Wild Things

Animals That Scientists Thought Never, Ever Have Sex Actually Do Have Sex

A lonesome bdelloid rotifer.

Courtesy of Damián H. Zanette via Wikipedia Commons

This article originally appeared in Sick Papes, a blog about exciting new science papers.

As all you puddle-suckers know, there are entire universes of microscopic organisms living in every drop of puddle water, moss moisture, and sidewalk juice. Indeed, it was within a drop of filthy gutter scum that Antony van Leeuwenhoek (1632–1723) observed single-celled and other microscopic organisms for the first time. He also provided the first microscopic drawings of human sperm, which lead to some great drawings of little tiny men scrunched up inside the head of a sperm. [Editor’s note: this man-in-sperm theory is no crazier than the truth of how humans develop.] The revolutionary nature of Leeuwenhoek’s observations are even more impressive considering how shitty the microscope that he used was: Made of a single hemisphere of glass fixed to a metal plate, these scopes had a fixed focal distance of about 0.5 millimeters but magnified up to 275 times, and apparently had a resolution approaching one micron.

Today’s Sick Pape focuses on one of the types of animals that Leeuwenhoek saw when he magnified a drop of water: rotifers. Rotifers are microscopic animals (not bacteria, not amoebas—multicellular animals) measuring between about 100–500 microns yet possessing a complex anatomy including a nervous system and a digestive system. And for the past two decades, rotifers in the genus Bdelloidea have been the subject of intense research for one reason: They never, ever have sex.

Sexual reproduction is thought to be essential for the long-term survival of a species. There are a handful of animals that will occasionally pop out a virgin birth, but this is typically done just for a ratings bump at Animal Planet during the holiday season. In general, there are no species that exclusively reproduce asexually and manage to last more than a few million years. While there may be short-term Darwinian benefits to cloning yourself, only sexual reproduction can provide the cosmic intermingling and chromosomal diversity that organisms need to survive the long evolutionary grind.

Bdelloid rotifers thus pose a major question: How has this species survived for hundreds of millions of years without ever having sex? Male bdelloid rotifers have never been observed, nor has anyone ever witnessed meiosis or mating. Instead, the females just happily clone themselves forever like a goddamn metaphor I can’t think of right now. The big question has always been: How come these ladies haven’t gone extinct? What’s their secret?

Over the past few decades, bdelloid rotifers have been whispering their secrets into the quivering ears of Matt Meselson’s lab. Meselson is one of the best scientists who has ever lived, responsible for “the most beautiful experiment in biology” as well as helping to discover mRNA and many other Old Testament breakthroughs (as well as crusading for years against biological weapons). Which is just to say, he knows what the funk he’s talking about. And his lab has shown that bdelloid rotifers have a variety of alien mechanisms for increasing their genetic diversity without having sex. Namely, these rotifers suck up random DNA from their surroundings and shove it into their own genomes, and they also shatter their entire genome and then rebuild it with errors.

And now, Meselson (who is currently in his 80s) and his lab have done something that is so rarely done these days: They have provided strong evidence that their entire premise was wrong and that bdelloid rotifers actually do reproduce sexually. I want to emphasize that none of their past research was wrong; each of their past experiments still hold up, all the crazy DNA-sucking-and-shattering is still real, it’s just that these strategies exist in addition to sex (albeit, very infrequent sex).

Meselson and his squad begin their Sick Pape with an ominous threat that would terrify the Edward Snowdens and Chelsea Mannings among us: “Certain methods … offer definitive means for detecting infrequent or atypical sex.” [Editor’s note: What you or I do in private is not the business of the Meselson lab or the state.] However, instead of using the CIA interrogation methods implied by this terrifying abstract, they stick to modest and routine population genetics strategies. Specifically, these folks test for patterns of genetic similarity between different rotifers that imply they are the product of sexual reproduction in the not-too-distant past, and they find it.

For those who want to come into the methodological weeds with me, the “certain methods” to detect infrequent sex is this: You sequence a few genes from each of several rotifers and then look for a pattern where Gene X from rotifer 1 is very similar to its homolog in rotifer 2, whereas Gene Y is similar to its homolog in rotifer 3. This implies that there has been recent sex that has jumbled up different chromosomes between individuals in the population. To be sure that this wasn’t the result of direct DNA transfer between rotifers 1 and 3, these auteurs do the same test at several genes and show that the same pattern holds for all these genes, making it very, very unlikely to have been direct horizontal transfer of DNA.

We honor Ana Signorovitch, the pape’s first author, and her colleagues in the Meselson lab for having the courage to disprove themselves—an uncommon mark of high integrity—and for shining their beautiful light on an invisible little corner of our world. Of course, the timing and mechanisms of sex remain a complete mystery, so please keep up the hard work for many more decades to come!

Signorovitch, A., Hur, J., Gladyshev, E., & Meselson, M. (2015). Allele Sharing and Evidence for Sexuality in a Mitochondrial Clade of Bdelloid Rotifers. Genetics. doi:10.1534/genetics.115.176719