Paired fins were a key novelty in the evolution of vertebrates, ultimately allowing paired appendages to arise that help lizards to scamper, birds to fly, and humans to stand upright and use tools in their hands. Now analysis of lamprey, shark, and salamander embryos could shed light on how paired fins arose, findings appearing online this week in the Proceedings of the National Academy of Sciences.
There are two leading concepts for how paired fins evolved. One is that they developed from gill arches, bony hoops that support the gills. The other is that they arose from a continuous single fin that once encircled fish, similar to median fins seen in the embryos of fish and amphibians. A number of recent studies revealed that several of the genes underlying the formation of paired fins also help guide the development of both gill arches and median fins, supporting both concepts for how paired fins evolved.
To shed light on the origin of paired fins, scientists analyzed the embryos of lampreys, jawless fish that diverged from other vertebrates before the evolution of paired fins. They focused on tissue known as the somatopleure, from which paired appendages develop as outgrowths.
Researchers knew that in animals such as reptiles, birds and mammals, the somatopleure helps form the body wall, the outer part of the body that encloses the body cavity. However, it was uncertain if the somatopleure behaved similarly in fish and amphibians, the vertebrates that developed the first paired appendages.
Evolutionary morphologist Ann Burke at Wesleyan University and her colleagues investigated the sea lamprey (Petromyzon marinus) and the Japanese lamprey (Lethenteron japonicum). They compared the embryonic development of both these jawless fish to jawed animals — a shark, the catshark, and a salamander, the axolotl.
“The main difficulty with our research was with working on animals that are not standard model organisms,” Burke says. “The lampreys only spawn once a year, so we only have a very small window of time to collect embryos and do experiments on them before we have to wait another year.”
Analysis of lamprey embryos revealed the somatopleure gets eliminated very early in embryonic development. In contrast, in both the catshark and axolotl, the somatopleure persists well into embryonic development to help make up the body wall much as it does in other animals with paired appendages.
The researchers suggest the quest to discover the origin of paired fins might perhaps focus on the first steps of how the somatopleure came to persist in the body wall. For instance, seeing if the somatopleure persists longest near the gills or near the flanks of embryos during their development could yield clues as to whether paired fins might be linked with gill arches or median fins, respectively. “By looking at the first steps of development of these paired fins, we may get insights into how they first evolved,” Burke says.