The family tree of life has a newly discovered branch. Genetic studies comparing mitochondrial DNA have revealed that what has long been thought to be the group from which insects arose, the Collembola -- wingless hexapods (or "six legs") commonly called springtails -- turns out not to be closely related to insects after all.
Instead, these creatures belong to a separate evolutionary lineage that predates even the separation of insects and crustaceans. The research was carried out at the U.S. Department of Energy's Joint Genome Institute (JGI) by scientists with the Lawrence Berkeley National Laboratory (Berkeley Lab), working with a team of Italian researchers.
"Based on the similarities in their body organization, their six legs, and other morphological characteristics, it has been generally accepted that the collembolans were the basal stock from which insects arose," says Jeffrey Boore, a biologist with Berkeley Lab's Genomics Division who heads JGI's Evolutionary Genomics Department. "Our study shows that they evolved separately from insects and independently adapted to life on land."
Collaborating with Boore on the project were Francesco Nardi, Giacomo Spinsanti, Antonio Carapelli, Romano Dallai and Francesco Frati, all with the Department of Evolutionary Biology at the University of Siena in Italy. Their research was reported in the March 21 issue of the journal Science.
"The collembolan appear to branch off the evolutionary line leading to the insects at a much earlier stage than previously thought -- earlier than some if not all of the crustaceans," says Nardi. "In fact, the few crustaceans that we've analyzed so far using mitochondrial genomics appear to be more closely related to the true insects than are the collembolans."
Mitochondria are organelles, found in living cells, that play a central role in vital life processes such as metabolism. They have been called "the powerhouses" of cells because they produce the ATP molecules that provide cells with chemical energy. But mitochondria were actually once independent organisms that long ago evolved into a symbiotic relationship with the nuclei of cells.
The former independence of mitochondria has been preserved in a tiny genome, separate from the genome of the host cell, with its own genes and its own system for DNA replication and translation into proteins. Much simpler than nuclear genomes but governed by the same rules of genetic conservation, and closely interacting with host-cell genomes, mitochondrial genomes can serve as a powerful tool for the study of evolutionary biology.
"The small size and compact arrangement of mitochondrial genomes make it possible to study and do comparative genomics with many different organisms," Boore says. "Plus, these genomes are usually circular, which allows them to be physically isolated from nuclear genomes. And their biochemistry is relatively well understood."
Insects have been thought to be the dominant group of Hexapoda, also thought to include the Collembola and a few other wingless groups. Together these groups have been included as part of the phylum Arthropoda (the name means "jointed feet"), which constitutes nearly 85 percent of all known species of animal life. Other arthropod taxa include Myriapoda (for example, centipedes and millipedes), Chelicerata (for example, spiders), and Crustacea (for example, crabs and lobsters).
Using classic approaches to evolutionary biology, such as comparative morphology or paleontology, it has long been held that Hexapoda is a "monophyletic" taxon, meaning all members of the group are descended from a single ancestor. However, by doing comparative analyses of the DNA from entire mitochondrial genomes, Boore and Nardi and their collaborators found that Collembola should not be included within Hexapoda.
"Based on our results and consequent phylogenetic reconstruction, we can say that collembolans, which have until now been classified as the sister group to the insects within the hexapods, should be separated so that they constitute a separate evolutionary line," says Nardi. "The next step will be to assign a class level status to Collembola, but this decision probably won't be made on the basis of a single study, even if the results we present seem to be quite strong."
Nardi says he and his Italian collaborators sought out Boore to work on this project because they consider him the "world's leading expert" in the study of mitochondrial genomics. "Jeff provides his expertise on mitochondrial genomics and is a bridge between us and the huge sequencing and technical capabilities of the JGI."
The JGI headquarters in Walnut Creek, California, houses one of the nation's fastest and most powerful genome sequencing operations. JGI is a collaboration between Berkeley Lab, Lawrence Livermore, and Los Alamos National Laboratories, funded by DOE's Office of Biological and Environmental Research.
Nardi says, "This has been a very fruitful collaboration so far, and we are planning to do more studies along this line."
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