‘Living Fossil’ Evolves Faster Than Previously Thought

The genome of the brachiopod Lingula anatina sheds light on how different branches of the animal kingdom converted sort tissue to hard structures.

AsianScientist (Sep. 23, 2015) – Scientists have sequenced the first genome of a lingulid brachiopod, Lingula anatina. Their paper, published in Nature Communications, presents the results of their analysis of over 34,000 genes comprising the L. anatina genome and sheds light on the process of biomineralization, which helped brachiopods dominate the seas during the early Cambrian period.

Brachiopods are marine invertebrates with external shells and a stalk. They are often confused with molluscs; however, the resemblance is superficial. Unlike bivalves—clams and mussels—that have shells on the sides of their bodies, brachiopod shells are on the top and bottom.

Brachiopods are one of the first known examples of animal biomineralization—a process whereby living organisms stiffen or harden tissues with minerals. The earliest discovered brachiopod fossils date to the early Cambrian period, approximately 520 million years ago. Brachiopods quickly spread all over the world and dominated the seas during the Paleozoic era (542-251 million years ago) and, by virtue of their mineralized shells, left an abundance of fossils.

The evolutionary origin of brachiopods and their relations to other species are still unclear. For years, scientists have been debating the phylogenetic position of brachiopods and molluscs, as well as their affinities for other animals in the same group, the Lophotrochozoa, comprising segmented worms, clams, oysters, snails, squids, and so on.

The phylogenetic analysis of the Lingula genome indicates that brachiopods are close relatives to molluscs, and more distant cousins to segmented worms; however, their relations to other lophotrochozoans still require further investigation.

The position of Lingula in the evolutionary tree. Credit: OIST.
The position of Lingula in the evolutionary tree. Credit: OIST.

“At the molecular level, brachiopods are very similar to molluscs. Both are protostomes—their embryos form mouths first and anuses thereafter. However, brachiopod embryonic development is very different from that of molluscs: it resembles that of deuterostomes, in which embryos form anuses first and mouths second,” says Mr. Luo Yi-Jyun, the first author of the paper, from the Okinawa Institute of Science and Technology Graduate University (OIST).

“The results of the Lingula genome project will help future research of these differences and the roles that specific genes play in development of various brachiopod body structures.”

Lingulid brachiopods had changed so little in appearance since the Silurian period (443-419 million years ago) that Darwin referred to them as ‘living fossils.’ However, unlike more famous ‘living fossils’ such as coelacanths which have the slowest molecular evolutionary rate among vertebrates, the Lingula genome has been evolving rapidly, despite the lack of changes in appearance.

Shells of fossilized and living Lingula show considerable diversity in chemical structure. Analysis of the soft tissues of fossils also suggests morphological changes among lingulid brachiopods. The authors of the paper also found significant changes in the genomic structure and gene families, contradicting the idea of a genuine ‘living fossils.’ Interestingly, the Lingula genes associated with basic metabolism show the slowest evolutionary change among lophotrochozoans.

One of the great mysteries of animal evolution is that vertebrates and Lingula, although evolutionarily distant, both use calcium phosphate and collagen fibers for biomineralization. However, genomic scale comparisons show that Lingula lacks genes for bone formation and has different types of collagen fibers. This study indicates that Lingula and bony vertebrates have evolved independently and employ different mechanisms for hard tissue formation. It is an interesting example of parallel evolution.

Professor Noriyuki Satoh, the head of the OIST Marine Genomics Unit and the last author of the paper, said, “This is one step toward untangling the mysteries of animal evolution. The study highlights the fact that various animals have taken evolutionary paths independently from one another.”

The article can be found at: Luo et al. (2015) The Lingula Genome Provides Insights into Brachiopod Evolution and the Origin of Phosphate Biomineralization.

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Source: Okinawa Institute of Science and Technology.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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