Scientists redefine animal classification system; change confirmed by genetics

The classification system for animals* has been hotly debated and frequently changed since it was created 300 years ago, but now researchers have actually found a genetic basis which confirms that part of the system we use today is actually pretty accurate—and they think this part can be defined even more specifically down to the genetic level.

An international team led by Professor Itai YanaAi of the Technion-Israel Institute of Technology Department of Biology made the discovery after using an extraordinarily powerful technique known as CEL-Seq. CEL-Seq monitors individual cells for their gene activity (as detected via mRNA)—and they applied it across 10 different species, with CEL-Seq being applied to 70 embryos per species.

In particular, they were interested in whether the animal classification of phylum—which separates animals into groups according to their body plans—is actually a useful tool for placing animals into groups, as well as what genetic attributes are the same and different between the different phyla.

Graphic of different animals during development

Credit: American Technion Society

“We selected species representing ten different animal phyla [of the 35 total phyla],” said Yanai in a statement. “For each phyla we determined the gene expression profile of all genes from the development of the fertilized egg to the free-living larvae. We found a surprising pattern of gene expression conservation in all species occurring at a pivotal, transitional period in development.”

Different bodies, different expressions

According to the paper, which is published in Nature, they actually discovered that while the animals they selected were extremely different in their body plans—there were fish, worms, flies, water bears, sponges, and more—all of them expressed two distinct patterns of gene expression (i.e. different patterns of turned on genes) as they developed from a fertilized egg into a full organism: One which creates a general template of an animal, and another which turns it into an animal of a specific species.

With this new knowledge, scientists have a new proposed definition for a phylum, which (in a jargon-heavy way) is defined as “a set of species sharing the same signals and transcription factor networks during the mid-developmental transition.”

Which is to say, they have made the definition for phylum more specific than organisms merely sharing a similar body plan. Now, they suggest that organisms in the same phylum also share unique genetic transition as they develop, which changes the levels at which genes are expressed and gives them unique body features.

“The transition we identified may be a hallmark of development only in animals,” the researchers wrote. “Or, future work may show that this is a general characteristic of development in all multicellular life.”

*For those of you who don’t remember biology: kingdom, phylum, class, order, family, genus, species is how we go from a general group of organisms, like the animal kingdom, to a specific organism, like Homo sapiens


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