Hidden in genetics: the evolutionary relationships of two groups of ancient invertebrates revealed

Kamptozoa and Bryozoa are two tribes of small aquatic invertebrates. They are related to snails and clams (collectively called mollusks), bristle worms, earthworms and leeches (collectively called annelids), and tapeworms (nemertea). But their precise position on the tree of life, and how closely related they are to these other animals, has always puzzled evolutionary biologists. Previous studies have consistently moved them. What’s more, while Kamptozoa and Bryozoa were originally considered one group, they were separated based on their appearance and anatomy. Now, scientists at the Okinawa Institute of Science and Technology Graduate University (OIST), in collaboration with colleagues from St Petersburg University and Tsukuba University, have revealed that the two phyla split from mollusks and worms earlier than previous studies have suggested, and thus they are indeed a separate group.

“We have shown that by using high-quality transcriptomic data, we can answer a long-standing question with the best of our current techniques,” said Dr. Konstantin Khalturin, Staff Scientist at OIST’s Marine Genomics Unit and lead author of the paper published in Progress in science.

A genome is the complete set of genetic information found in each cell. It is divided into genes. These genes are made up of DNA base pairs and each gene contains the instructions needed to make a protein, thus leading to the proper care and maintenance of a cell. To carry out the instructions, the DNA must first be transcribed into RNA. A transcriptome results from this, like the reflection of a genome, but written in RNA base pairs instead of DNA.

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

Because Kamptozoa and Bryozoa are closely related to mollusks, annelids and nemertea, minor errors in the dataset or missing data could result in incorrect placement on the evolutionary tree. In addition, while collecting these small animals, it is easy to pick up other organisms, such as algae, that contaminate the sample. dr. Khalturin emphasized that they were careful to avoid contamination and later screened their dataset for RNA from algae and small animals to remove anything that might have come from them.

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

Using these transcriptomes, they predicted proteins and compared them with comparable data from 31 other species, some of which were closely related to Kamptozoa and Bryozoa, such as mussels and bristle worms, and others more distant, such as frogs, starfish, insects, and jellyfish. . The high-quality data sets allowed them to compare many different genes and proteins simultaneously. dr. Khalturin praised the powerful computing capabilities that the researchers at OIST were able to use.

“Our main finding is that the two phyla belong together,” said Dr. Khalturin. “This result was originally proposed in the 1800s by biologists who grouped animals based on what they looked like.”

Although Dr. Khalturin stated that this question had now been answered to the best of his ability, he also stressed 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 life diversified.

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