Most sea squirts are little more than rubbery blobs that stick to shells and rocks on the sea floor. Yet a new genetic analysis puts them into the evolutionary spotlight: these creatures, also known as tunicates, are the closest relatives to vertebrates. The finding strengthens the case for revising the traditional view of how vertebrates arose, says Hervé Philippe, an evolutionary biologist from the University of Montreal, Canada.
Vertebrates belong to a group of animals called chordates. Each has a dorsal nerve chord, a notochord, gill slits, and a tail of sorts, at least during part of their life cycle. Chordates also include two kinds of invertebrates: tunicates, and a small, eel-like creature called amphioxus. With its segmented muscles, amphioxus looks more like a vertebrate than does a sea squirt. And early genetic comparisons reinforced this resemblance. Thus, researchers had assumed the sea squirt was a distant cousin to the other chordates.
But some researchers had begun to question this arrangement, based on a comparison of a few more genes. In the new study, Philippe and his colleagues took an even broader look. They compared 146 genes in many chordates, including hagfish and lamprey (considered the most primitive vertebrates) and larvaceans, which are sea-faring relatives of sea squirts.
When the researchers built an evolutionary tree from these comparative data, the amphioxus branched off earliest, followed by tunicates and larvaceans, with vertebrates being the most recent to evolve, the team reports 23 February in Nature. If sea squirts, and not amphioxus, truly are the vertebrate's closest relative, then researchers studying vertebrate evolution may have to reinterpret morphological, developmental, and paleontological clues about how vertebrates got their start, concludes Philippe.
Given how many genes were considered, "I think [the paper] will turn some heads," says William Jeffery, an evolutionary developmental biologist at the University of Maryland, College Park. But others think the jury is still out. Tunicates and larvaceans evolve rapidly and have gained and lost so many genes that it's very hard to position them properly in an evolutionary tree, they note. "I think it's premature to say that they have proved the position of tunicates," says Billie Swalla, an evolutionary developmental biologist at the University of Washington, Seattle.