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The evolutionary relationships of two ancient invertebrate groups have been revealed

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Kamptozoa and Bryozoa are two phyla of small aquatic invertebrates. They are related to snails and clams (collectively called mollusks), silkworms, earthworms and leeches (collectively called annelids) and ribbon worms (nemertea). But their precise location on the tree of life and their close relationship with these other animals have always puzzled evolutionary biologists. Previous studies have constantly displaced them. Also, while Kamptozoa and Bryozoa were originally considered to be 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), along with colleagues from St. Petersburg University and Tsukuba University, revealed that the two phyla separated from mollusks and worms earlier than suggested by previous studies, and thus effectively 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 each cell. It is broken down into genes. These genes are made up of base pairs of DNA and each gene contains the instructions needed to create a protein, and thus leads to proper care and maintenance of a cell. In order for the instructions to be carried out, the DNA must first be transcribed into the RNA. The result is a transcriptome, 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 greater genetic differences. Using this data, the researchers have improved our understanding of animal evolution, but some questions are still difficult to answer.

Since Kamptozoa and Bryozoa are closely related to mollusks, annelids, and nemertea, small errors in the dataset or missing data can lead to misplacement on the evolutionary tree. Additionally, when these tiny animals are collected, 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 algae and small animal RNA to rule out anything that might have come from them.

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

Using these transcriptomes, they predicted proteins and compared them with similar data from 31 other species, some of which were closely related to Kamptozoans and Bryozoans, such as clams and silkworms, and others further away, such as frogs, starfish. , insects and Medusa. The high-quality datasets meant they could compare many different genes and proteins at the same time. Dr. Khalturin attributed to the OIST the powerful computational capabilities that researchers had access to.

“Our main finding is that the two phyla go together,” said Dr. Khalturin. “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 ability, he also pointed out that the dataset could answer other fundamental evolutionary questions, such as the more precise location of mollusks and annelids on the tree of life. and how diverse life.

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