In a study that compares the genomes of aquatic life forms, researchers have found evidence to shuffle the branches of the tree of life. For more than a century, scientists thought that complex cell types, like neurons and muscles, evolved only once, after simple animals that lack these cell types branched from the rest of animals on the evolutionary tree. A team of researchers from the National Human Genome Research Institute (NHGRI), part of the National Institutes of Health, has provided new evidence from the genomic study of a ctenophore speciesa comb jellythat challenges this long-held view.
The cornerstone of the study, published in the Dec. 13, 2013, advance online issue of Science, is the researchers' sequencing, assembly, annotation and analysis of the genome of Mnemiopsis leidyi, a comb jelly native to the coastal waters of the western Atlantic Ocean. Whole-genome sequencing data shows that comb jellies branched from the rest of the animals before the sponge, a simple animal without complex cell types, according to the study. The results also show that critical cell types, such as neurons and muscle cells, were either lost multiple times during evolution or evolved independently in the ctenophores.
For the past 30 years, researchers have used whole-genome sequencing of organisms to advance their understanding of evolution. They do this by comparing the order of the chemical bases of DNA150 million base pairs for comb jellies versus 3 billion in humansthat comprise the organism's genome. While whole-genome sequencing data have been available for four of the five major animal lineagessponges (Porifera), flat invertebrates (Placozoa), jellyfish (Cnidaria), and animals with left-right symmetry, including humans (Bilateria)Ctenophora remained the last major animal lineage for which there were no sequenced genomes.
"Having genomic data from the ctenophores is crucial from a comparative genomics perspective, sinc
|Contact: Raymond MacDougall|
NIH/National Human Genome Research Institute