Pufferfish and ancestral genomes

The fugu is a famous fish, at least as a Japanese sushi dish containing a potentially lethal neurotoxin that was featured on an episode of The Simpsons. Fugu is a member of the pufferfish group, which have another claim to fame: an extremely small genome, roughly a tenth the size of that of other vertebrates. The genome of several species of pufferfish is being sequenced, and the latest issue of Nature announces the completion of a draft sequence for the green spotted pufferfish, Tetraodon nigroviridis, a small freshwater species.

Tetraodon has about the same number of genes as we do, 20,000-25,000, but they are contained in a total genome length of 340Mb vs. our huge 3.1Gb. One major difference is that in Tetraodon, transposable elements are rare: they have 73 types, present in less than 4000 copies, but humans have about 20 different types present in millions of copies. Transposable elements may be reverse transcriptases that blindly copy RNA sequences back into the DNA (called LINES) or shorter sequences that are processed by LINES, called SINES. These really are parasitic bits of selfish DNA, and somehow, pufferfish seem to be largely free of them.

One of the interesting things one can do with a pair of genome sequences is to start mapping synteny. Synteny represents the preservation of small regions of order within a chromosome; while the overall organization may have been scrambled by millions of years of chromosome breaks and fusions and duplications and deletions, we can still identify smaller blocks that maintain the same series of genes within them. For example, if we look on a chromosome of one organism and we see the series of genes A-B-C-D-E-F, and we look in another organism and find a chromosome with the genes W-X-C-D-E-Y-Z, we can see that the C-D-E chunk can be mapped directly to one region of that second organism's chromosome.

Continue reading "Pufferfish and ancestral genomes" (on Pharyngula)