. dioica genome is believed to be up to 70 million base pairs long, which is about forty times smaller than the human genome. The researchers estimate that O. dioica has about 15,000 genes.

"It is still premature to establish a link between the small genome and the short generation time of Oikopleura, even though a correlation between longevity and genome size has been found in birds," the researchers write in a short paper in Science. The life of Oikopleura spans two to four days depending on the sea temperature.

The organism is also an attractive candidate to study the emergence of vertebrates. In terms of evolution, O. dioica lies between vertebrates and invertebrates. It has a spine and a dorsal nervous system, but it lacks bones and teeth and its brain is very small.

Oikopleura, which is composed of two Greek words—oikos meaning house and pleura meaning side or rib—can be found in all oceans. They build a house many times the size of their body to filter prey from the water. Their long thin tail is used to create a current within the house drawing in water over the filters. The filters can become clogged relatively quickly. When this happens, the organism abandons the house and builds a new one. Discarded houses slowly descend like rain into deeper waters—known as marine snow.

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Seo, H.-C. et al. Miniature genome in the marine chordate Oikopleura dioica. Science 294, 2506 (December 21, 2001).

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