Radiation is one of the most lethal stressors that a living creature can face. In humans, acute exposure to even just a few grays (units of absorbed radiation) can cause nausea, organ failure, and death. And yet, in some microscopic animals, the limits of this radiation resistance are incomprehensibly higher than what we humans can tolerate. Among these super-survivors are minuscule aquatic animals, so small that they can\u2019t even be seen with the naked eye: bdelloid rotifers.<\/p>\n
For evolutionary biologists, molecular geneticists or anyone intrigued by life at its extremes, bdelloid rotifers are a fascinating case study in the adaptive strategies of life on Earth that push the boundaries of what we thought possible.<\/p>\n
What Are Bdelloid Rotifers?<\/p>\n
Bdelloid rotifers aren\u2019t household names. They are tiny, mostly freshwater animals, usually less than a millimeter long, that can be found worldwide in freshwater habitats, on wet mosses, in soil films and even in ephemeral puddles that dry out periodically.<\/p>\n
But under a microscope, they are incredibly complex for their size: they have muscular pharynges, ciliated structures used for movement and feeding, as well as entire organ systems. Notably, one of their most unique defining features is that they\u2019re ancient asexuals, meaning that they appear to reproduce almost entirely without sex.<\/p>\n
However, what makes them especially extraordinary is that they can survive exposure to ionizing radiation doses up to 1,000 times greater than the lethal dose for humans \u2014 and recover completely. Somehow, an organism smaller than what we can even see can repair DNA damage that would be catastrophic in most other animals.<\/p>\n
To quantify just how extreme this is, humans exposed to anything around 5 to 10 grays (Gy) of radiation, even in a short time, are very likely to succumb to acute radiation syndrome without medical intervention. By contrast, bdelloid rotifers can endure doses in excess of 5,000 Gy, and the majority of their reproductive capacity will be preserved after recovery.<\/p>\n
It\u2019s important to note that this ability is not entirely unique among extremophiles; however, among multicellular animals, bdelloid rotifers\u2019 tolerance is virtually unmatched.<\/p>\n
Bacteria such as Deinococcus radiodurans<\/a> can also survive enormous doses, and certain tardigrades<\/a> (water bears) also endure high radiation levels. What distinguishes bdelloids is the specific combination of their multicellularity, reproductive resilience and ability to resume active life after extreme damage.<\/p>\n Tardigrades, for instance, can endure high radiation while in cryptobiotic states, but they would perish if exposed to the same levels while metabolically active. Bdelloids, by contrast, can repair radiation damage while active.<\/p>\n Many wonder how such a feat is possible for such a small organism. Intriguingly, researchers have discovered that their radiation resistance appears to be tied to their ability to survive desiccation (drying out completely). That is, the same mechanisms that enable them to repair the DNA damage caused by extreme dehydration are also what help them withstand ionizing radiation.<\/p>\n In a 2014 study<\/a> published in the Journal of Evolutionary Biology, researchers uncovered that bdelloid rotifers can survive prolonged desiccation by entering a state of suspended animation. In this state, their metabolism drops to near zero as their DNA undergoes fragmentation. But as soon as they\u2019re reexposed to water, they rehydrate and swiftly repair the damage with no issue.<\/p>\n And as the researchers discovered, this robust DNA repair machinery is also incredibly effective against radiation-induced DNA breaks.<\/p>\n Bdelloid Rotifers\u2019 DNA Repair Secret<\/p>\n Ionizing radiation kills cells primarily by breaking the strands that make up organisms\u2019 DNA. When DNA is broken in many places at once \u2014 as it does with high radiation doses \u2014 biologists previously assumed that there was no way for most multicellular organisms to reassemble their genomes correctly.<\/p>\n