Sea squirt relative demonstrates that sometimes less is more: losing genes can be adaptive.
As far as sea squirts and their close relatives go, the genus Oikopleura represents a decidedly strange group of organisms, both from the standpoint of physical attributes and genetics. It belongs to a larger group of invertebrate animals that are closely related to all vertebrates: the tunicates. But unlike most others in that group, it does not undergo metamorphosis from a free-swimming larva to a fixed-to-the-bottom, or sessile, adult.
Cañestro quickly learned that the balloons are not all that Oikopleura casts off. The researcher initially wanted to study the species Oikopleura dioica because, given its position on the evolutionary tree, it might reveal something about what the common ancestor of vertebrates looked like. But soon he discovered that this creature had lost a whole set of genes, including retinoic-acid-signaling genes that are essential for vertebrate heart and brain development.
Members of Oikopleura genus live fast and die young, going from egg to reproductive adult in about one week. “Because there’s been this selection for rapid development, they have taken a lot of shortcuts,” says John Postlethwait, an evolutionary biologist at the University of Oregon, who was also not involved in the November 2021 study but was Cañestro’s postdoctoral adviser more than a decade ago.
Oikopleura’s heart is simpler than that of sea squirts because it is not cylindrical. Rather it is made up of a flat sheet of cells that contract in concert with the continuously beating tail to pump water through the filter-feeding apparatus. The tail cannot easily push water through a cylindrical tube, but the laminar heart beats against the stomach in such a way that the heart and tail more easily coordinate the movements of water through the body cavity.
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