The Microbe Blog (at http://www.smallthingsconsidered.us)

Elio Schaechter and Merry Youle, fellow writer and editor

http://schaechter.asmblog.org/schaechter/2009/04/the-scandalous-bdelloidrotifers.html

Sexually reproducing organisms that abandon sexual reproduction are typically doomed to early extinction. Reports of "ancient asexuals"-eukaryotes that have lived without sex for millions of years-have often not held up to close scrutiny. But then there are the bdelloid rotifers, an "evolutionary scandal." The status of these tiny freshwater invertebrates as ancient asexuals has been confirmed repeatedly. And that's but one of their remarkable quirks.

Rotifers are small, typically less than 0.5 mm. Bdelloid rotifers were first described in detail in 1702 by Antonie van Leeuwenhoek-but no one realized they were metazoans until 1838. Taxonomically, the bdelloids have an entire class to themselves, with four families and some 460 described species. All bdelloids are female; no males or hermaphrodites have ever been observed. They lack meiosis. Eggs are produced from primary oocytes by mitosis. Their sister class, the monogonont rotifers, have retained the capacity for sexual reproduction, but they don't bother to produce males except when needed for the production of overwintering "resting eggs."

Bdelloids dwell in the most ephemeral of freshwater habitats. Not just in small puddles, but in the transient layer of moisture sometimes found on moss or lichens-even on mushrooms. Dessication is a routine occurrence which they handle with their own particular style of anhydrobiosis. Uniquely, they can withstand dessication at any stage in their life cycle. In the lab, they've been shown to revive after 9 years of "life without water." Few are the competitors, predators, or parasites that can keep up with them.

Likewise, bdelloid rotifers are highly resistant to ionizing radiation, far more so than any other animal studied. A comparison: in the most resistant arthropod known, a 200-Gy dose produces more than 99% sterility; in bdelloids, a 560-Gy dose causing approximately 500 double-strand breaks per genome reduces fecundity by only 20%. The researchers point out that resistance to ionizing radiation may be an accidental by-product of resistance to desiccation. Both factors cause double-strand DNA breakage. Although other factors are involved in resistance to desiccation and radiation, the capacity to repair double-strand DNA breaks is essential.

The bdelloid genome contains some mobile elements (e.g., DNA transposons) that are typically acquired by horizontal gene transfer (HGT). These elements are found primarily in the telomeric regions of the chromosomes, and only rarely in gene-rich regions. Most unexpectedly, in the telomeric regions one also finds dozens of nonmetazoan genes acquired by HGT. There are bacterial genes, fungal genes, and plant genes-some with no metazoan orthologs. Some are degraded, many are intact, some are intact and transcribed. Most of the intact genes encode proteins with simple enzymatic functions that are not part of multicomponent pathways. So far, it has not been possible to determine donor species or time of transfer.

Given that few instances of HGT have been confirmed in metazoans and that most of those involve long-term associations with organelles, endosymbionts, or intracellular parasites, it is stunning to find foreign genes by the dozen in these rotifers. The authors speculate: "It may be that HGT is facilitated by membrane disruption and DNA fragmentation and repair associated with the repeated desiccation and recovery experienced in typical bdelloid habitats, allowing DNA in ingested or other environmental material to enter bdelloid genomes."

This fortuitous combination of ephemeral habitat and accurate DNA repair mechanisms has likely contributed to the evolutionary success of the bdelloids. They didn't play the evolutionary game by the rules, but still they won. Scandalous.

Gladyshev, E. A., M. Meselson, and I. R. Arkhipova. 2009. Massive horizontal gene transfer in bdelloid rotifers. Science 320:1210-1213.

Talmudic Question of the Month*
by Qetzal

Why are essentially all complex multicellular organisms eukaryotes?

Answers? Add a comment online to Talmudic Question #36, July 31, 2008.

http://schaechter.asmblog.org/schaechter/2008/07/talmudic-ques-1.html