The evolutionary relationships of ancient invertebrates have been revealed

Kamptozoa and Bryozoa are two types of small aquatic invertebrates. They are related to snails and clams (collectively called molluscs), bristleworms, earthworms and leeches (collectively called annelids), and tapeworms (nemertea). But their exact 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. Moreover, while Kamptozoa and Bryozoa were originally considered a single group, they were separated based on their appearance and anatomy. Now, using cutting-edge sequencing technology and powerful computational analysis, scientists from the Okinawa Institute of Science and Technology (OIST), in collaboration with colleagues from the University of St. Petersburg and the University of Tsukuba, have revealed that the two groups are separated from molluscs and worms by -earlier than previous studies had suggested, and thus they do form a distinct 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 Halturin, staff scientist in OIST’s Division of Marine Genomics and first author of the paper published in Scientific progress.

The 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 and thus lead to the proper care and maintenance of the cell. For instructions to be carried out, DNA must first be transcribed into RNA. The transcriptome is the result of this, similar to the reflection of a genome, but written in RNA base pairs rather than DNA.

This genetic information is different in different 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 knowledge of animal evolution, but some questions still prove difficult to answer.

Because Kamptozoa and Bryozoa are closely related to molluscs, annelids, and nemerthea, small errors in the data set or missing data may result in incorrect placement in the evolutionary tree. Also, while collecting these small animals, it is easy to pick up other organisms such as algae that contaminate the sample. Dr. Halturin emphasized that they were careful to avoid contamination and later screened their dataset for algae and small animal RNA to remove anything that might have come from them.

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

Using these transcriptomes, they predicted proteins and compared them with similar data for 31 other species, some of which were closely related to Kamptozoa and Bryozoa, such as clams and bristleworms, and others that were more distantly related, such as frogs, starfish, insects, and jellyfish. The high-quality datasets meant they could compare many different genes and proteins simultaneously. Dr. Halturin acknowledged the powerful computing capabilities that researchers have access to at OIST.

“Our main finding is that the two species are similar,” Dr Halturin said. “This result was originally proposed in the 19th century by biologists who grouped animals based on how they looked.”

While Dr Halturin said this question had already been answered to the best of his ability, he also stressed that the data set could answer other fundamental evolutionary questions – such as the more precise location of molluscs and tree ringworms of life and how life is diverse.

Reference: Khalturin K, Shunatova N, Shchenkov S et al. Polyzoa returns: The effect of complete gene sets on the placement of Ectoprocta and Entoproct. Sci Adv. 8(26):eabo4400. doi: 10.1126/sciadv.abo4400.

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