If the bdelloid rotifer could talk, she’d tell you sex is overrated. These microscopic relatives of the flatworm haven’t had so much as a sniff of sex for some 80m years – a commitment to celibacy unmatched in the animal kingdom.
All 450 species of this class of rotifer are female. No males have ever been described. Bdelloids make their home in brackish water, such as puddles (cute) and sewage treatment tanks (less so), which might not be terribly enticing on a dating profile. But these self-replicating sisters care not, for they’ve cracked the conundrum of how to survive without the genetic shuffling of sex.
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What's so unusual about bdelloids?
The standard explanation for the existence for sex is that it increases genetic diversity, which helps species stay ahead of harmful mutations and ever-evolving parasites. Asexual species are therefore expected to have a shelf life of around 100,000 generations. The bdelloids’ flouting of this rule has earned them a reputation as ‘evolutionary scoundrels’, forcing us to consider the whole question of sex.
How do bdelloids live so long if they don't have sex?
Their evolutionary longevity is an area of continuing investigation. One of the secrets seems to be that they ‘steal’ genes from other life-forms, possibly through the stuff that they eat. The bdelloid diet isn’t exactly enviable – which, given their living conditions, is hardly big news. They mostly survive on organic detritus – dead bacteria, algae and protozoans. In other words, pretty much anything they can fit in their mouths.
Some scientists think that the bdelloids can extract DNA from their dinner and spruce up their own genome by a process called ‘horizontal gene transfer’. Studies have shown that up to 10 per cent of the active genes in these rotifers could be pirated from other species. All told, the bdelloids appear to have adopted a Frankenstein collage of foreign DNA from more than 500 other species. Whether that’s through ingestion or not is up for debate, but these pilfered genes could be the reason for another bdelloid superpower: their epic resistance to drought and radiation.
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How tough are bdelloids?
If nature had an edition of the TV reality show Survivor, the bdelloid rotifers would probably be the last animals standing. These extremophiles can survive several years of desiccation and high blasts of radiation, beating even the notoriously indestructible tardigrade in the hard-nut stakes.
A Russian scientist recently resuscitated rotifers that froze in ancient Siberian permafrost during the latter part of the Pleistocene – some 24,000 years ago. Once thawed, the first thing these ancient females did was start pumping out perfect clonal daughters.
Why can bdelloids survive extreme environments?
The bdelloids are able to survive and produce viable replicas of themselves because their mosaic of stolen genes codes for enzymes that give them the remarkable ability to repair their shredded DNA. The temporary water sources they call home routinely dry out, leaving the bdelloids essentially mummified for potentially years while they wait for the next rainfall.
Such dehydration has the same damaging effect on their DNA as radiation (if less extreme), and their stolen genetic repair kit helps them patch things back up. As an added bonus it’s likely this is the bdelloids’ secret to surviving millennia without males. The process of shattering and reconstructing their genome looks like it provides the bdelloids with the same evolutionary benefits as sex.
This is huge news. Until the bdelloid rotifer came along, sex had cornered the market on shuffling the genome. Now we’ve discovered there’s at least one other way (males take note).
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Main image: a bdelloid rotifer. Credit: Frank Fox, CC BY-SA 3.0 DE, via Wikimedia Commons