December 13, 2013
Comb Jellyfish Offer Surprising Insight Into Evolution Of Life
A new study from researchers at the National Human Genome Research Institute (NHGRI) has indicated that a reshuffling of the evolutionary tree for the animal kingdom may be in order.
The study, which was published in the journal Science, challenged the idea that complex cell types, like neurons and muscle cells, evolved only once, after simple animals that don’t have these cells diverged from the rest of the animal kingdom.
The study researchers reached their conclusion through the sequencing and analysis of the genome of Mnemiopsis leidyi, a comb jelly found in the coastal waters of the western Atlantic Ocean. The genetic analysis showed that comb jellies, a species of ctenophores, diverged from other animals before developing genes for muscles.
Until this study, whole-genome sequencing data have been available for four of the five major animal lineages: sponges, flat invertebrates, jellyfish and Bilateria (animals with left-right symmetry such as humans). Ctenophora was the last major animal lineage without a fully sequenced genome.
"Having genomic data from the ctenophores is crucial from a comparative genomics perspective, since it allows us to determine what physical and structural features were present in animals early on," said study author Andy Baxevanis, a senior scientist in NHGRI's Division of Intramural Research. "These data also provide us an invaluable window for determining the order of events that led to the incredible diversity that we see in the animal kingdom."
While comb jellies possess muscle cells, the DNA analysis found that they don’t have the majority of genes that specify muscle types in other animals. According to the researchers, this finding indicated that comb jellies’ muscle cells evolved after they diverged from the rest of the animal kingdom.
"Our analysis of the Mnemiopsis genome thoroughly corroborates previous studies suggesting that ctenophores might be the sister group to the rest of the animals," said study author Joseph Ryan, a researcher at the Sars International Center for Marine Molecular Biology at the University of Bergen in Norway.
"With our whole-genome sequencing data in hand, it is now clear that the cell types that make up muscles and nervous systems were either lost in some animal lineages or that, despite the complexity of these cells, they very well may have evolved multiple times."
The study team said their findings are important for expanding the knowledge of genomes across the animal kingdom, particularly for learning about evolutionary adaptations at the molecular and cellular levels.
"The whole-genome sequence of the comb jelly provides a nontraditional model through which new insights about genes and their functions, including those in our own genome, may become better understood,” said Dr. Daniel Kastner, and NHGRI scientific director who was not among the study authors.
"Our study demonstrates the power of comparative genomics research having an evolutionary point of view, probing the interface of genomics and developmental biology," said Jim Mullikin, study author and director at the National Institutes of Health (NIH) Intramural Sequencing Center (NISC) in Rockville, Md. "The data generated in the course of this study also provide a strong foundation for future work that will undoubtedly lead to novel findings related to the nature of animal biology."