11 April 2013


The ethereal comb jellies, or ctenophores, have taken a far-out trip down Lineage Street and ended up somewhere nothing else has ever been.
The ctenophore Mnemiopsis leidyi. Photo by EricksonSmith at Flickr.
Researchers have for the first time decoded the mitochondrial genome of a comb jelly——specifically the wide-ranging (often invasive) species Mnemiopsis leidyiAnd it's the weirdest genome imaginable. From the paper in Mitochondrial DNA:
At just over 10 kb, the mt-genome of M. leidyi is the smallest animal mtDNA ever reported and is among the most derived.   

Mitochondrial DNA is passed down only from mothers and doesn't suffer mutations well. Those that appear are pruned out, leaving the mitochondrial tree shorter (derived). As lead author Walker Pett told me:
Ctenophores have taken this process to the extreme. Mnemiopsis has the smallest, fastest evolving, most highly modified mitochondrial genome of any animal. It has lost half of its genes, and the remaining genes are so different from those in ctenophores' closest relatives that some of them are almost unrecognizable.

The pattern isn't restricted to mitochondrial DNA, either. Pett continues:

Ctenophore DNA appears to be extremely fast evolving in general, which makes it difficult to place ctenophores on a phylogenetic tree. Surprisingly, it is still an open question whether the earliest animals were sponges or ctenophores, in part because the DNA of ctenophores has mutated so much that it is difficult to determine which animals are their closest relatives.
Other owners of odd mitochondrial DNA: Upper left, clockwise: box jelly (Tamoya ohboya) | Ned DeLoach via tessarazoa at Flickr; tunicates (Clavelina moluccensis) | Nhobgood at Wikimedia Commons; scyphozoan (Chrysaora colorata)  | Sanjay Acharya at Wikimedia Commons; chaetognath (Chatognath spadella) |  Zatelmar at Wikimedia Commons.
Pett says that other planktonic (often gelatinous) animals, like box jellies, Scyphozoans, tunicates, and chaetognaths also have some of the strangest modifications to their mitochondrial DNA. He wonders if they might share common boom-and-bust cycles in their population biology.

However the ctenophores got their weirdness, you can see from the video below it also begat extreme beauty.

Iridescent Ctenophores from Parafilms on Vimeo.

The paper:

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