Another genome is sequenced and the hypothesis of front-loading evolution continues to grow stronger. This time it is the genome of Trichoplax adhaerens, the sole member of the placozoan phylum. The genome analysis “supports placing the placozoans after the sponge lineage branched from other animals.”

Although I have yet read the paper, sample these tasty morsels:

Trichoplax is a two-millimeter flat disk containing fluid sandwiched between two cell layers. It lacks organs and only has four or five cell types. Yet, despite its apparent simplicity, its genome encodes a panoply of signaling genes and transcription factors usually associated with more complex animals.

Trichoplax has no neurons, but has many genes that are associated with neural function in more complex animals. “It lacks a nervous system, but it still is able to respond to environmental stimuli. “It has genes, such as ion channels and receptors, that we associate with neuronal functions, but no neurons have ever been reported,” explained Rokhsar.

Of the 11,514 genes identified in the six chromosomes of Trichoplax, 80 percent are shared with cnidarians and bilaterians. Trichoplax also shares over 80 percent of its introns—the regions within genes that are not translated into proteins—with humans. Even the arrangement of genes is conserved between the Trichoplax and human genomes. This stands in contrast to other model systems such as fruit flies and soil nematodes that have experienced a paring down of non-coding regions and a loss of the ancestral genome organizations.

Apart from the growing evidence that preadaptation played a huge role in the evolution of neurons (yet another topic I need to discuss), I’m starting to notice a common theme – compared to the genomes of flies and worms, the human genome is looking much more like the ancient metazoan genome.

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