Because there is no source of such intense ionizing radiation on Earth, except if we make it, there is no way these organisms could have evolved to be radiation resistant, says Mark Welch. Instead, they propose that bdelloids DNA repair capacity evolved due to a different environmental adaptation tolerance of extreme dryness.
Bdelloids, which live in ephemeral aquatic habitats such as temporary freshwater pools and on mosses, are able to survive complete desiccation (drying out) at any stage of their life cycle. They just curl up and go dormant for weeks, months, or years, and when water becomes available, they spring back to life. Mark Welch and his colleagues showed that desiccation, like ionizing radiation, breaks up the rotifers DNA into many pieces. Presumably, the same mechanisms they use to survive desiccation as part of their life cycle also protect them from ionizing radiation.
Thats the next thing we are looking at. How are the bdelloids able to repair this many double-stranded breaks in their DNA? Do they have better enzymes, more enzymes? Mark Welch says.
One feature that may confer exceptional DNA repair capacity on the bdelloids is described in the teams second PNAS paper. Here, they give evidence that the bdelloid rotifer, like most animals, originally had two copies of each chromosome. But at some point in its evolution, it underwent a whole-genome duplication, giving it four copies of each chromosome and hence of each gene. Normally, lineages that undergo whole-genome duplication lose the duplicate genes over time. The bdelloid, though, has kept most of its duplicate genes throughout its evolutionary history.
We believe they have kept most of their duplicate genes because they are serving as templates for DNA repair, says Mark Welch. One possible result of DNA repair is gene conversio
|Contact: Diana Kenney|
Marine Biological Laboratory