Maybe it's a side effect: "In at least some cases, extreme IR resistance is associated with adaptation for extreme desiccation resistance." Except that it isn't always associated with that.
So maybe, contra their claim, there have been eras or regions which featured lots of ionizing radiation. As for regions--remember the South African natural reactor? As for eras--from time to time supernovas go off. I don't have a good handle on what stars or star relics used to be close to us but aren't anymore, but IIRC we're supposed to be in a supernova's "bubble" now, though that one was probably too far away to have much effect.
They tried using a cobalt-60 source, but that only lasted 20 generations, so they shifted to using a Linac. (I wonder if Henry Kaplan dreamed of that application.)
One interesting little detail, which one might have predicted (and I did): "isolates from each population after 100 cycles of selection exhibited a growth deficiency in rich medium without IR selection." There's a price to pay for radiation resistance.
And wrt the "side effect" theory:
"isolates from each population exhibited no increase in desiccation resistance compared to the Founder strain in our assay. The lack of desiccation resistance suggests that these evolved populations are not exhibiting convergent evolution toward the highly desiccation and IR-resistant phenotype of D. radiodurans, and is in fact developing highly selective resistance to IR. In accordance with this hypothesis, isolates from round 100 of selection do not have significantly altered ratios of intracellular Mn and Fe, which has been hypothesized to be an indicator of IR resistance in nature"
The changes are more extensive than I'd have guessed. "mechanisms of IR resistance have expanded well beyond modifications to DNA repair mechanisms." They can stand 3-4 times as much gamma radiation as their un-bred ancestors
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