Cross-posted from The Molecular Ecologist.
Anyone who’s been anywhere near my Twitter feed in the last month knows I’m pretty darned happy with Star Trek: Discovery, the latest iteration of the five-decade-old science fiction franchise. Discovery manages to build something new with the key elements of Trek and provide “fan service” by calling up beloved old characters and plot points. In addition to putting tribbles back onscreen, it’s also providing another kind of fan service that has long been a Star Trek specialty — wrapping contemporary scientific discoveries into its science-fictional future. Often in the process, the real-world science gets a bit, um, stretched. But Discovery‘s take on cutting-edge science has what is, I think, a new distinction for Star Trek: it references a specific result in genomics research that didn’t survive a year after its initial publication.
I’ll explain below the jump, and rank Discovery‘s geno-flummox against Trek’s track record on genetics and evolution. I’ll have to get into spoilers for the first five episodes of Star Trek: Discovery, so consider yourself warned.
The Tardigrade and the “communications arising”
Okay, so here we go. For those of you who don’t care about spoilers because you don’t watch Star Trek, the plot of Star Trek: Discovery to date has centered on the development, aboard the eponymous starship Discovery, of a method for transporting a spacecraft instantaneously to any point in the universe. (Starships on Trek have faster than light drives, but not instantaneous ones — it still takes most of a day to get from Earth to Alpha Centauri.) The instantaneous drive runs on the spores of a fungus that are somehow connected to the multi-dimensional fabric of the universe.
I know, it’s bonkers. The good news is that this means a central character on the Discovery is a mycologist. The more-bonkers news is that to actually control the space-hopping, the fungal spores need to interact with a giant tardigrade. (Or tardigrade-like alien, anyway.) And here — yes really — is where we run into trouble: the tardigrade interacts with the spores through horizontal gene transfer.
Here, via the official Discovery Twitter feed, is a convenient expository video clip:
“Like its microscopic cousins on Earth, the tardigrade is able to incorporate foreign DNA into its own genome,” says Discovery‘s protagonist, Michael Burnham. She’s all but quoting popular science headlines from 2015, when a team of researchers at the University of North Carolina Chapel Hill reported sequencing a tardigrade genome and finding that about 1/6 of the sequence had been acquired from other organisms, mostly bacteria. Horizontal gene transfer isn’t unheard of in animals, but not to anywhere near that extent. Genome sequencing can easily pick up DNA sequences from contaminants — and a tardigrade, like other animals, contains plenty of live and active bacteria with their own un-horizontally-transferred DNA code. The UNC team took some effort to differentiate genuinely HGT’d genetic code from contamination, but the latter is still the more parsimonious explanation. And, indeed, other researchers re-analyzed the raw data and collected new sequences of their own and found that HGT’d DNA accounted for at most two percent of a tardigrade genome — about what’s been seen in other animals.
To boldly sequence what no one has sequenced before … or maybe not
So Discovery has made a genome-sequencing error into Star Trek canon. But it’s hardly out of line with what came before. As much as it pains this Trekkie biologist to admit, the franchise has long had a fairly shaky grasp on the details of genetics and biological evolution.
Genetics pops up in multiple episodes of the original Star Trek TV series, including the origin of one of the franchise’s greatest villains, the genetically engineered super-human Khan Noonian Singh. DNA sequencing is specifically mentioned in an episode called “The Immunity Syndrome”), in which the starship Enterprise is trapped by a thousand-mile-long spaceborne amoeba. To find an escape, the ship’s First Officer Spock flies a shuttlecraft into the amoeba, taking equipment including a “DNA code analyzer”, and determines that the monster is about to undergo mitosis based on inspecting “the chromosome structure”. That’s really pretty good for a 1967 pulp science fiction show — though how a DNA sequencer (presumably) designed for terrestrial life could cope with the molecular structure of a cell the size of a small moon is unclear. Also, you hardly need to sequence DNA to see chromosomes lining up for mitosis, especially when said chromosomes are hundreds of miles long.
“The Immunity Syndrome” is a model of biological coherence compared to later entries, though. In “Genesis,” a late-season episode of Star Trek: The Next Generation, gene therapy gone wrong creates a virus that transforms the crew of the Enterprise into various nonhuman animals. The explanation given is that the virus activates a DNA sequence from introns, bits of DNA sequence within a gene that are edited out when the gene’s sequence is transcribed for translation into protein. Introns are a real thing. Introns, it is further explained, contain inactive DNA sequence from our evolutionary ancestors, which could turn us into those ancestors if re-activated. This is very, very not true. Even if introns contained ancestral sequences — which they really don’t — that sequence would mutate into uselessness over generations of disuse. And even if it somehow didn’t, it couldn’t turn a human being into a spider, like poor Lieutenant Barclay. That triple-lutz of impossibility is because of how evolutionary relationships work. Humans and spiders share a common ancestor, but that common ancestor was nothing like a modern spider. The DNA sequence that makes a spider uniquely a spider, as opposed to a human being, has evolved since that common ancestry. So it couldn’t be in the introns carried by a human, if introns carried an ancestral genetic code, which, again, they don’t.
I could go on at tedious length. In one episode of Star Trek: Voyager, two characters merge into a single being because a teleportation accident facilitates the same process by which eukaryotic cells acquired mitochondria and chloroplasts. The recurrent villains of Star Trek: Deep Space Nine are a race of shape-shifters who are able to duplicate another living thing down to its DNA sequence but somehow retain their own biological and psychological identities. In every version of Trek, any time a character uses a computer for genetic analysis the computer displays DNA sequence as a double helix, which no real-world genetic analysis software has ever done. But far and away Star Trek‘s most common offense against biology — its original genetic sin, if you will — is that on almost every strange new world explored by Starfleet, new life turns out to be pretty much exactly like the life we already know, at the molecular level.
It starts with the idea that you could sequence the DNA of a giant space amoeba using equipment designed for earth-life — the vast majority of alien life on Star Trek is based on the same molecular building blocks as our own. Much less plausibly, human-like alien species on Star Trek are generally so similar to Homo sapiens that we can hybridize. Mr. Spock has a Vulcan father and a human mother, and he’s only the first example of interstellar interbreeding in the franchise. (The fan-run Trek reference site Memory Alpha has a loooong list of examples.) The franchise tried to explain this in an episode of Star Trek: The Next Generation in which it’s discovered that most of the Galaxy was “seeded” with simple life) by an ancient, highly advanced, now extinct humanoid civilization — arguing, basically, that cross-compatibility is possible because of shared ancestry. Considering that humans are entirely reproductively isolated from chimpanzees, who’ve been on an independent evolutionary trajectory for a mere six million years, it’s hardly credible that we’d be able to hybridize with species, even human-shaped ones, who’ve been evolving independently for at least four billion years.
Fact-checking is a dish best served cold
So, honestly, it’s hard to watch almost any episode of Star Trek without my biology-sense tingling. But here’s the thing: the bio-bollocks is often deeply entangled with what makes Trek great. The episode of Voyager in which two characters are temporarily transmuted into one touches on questions of personhood, and what makes us unique, self-determining individuals. The shape-shifting villains of Deep Space Nine created innumerable opportunities for stories about paranoia and power in wartime and the risks of trading freedom for security. The biological impossibility of Mr. Spock’s parentage makes him a touchstone for anyone who’s lived with dual identities or a sense of alienation from their community. The de-evolution virus … well, okay, that one I can’t justify. But by and large, when Star Trek has stretched and often broken the limits of biological realism, it’s done so to tell stories that are worth the telling — and that inspired many a nerdy kid to stick with science long enough to learn how fictional Star Trek really is.
All of which is to say, I can’t wait to find out what happens with the gene-trading space fungus in the next episode.
Arakawa K. 2016. No evidence for extensive horizontal gene transfer from the draft genome of a tardigrade. Proceedings of the National Academy of Sciences, 113(22), E3057-E3057. doi: 10.1073/pnas.1602711113
Bemm F, CL Weiß, J Schultz and F Förster, 2016. Genome of a tardigrade: Horizontal gene transfer or bacterial contamination? Proceedings of the National Academy of Sciences, 113(22), E3054-E3056. doi: 10.1073/pnas.1525116113
Boothby TC, JR Tenlen, FW Smith, JR Wang, KA Patanella, EO Nishimura, SC Tintori, Q Li, CD Jones, M Yandell and DN Messina. 2015. Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade. Proceedings of the National Academy of Sciences, 112(52), 15976-15981. doi: 10.1073/pnas.1510461112
Koutsovoulos G, S Kumar, DR Laetsch, L Stevens, J Daub, C Conlon, H Maroon, F Thomas, AA Aboobaker and M Blaxter, 2016. No evidence for extensive horizontal gene transfer in the genome of the tardigrade Hypsibius dujardini. Proceedings of the National Academy of Sciences, 113(18), 5053-5058. doi: 10.1073/pnas.1600338113
Image: Michael Burnham (Sonequa Martin-Green), studying the giant space tardigrade. (CBS). Used under the “fair use” exception as detailed by 17 USC Section 107 (1).