The family tree of a group of sea creatures with an ancient pedigree is being called into question. A study led by a researcher at Scripps Institution of Oceanography at UC San Diego has uncovered new details about the organism’s historical lineage and evolutionary fingerprints.
Sea lilies and featherstars, little-studied invertebrates called crinoids, are related to sea stars, sea urchins, and sea cucumbers. All sea lilies are found in deep water, and attach to the bottom via long stalks that resemble forms that dominated many ocean habitats hundreds of millions of years ago. Featherstars, common in places such as the coral reefs of the Indian and Pacific oceans, are much more diverse today than sea lilies. Featherstars are thought to have evolved from one of the sea lily groups, as they have a stalk as juveniles, but then break free.
Studying the genetic makeup of 59 different sea lily and featherstar species, Scripps Marine Biology Professor Greg Rouse and an international team of researchers found that several of the sea lily groups are indeed primitive, lying at the base of the evolutionary tree of living crinoids. Other sea lily groups, however, showed up as branches within the featherstar part of the tree.
“It’s an evolutionary reversal of fortunes. Some ancient sea lily lost its stalk at an early age, but then some of its featherstar descendants turned around and kept their stalk to adulthood,” said Rouse, curator of the Scripps Benthic Invertebrate Collection. “This appears to represent a phenomenon known as paedomorphosis, in which a juvenile trait of an ancestor is retained in the adult descendant.”
The study “Fixed, free, and fixed: The fickle phylogeny of extant Crinoidea (Echinodermata) and their Permian–Triassic origin,” is published in the January issue of Molecular Phylogenetics and Evolution.
Rouse and his colleagues sequenced the DNA from specimens collected by various means, including snorkeling, scuba, submersibles, remotely operated vehicles, and museum collections. Their results suggest that all living crinoids descended from one or a few ancestors that squeaked through the Permian mass extinction that wiped out much of life on Earth and in the sea some 250 million years ago, before the age of dinosaurs. They also discovered that several major branches of the featherstar tree need rearranging.
“We’ve got a lot of work to do before we can finally nail down the evolutionary story of crinoids,” Rouse believes.
Coauthors of the paper include: Lars Jermiin of Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO) Ecosystem Sciences and University of Sydney; Nerida Wilson of the Australian Museum; Igor Eeckhaut and Deborah Lanterbecq of the University of Mons, Belgium; Tatsuo Oji of Nagoya University; Craig Young of the Oregon Institute of Marine Biology; Teena Browning of the Department of Climate Change, Canberra, Australia; Paula Cisternas of the University of Sydney; Lauren Helgen of the Smithsonian Institution; Michelle Stuckey of CSIRO Ecosystem Sciences; and Charles Messing of Nova Southeastern University.
The study was funded by the National Science Foundation’s Assembling the Tree of Life program, the Australian Research Council, and the Belgian Fonds de la Recherche Scientifique.
– Mario C. Aguilera