Genome of marine organism tells of humans' unicellular...(hoanoflagellates are not - a situation K...)

The cells are egg-shaped with a single long tail or flagellum at one end surrounded at its base by a collar of tentacles - choano comes from the Greek word for collar - that capture bacteria. The flagellum propels the choanoflagellate through the water and also washes bacteria towards the tentacles. Because choanoflagellates resemble the feeding cells of sponges, which are among the most primitive of animals, biologists 165 years ago proposed that these organisms were very distant ancestors of multicelled animals.

King and Rokhsar successfully proposed the choanoflagellate for sequencing several years ago as part of the Department of Energy's Microbial Genome Program, and in the intervening years, King worked on isolating enough uncontaminated DNA for sequencing. The draft genome, completed and annotated in 2007, consists of about 9,200 genes. It is similar in size to the genomes of fungi and diatoms, but much smaller than the genomes of metazoans. Humans, for example, have about 25,000 genes.

Interestingly, the choanoflagellate has nearly as many introns - non-coding regions once referred to as "junk" DNA - in its genes as humans do in their genes, and often in the same spots. Introns have to be snipped out before a gene can be used as a blueprint for a protein and have been associated mostly with higher organisms.

The choanoflagellate genome, like the genomes of many seemingly simple organisms sequenced in recent years, shows a surprising degree of complexity, King said. Many genes involved in the central nervous system of higher organisms, for example, have been found in simple organisms that lack a centralized nervous system.

Likewise, choanoflagellates have five immunoglobulin domains, though they have no immune system; collagen, integrin and cadherin domains, though they have no skeleton or matrix binding cells together;
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