Scientists shake up the arthropod family tree

A team of scientists compared genetic sequences from 75 arthropod species and drew a new family tree for the most successful phylum of animals on Earth.

A research team from the Natural History Museum of Los Angeles, the University of Maryland and Duke University has outlined a new family tree that includes every major arthropod lineage. Their study, "Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences," was published Feb. 10, 2010, in the advanced online publication section of the journal Nature's website. The team painstakingly collected specimens from 75 arthropod species, stripped them down to their DNA, and searched across the data for common protein-coding gene sequences. This groundbreaking work, which was supported by the National Science Foundation, took over a decade to complete. More genes and more species were used in this research, making it four times larger than any previous study, according to background information in the press release. The University of California Museum of Paleontology (UCMP) website calls arthropods the "real rulers of the Earth," because they comprise three-fourths of all currently known living and fossil species and have spread successfully over diverse land, sea and air environments. With over one million species, arthropods are the most successful group of animals on the planet, the UCMP article states. Scientists spend a lot of effort sorting out family links among arthropods because this largest, most successful phylum's wide-ranging proliferation makes it and its members important in many fields of the earth and life sciences. The new arthropod tree, based upon statistical analysis of molecular data, overturns many earlier taxonomic classifications of the animals that were based upon shapes and features. Hexapoda (or insects) is the only group of arthropods whose classifications on the new genetic tree generally conform to the older taxonomic classifications, according to University of Maryland entomologist Jeffrey Shultz, who led the efforts to choose and collect the species needed for this study's innovative comparison. What were the biggest surprises in arthropod family relationships? According to the authors: One of the biggest surprises on the the new tree is that the Xenocarida ("strange shrimp"), which includes two centipede-like marine animals known as cephalocarids and remipedes, are most closely linked with Hexapoda, the insects within a new group named Miracrustacea, or "surprising crustaceans," rather than with the crustaceans (such as lobster, crabs, and shrimp). Another big surprise, is that triops, a 2-inch crustacean that resembles a cross between a mayfly and a horseshoe crab that was thought to be an early crustacean, was shown to have a much more recent origin. The new analysis also put millipedes and centipedes together with crustaceans and insects in a group taxonomists had long ago named (the subphylum) Mandibulata. Duke biology professor Cliff Cunningham, who led the study said:

"There are still many holes that need to be filled in, but at least the shape of the tree seems right. Now the developmental biologists can really piece things together."

"The only thing people thought they knew before molecular data was available was that the Myriapods were with the insects, but that turned out to be wrong, added Shultz. "Even the grouping Crustacea is no longer correct, since it includes the six-legged insects." he noted.