How bdelloids have been able to gobble up such a variety of genes from their environment and incorporate it into their genome is a good question. Typically in animals, the germ linethe heritable egg and sperm cellsare protected from environmental assaults, such as intrusion of foreign DNA, by the rest of the body cells, which are not heritable and serve to "sequester" the germ line. Ideas on why the bdelloids' germ line is so exposed to environmental exchange, Arkhipova says, "are all speculative. But we talk about this a lot!"
One clue is the unusual ability of bdelloids to survive total desiccation (drying out), which is fatal for most organisms. When water disappears from their environment, bdelloids enter a kind of suspended, dehydrated state, and can stay there for months or even years. But once water returns, they spring back to action, move around, eat things, and start reproducing again. During the desiccation phase, Arkhipova says, "you would imagine there is potential for membrane damage and DNA damage in the rotifer. And not only the rotifer desiccates, but also everything it just consumed." If the DNA of both the rotifer and its food are broken up during desiccation, "this would provide an opportunity for the (foreign) DNA to enter the rotifer's germ line. During rehydration, the DNA breakage is somehow repaired, and the foreign DNA may get incorporated," she says.
This idea was inspired by recent work by Gladyshev and Meselson (2008) that showed bdelloids are exceptionally good at recovering from ionizing radiation, which shatters their DNA. The rotifers' talent for repairing DNA breaks may have evolved due to their desiccation-prone lifestyle. "The effects of radiation and desiccation may be quite similar and involve damage to chromosomal DNA as well as membranes," Arkhip
|Contact: Diana Kenney|
Marine Biological Laboratory