How fish got their spines

Two males from the cichlid fish Astatotilapia Burtoni (the model organism used in Hoech et al.'s study of spine and soft-ray development) Credit: Joost Woltering The movie "A Fish Called Wanda" shows Otto as a villain who effortlessly eats all the fish in Ken's tank. However, reality is much more difficult. One fan, at least, was admitted to hospital with a stuck fish in his throat. This was also a painful lesson in Ichthyology (the scientific study on fishes), namely that some fishes can be protected by their fin spines. Many fish species have two types of fin elements. There are the 'ordinary' soft fin Rays which are flexible and blunt and are used primarily for locomotion. And there are fin spines which are sharp and highly ossified. The fin spines make the fish more difficult to eat, which is a strong evolutionary advantage. The most diverse fish lineage, with over 18,000 members. These fishes have even developed separate "spiny fins" that are made up of only spines. The evolution of fin spines is a key factor in the diversity and success of fishes. Researchers from the University of Konstanz, led by Joost Woltering and Rebekka Hchworks, first author of the study in PNAS, have published a study that shows how fin spines develop during embryonic development. The researchers also discuss how the different lineages of fish could have their own ancestral soft-rays that can evolve into spines. The study is focused on the Astatotilapia Burtoni, a model species of spiny-rayed fish. It has well-developed soft-rayed fin parts and spiny spines. Different development genes for soft-rays and spines The team first determined the genetic profile of spiny and soft-ray fins in embryonic development. Hch says, "What we saw in these first experiments was that a group of genes we knew from fin and joint development become differently activated in spines than soft-rays." These genes are called master regulator genes. They have been shown to influence morphology in both the limb and skeleton. These genes are thought to be responsible for determining whether fin elements emerge looking like a spine, or soft-ray in fish fins. Soft-rays can transform into spines, and vice versa The team then identified the genetic pathways that activate these master regulator genes, and which determine their activity at various locations across the fins. "Importantly we were able address the roles of those pathways using chemical tools so-called inhibits and activators as well as CRISPR/Cas9 gene scissors to test how spiny or soft-rayed fin domains can be established during development," Woltering, an assistant professor in Biology at the University of Konstanz, and senior author of this study. The scientists were able alter the number and soft-rays of the fins through their experiments. The BMP (bone-morphogenetic protein signaling) was modulated to produce the most dramatic results. Woltering says that we saw changes in activation of master regulatory genes as well as so-called homeotic transforms in which soft-rays became spines or vice versa. Another observation was that the morphology and coloration of the fins changed in these fish. "Male Cichlids have bright yellow spots, but they are limited to the soft-rayed portion. Woltering says that we found that the fins also changed from a soft-ray to a spine when the soft-ray became a spine. This observation shows that spines and soft-rays in spiny-rayed fish are integral parts of a larger developmental program that determines many of the fin's visible features. Different fish lineages use the same principle The team realized that the evolution of the spiny fin had resulted in a redeployment of a highly conserved patterning system as they worked together to piece together the puzzle. "In fact, the genetic code which determines where spines will appear in the fin domain is active even in fins without spines. This suggests that an ancestral genetic pattern has been rearranged to make spines," Hch says. The authors used this new insight to study fin patterning in catfish. This is a group of fish that has evolved their own fins. The genetic code for spines in the catfish was identical to the one found in the cichlid. While there are some differences between spiny fish species it indicates that there is an evolutionary advantage to a conserved fin pattern. Next steps The team will continue to research the genes that are downstream of the spine and soft-ray control gene genes. This will allow them to discover how they affect fin morphology and control ossification as well as cellular growth pathways. Woltering concludes, "In the end we want to gain better understanding of how new anatomy structures arise that make certain species more successful and how this contributed the amazing evolutionary diversity of the fish linesages." Learn more about how the pufferfish got its bizarre spines Additional information: Rebekka hch et al., "Spiny-rayed fins in acanthomorphs are established through a BMP–gremlin–shh signaling system," PNAS (2021). Information from the Journal: Proceedings of National Academy of Sciences Rebekka Hch and colleagues, "Spiny- and soft-rayed Fin Domains in Acanthomorph Fish are Established Through a BMP–gremlin–shh Signaling Network," (2021). www.pnas.org/cgi/doi/10.1073/pnas.2101783118

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