Deuterostomes are a vast "superphylum" of animals that includes things as diverse as sea cucumbers, snakes, and people. By analyzing the genomes of deep-sea invertebrates called acorn worms, researchers hope to elucidate various characteristics of the most recent common ancestor of all deuterostomes – which lived more than half a billion years ago. The findings were published in Nature this week.
Three different groups of animals make up the deuterostomes: acorn worms, echinoderms (sea stars, sea urchins, and their relatives), and chordates (all vertebrates and some invertebrates, such as sea squirts). Acorn worm adults live on the ocean floor and burrow into the sediment below. Like us but unlike echinoderms, acorn worms have soft bodies, bilateral symmetry, and gill slits, which in people are only present during the embryonic stage. These similarities make acorn worms key comparators for inferring the ancestral genomic features of deuterostomes.
"Acorn worms are our most ancient deuterostome relatives dating back to the origin of deuterostomes around 570 million years ago," Oleg Simakov from the Okinawa Institute of Science and Technology Graduate University explains to IFLScience over email. "The sampling of acorn worms provides a solid comparative platform, informing us about the early evolution of deuterostomes." While many chordate (and especially vertebrate) genomes have been sequenced – cats, horses, whales, and ancient humans, just to name a few – there aren’t nearly as many genome sequences available for other deuterostomes.
So Simakov, together with a large international team, sequenced and analyzed the whole genomes of two acorn worms from lineages that separated at least 370 million years ago: Ptychodera flava from the Pacific (pictured above) and Saccoglossus kowalevskii from the Atlantic (below). The former develops indirectly through a feeding larva that metamorphoses into a juvenile worm months later; the latter develops directly into a juvenile worm within days. You can watch videos of the two squishy species here and here. The researchers then compared these genomes with those of other animals.
Saccoglossus kowalevskii. John Gerhart
What unites all deuterostomes – and separates us from protostomes like mollusks and arthropods – are a few fundamental details of our early development way, way back in the womb. So while adult humans look nothing like adult acorn worms, our embryos are more similar. "The main finding of Simakov and colleagues’ study is that deuterostome genomes, like their embryology, show extensive conservation across great evolutionary timescales," Brown University’s Casey Dunn writes in an accompanying News & Views article.
The team found shared traits that were likely inherited from the last common deuterostome ancestor, including more than 30 novel deuterostome genes with sequences that vary dramatically from those of other multicellular animals.
They also identified a conserved cluster of six deuterostome-specific genes, including those involved in the development of what’s called pharyngeal gill slits – a series of openings in the body wall found in all deuterostomes at one point or another. (Again, for us these are only present in the embryo, and while echinoderms used to have them, they lost them over evolutionary time.) These features were the foremost morphological innovation of early deuterostomes – and they’re probably central to the filter-feeding lifestyle of our early, early ancestors.