Evolutionary Relationships of Two Ancient Invertebrate Groups Revealed

Evolutionary Relationships of Two Ancient Invertebrate Groups Revealed

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Kamptozoa and Bryozoa are two phyla of small aquatic invertebrates. They are related to snails and clams (collectively called molluscs), silkworms, earthworms and leeches (collectively called annelids), and ribbon worms (nemertea). But their precise position on the tree of life and their close connection to these other animals have always puzzled evolutionary biologists. Previous studies have consistently moved them. Additionally, while Kamptozoa and Bryozoa were originally considered to form a single group, they were separated based on their appearance and anatomy. Now, using state-of-the-art sequencing technology and powerful computational analysis, scientists from the Okinawa University Institute of Science and Technology (OIST), together with colleagues from Saint Petersburg University and the University of Tsukuba, revealed that the two phyla separated from molluscs and worms earlier than previous studies suggested, and so they do indeed form a separate group.

“We have shown that by using high-quality transcriptomic data, we can answer a long-standing question to the best of our current techniques,” said Dr. Konstantin Khalturin, researcher at the Marine Genomics Unit of the OIST and first author of the article published in Scientific advances.

A genome is the complete set of genetic information present in every cell. It is subdivided into genes. These genes are made up of base pairs of DNA and each gene contains the necessary instructions to create a protein, and thus leads to the proper care and maintenance of a cell. For instructions to be carried out, DNA must first be transcribed into RNA. A transcriptome is the result, like a reflection of a genome but written in RNA base pairs rather than DNA.

This genetic information differs between species. Those that are closely related have very similar genetic information, while greater evolutionary distance leads to more genetic differences. By using this data, researchers have improved our knowledge of animal evolution, but some questions still prove difficult to answer.

As Kamptozoa and Bryozoa are closely related to molluscs, annelids and nemertea, small errors in the dataset or missing data can lead to incorrect placement on the evolutionary tree. Additionally, when collecting these tiny animals, it is easy to detect other organisms, such as algae, contaminating the sample. Dr Khalturin pointed out that they took care to avoid contamination and then sifted through their dataset for RNA from algae and small animals to rule out anything that might have come from them.

In total, the researchers sequenced the transcriptome of four species of Kamptozoa and two species of Bryozoa, but at a much higher level of quality than previously achieved. While previous datasets had 20-60% completeness, in this study the completeness of the transcriptome was greater than 96%.

Using these transcriptomes, they predicted the proteins and compared them to similar data from 31 other species, some of which were closely related to Kamptozoans and Bryozoans, such as clams and silkworms, and others more away, such as frogs, starfish, insects, and jellyfish. The high-quality datasets meant they could compare many different genes and proteins simultaneously. Dr. Khalturin credited the powerful computing capabilities researchers could access to the OIST.

“Our main finding is that the two phyla go together,” Dr. Khalturin said. “This result was originally proposed in the 19th century by biologists who grouped animals based on their appearance. »

Although Dr Khalturin said this question has now been answered to the best of our abilities, he also pointed out that the dataset could answer other fundamental evolutionary questions – such as the more precise location of molluscs and annelids on the tree of life, and how diversified life.

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