In press at PNAS is the report of the sequence of the mitochondrial genome sequence of the very unusual metazoan Trichoplax adhaerens. Trichoplax is odd because it is an extremely simple version of a multicellular organism. It has no real organs, no axis of symmetry, a very small nuclear genome and only four distinct cell types.
This has led to the hypothesis that it is a basal metazoan. That is the first lineage to diverge from all the other metazoans. There could of course be other explanations for the apparent simplicity of Trichoplax such as it is a result of a loss of various features found in its ancestors. The mitochondrial genome sequence seems to confirm that Trichoplax is indeed a basal metazoan.
The mitochondrial genome is the largest yet discovered in metazoans at 24-kb, although this is smaller than the largest mitochondrial genome yet discovered in eukaryotes. That distinction goes to Reclinomonas
Trichoplax adhaerens is a fascinating organism and with a nuclear genome on the way it seems likely to reveal more interesting discoveries.
Refs:Stephen L. Dellaporta, Anthony Xu, Sven Sagasser, Wolfgang Jakob, Maria A. Moreno, Leo W. Buss, and Bernd Schierwater.
Mitochondrial genome of Trichoplax adhaerens supports Placozoa as the basal lower metazoan phylum
PNAS published May 26, 2006, 10.1073/pnas.0602076103
Trichoplax: my favorite animal (PDF)
The Trichoplax story (PDF)
PDFs from Bernd Schierwater lab