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Unparalleled Reproductive Methods and Developmental Mechanisms of the Annelid Silicea
Toru Miura (Professor, Department of Biological Sciences / Misaki Marine Biological Station)
The earth is home to a wide variety of organisms,each of which has evolved life history strategies suited to the environment in which it lives.
Among them, there are many unusual ones that are unthinkable according to our common sense of biology.
The marine environment is an unknown world, especially for us humans who live on land.
In addition to the discovery of new species, there are many unknown life phenomena even among known species.
In the annelid Silis, another head is formed in the middle of the body during growth.
This is a phenomenon unthinkable according to conventional embryological knowledge.
We have clarified what kind of developmental control mechanism is responsible for this phenomenon.
At Misaki Marine Biological Station, we are studying the evolutionary process of animals, especially how innovative traits were acquired during evolution, from a developmental perspective. Some multicellular animals (metazoans), such as jellyfish, have a radiosymmetric system, but there is no end to the morphological diversity of animals that have a head-tail axis and are left-right homologous. As the term "head-tail axis" implies, there is usually one head and one tail per individual. However, there are some animals that have "another head" in the middle of their body. This is the case of the "Cirrhis," which I will introduce in this article.
Cirrus is a member of the family Cirsidae, which is a phylum of annelids belonging to the polychaete order. The life history of the ground squirrel is extremely unique. The individual-like part that is torn off and swims out is called a "stolon," and this process is called "stolonization. It is common knowledge that only one head is formed at the most anterior end during embryogenesis in left-right sympatric animals, but in Cirsium, an "additional head" is formed in the middle of the body, which is an extremely rare occurrence.
We first examined the morphology and histology of this extremely interesting phenomenon in detail to determine what kind of biological events (e.g., developmental processes) are taking place, and then analyzed the expression patterns of important genes involved in determining and forming the head and body axis in detail. We chose the Green Anthias Megasyllis nipponica, which is widely distributed in the seas around Japan and for which breeding methods have already been established. In many animals, homeotic (Hox) genes, important developmental regulators, are expressed in a specific order along the anterior-posterior axis. This determines the identity of body parts (i.e., which part becomes what). Thus, in stronization, we predicted that the pattern of Hox genes along the anterior-posterior axis would be duplicated, resulting in a form similar to two individuals connected in series. However, the expression pattern of Hox genes was found to be unchanged during this process. On the other hand, the "head formation factor" gene, which is responsible for forming the head, was shown to be expressed in the planned head region of the stolon. This indicates that the stolon is a unit specialized for reproduction (spermatogenesis and egg release), as if a head were suddenly attached to the tail, rather than a complete unit.
Individual (female) of Midori silis with well-developed stolons. The posterior end of the body has developed stolons filled with numerous eggs. Eye spots, antennae, and swimming bristles are developed for swimming and reproduction.
There are still many unresolved questions such as how the expression of head formation factors is induced in the back of the body during the reproductive cycle in the life history, what is the endocrine regulation, and how sex (testis development versus ovary development) is determined. Together with my students, I hope to unveil the mysteries of these strange creatures, which are abundant along the coastlines surrounding Japan.
The results of this research were published in Scientific Reports, 13, 19419, (2023) by M. Nakamura, et al.
This work was published in M. Nakamura, et al., Scientific Reports, 13, 19419, (2023).
(Press release, November 22, 2023)
Published in The Rigaku-bu News, March 2024
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