Saturday, January 09, 2010

Water fleas have new introns

The study of intron gain and loss can be frustrating, because such events are very rare. Documented cases of intron gain have been particularly elusive. That makes the recent study by Li et al. especially exciting ("Extensive, Recent Intron Gains in Daphnia populations," in Science 2009, from Michael Lynch's group at Indiana). They have found that intron gain is remarkably common in Daphnia, and that the new introns lack features expected from most hypothesized mechanisms of intron gain. The independent gain of introns in parallel at the same site in different lineages is also observed, and also unexpected. These authors hypothesize that intron gain may arise fortuitously as a consequence of DNA damage, but this remains to be established. Whatever the mechanism, the observation that new introns can arise at reasonable rates in at least one species provides both an important clue to the origins of introns and a system for further investigation.

A consideration of the allele-frequency spectrum suggests that these new introns in Daphnia (also known as the water flea) are indeed deleterious, bringing to mind a famous poem by Jonathan Swift ("On Poetry: A Rhapsody", pub. 1733):
"So nat'ralists observe, a flea
Hath smaller fleas that on him prey,
And these have smaller fleas that bite 'em,
And so proceed ad infinitum."