Newswise — Teleost fish form a highly diverse group, and among them, seahorses exhibit a distinct morphology. The seahorse's unique features include characteristic spines and a brood pouch that contains specialized epithelial cells known as flame cone cells, which are covered by a mucous cap. Surprisingly, these flame cone cells are absent in their close relatives, the barbed pipefish Urocampus nanus, and the seaweed pipefish Syngnathus schlegeli, both belonging to the Syngnathidae lineage. Despite much research into the function of flame cone cells, their evolutionary origins have remained shrouded in mystery.
However, a recent breakthrough has emerged, thanks to a team of scientists led by Assoc. Prof. Mari Kawaguchi and Prof. Shigeki Yasumasu from Sophia University's Department of Materials and Life Sciences. In their study, they investigated the seahorse species Hippocampus abdominalis and made a fascinating discovery—a unique gene known as the proline-glycine rich (pgrich) gene. This gene appears to be an 'orphan' gene, meaning it lacks identifiable homologous sequences in other species or lineages. The researchers believe that the pgrich gene is closely associated with the development of the flame cone cells within the seahorse's brood pouch.
These groundbreaking findings were published in the prestigious journal Cell and Tissue Research on 25th May 2023. This discovery brings us one step closer to understanding the intricate mechanisms behind the seahorse's extraordinary traits and sheds light on the evolution of this peculiar lineage of fish.
Dr. Kawaguchi, the lead researcher, highlighted the compelling reasons for embarking on this investigation, stating, "Seahorses possess intriguing morphology, with male individuals carrying embryos in their brood pouch—an exceptionally rare phenomenon in the animal kingdom. This unique characteristic makes seahorses an ideal model organism for studying evolution. Our primary objective was to identify the specific genes responsible for the formation of flame cone cells within the brood pouch."
To validate their hypothesis, the research team adopted a two-pronged approach. Firstly, they employed histological staining combined with electron microscopy to confirm the presence of flame cone cells on the outer epithelium of the brood pouch in H. abdominalis, while these cells were notably absent in U. nanus and S. schlegeli, two closely related species. Building on this confirmation, they then utilized in situ hybridization and immunohistochemical techniques to demonstrate that the pgrich gene was actively expressed and its protein was specifically localized within the flame cone cells on the body surface.
This integrated methodology allowed the scientists to establish a clear link between the unique pgrich gene and the formation of flame cone cells, providing valuable insights into the distinctive reproductive biology of seahorses and shedding light on the evolutionary origins of these fascinating creatures.
The protein product of the pgrich gene, known as PGrich, displayed partial similarity with the translated amino acid sequence derived from the noncoding region of the greater pipefish elastin gene's antisense strand. Through meticulous sequence analyses, the research team made an intriguing discovery—they found numerous transposable elements surrounding the pgrich gene. Based on their findings, the team hypothesizes that the pgrich gene might have originated from the elastin gene in pipefish, and over time, it acquired a novel function in the development of the unique flame cone cells seen in seahorses.
As the team continues their efforts to unravel the evolutionary history of the seahorse brood pouch, Dr. Kawaguchi concludes, "Understanding the evolutionary journey of the pgrich gene could hold valuable insights into the emergence of orphan genes and shed light on the developmental process of the brood pouch within this lineage. Seahorses have captured the fascination of people worldwide, especially in home aquariums, and comprehending these remarkable phenomena will undoubtedly enhance our appreciation and wonderment for these captivating fish!"
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