SOLVING EVOLUTIONARY MYSTERIES WITH STICKLEBACK FISH
Scientists have been puzzled by a question for centuries: how are genes regulated? They discovered the answer in a freshwater lake.
The stickleback’s fish ancestors first lived in the ocean. Stickleback fish were later isolated into separate colonies caused by geological changes about 10,000 years ago, nearing the end of the Ice Age. In these new environments, the fish adapted to the environmental conditions, which led to an evolutionary change. Scientists pondered, what mechanisms happened inside the organism that allowed the fish to evolve in their new environment?
First, scientists observed the differences from the marine stickleback fish compared to the evolved freshwater fish. The freshwater fish got smaller, their color changed, and most strikingly, their skeletons changed. In the marine environment the main predators were large fish while in the freshwater the main predators were dragonfly larvae. So, in the marine environment pelvic spines were beneficial since the main predators were large fish, which struggled to swallow the bony pelvic spines – while in the freshwater environment the main predators were dragonfly larvae, which caught the sticklebacks by latching onto their spines. In the new freshwater environment the pelvic spines actually reduced fitness and lessened the fish's chances of survival and reproduction since they were easier to be caught by the environment’s predators. After further observing the freshwater sticklebacks, scientists saw that they did not have pelvic spines, unlike their marine ancestors. The question still remained: how did such a dramatic change occur?
Changes in form are due to changes in development, and changes in development are due to genes. To search for answers to their questions, scientists looked into the stickleback’s genes. The marine and freshwater fish look different, yet they have identical genes. They looked into the DNA markers of the genes and found that the gene that codes for pelvic spines in sticklebacks is located on the seventh chromosome in the Pitx1 gene. They compared the Pitx1 nucleotide coding sequence of the marine and freshwater fish, finding the codes to be identical. Now the scientists were left with a new question: if the code is identical, then what caused the fishes to have differing structures?
Next, they put blue dye in the Pitx1 gene of both fish to try to find an answer to their new question. They discovered that in the marine stickleback there was blue dye by the pelvic spine, showing that the gene had been expressed, but in the freshwater stickleback there was no blue dye by the pelvic spine. This now proved that though the genes are the same, the expression of the genes is significantly different. This is how scientists discovered regulatory switches.
Regulatory switches are essentially “on and off” switches which control whether the gene is turned on and expressed or turned off and not expressed. In marine sticklebacks the Pitx1 switch was turned on, allowing for the gene to be activated and producing pelvic spines. Since the pelvic spines were not produced in the freshwater sticklebacks, this meant that there was a mutation in the regulatory switch, causing the gene not to be expressed. The freshwater sticklebacks had deleted the pelvic spine regulatory switch, helping them gain a new advantage in their environment.
This new discovery was made by scientists David Kingsley, Michael Bell, and the rest of their team at Howard Hughes Medical Institute. The major impact of this discovery wasn’t just finding out that regulatory switches can be mutated, changing the fish's structure without changing its DNA -- the study also implied that the regulatory switch controls gene expression that can be applied to all species of animals. Scientists now took the findings from these stickleback fish and uncovered the mystery of how certain species changed and evolved without their DNA changing. The stickleback fish answered the question that had been puzzling scientists, and now allowed for more clarity in the complex world of genetics and evolution.
1. Adams, Amy. Stickleback fish study uncovers evolutionary secrets. Stanford Report, 2004, https://news.stanford.edu/news/2004/april21/fish.html#:~:text=Stickleback%20fish%20study%20uncovers%20evolutionary%20secrets&text=Whales%2C%20snakes%20and%20some%20lizards,bring%20about%20this%20evolutionary%20change
2. Biointeractive. “The Making of the Fittest: Evolving Switches, Evolving Bodies.” HHMI BioInteractive, 16 October 2012, https://www.biointeractive.org/classroom-resources/making-fittest-evolving-switches-evolving-bodies. Accessed 7 March 2022.