Most life on earth depends on sunlight for energy. Flowers turn to face the morning sun; lizards bask to regulate their body temperatures; and humans rely on light therapy to cure their winter blues. Too much sun, however, can be harmful to animals, causing sunburn. We experience sunburn as pain, fatigue and red flaky skin, but biologically these are signs and symptoms of DNA damage from ultraviolet radiation. To combat these stresses, some animals such as cyanobacteria and sea urchins use natural sunscreen products produced by algae and microbes. A closely related transparent compound, gadusol, was discovered over 40 years ago in fish eggs, but where it came from and whether it worked to protect the fish from the sun's rays remained untested. This led Marlen Rice and colleagues from University of Utah, USA, to investigate gadusol's role as a maternally provided sunscreen in zebrafish.
First, to show that gadusol is produced by mother fish and can protect embryos effectively from sun damage, the researchers created mutant zebrafish by chopping out the eevs gene, which is essential for making gadusol. The embryos produced by the modified fish lacked maternally provided gadusol, and when the team measured the effect on swim bladder inflation – a hallmark of healthy development essential for survival – they found that all of the embryos that lacked gadusol failed to inflate their swim bladders at 5 days of age, and only 2% of larvae survived to 28 days. The fish lacking gadusol were extremely vulnerable to sun damage. The researchers next exposed the modified fish embryos to ultraviolet light and checked for the presence of damaged DNA. The embryos lacking gadusol suffered more DNA damage, further demonstrating gadusol's effectiveness as a sunscreen. Moreover, Rice and colleagues found that the embryos lacking gadusol had activated more genes that provide protection from stress than the embryos that had inherited sunscreen from their mothers.
Next, the team compared gadusol with other potential sunscreens, namely the skin pigment melanin and the nearly transparent shell of the zebrafish's egg, the chorion. To test the role melanin plays in providing protection from the sun, they created a mutant zebrafish by removing the mitfa gene, which disrupts a key step in melanin production. The fish without melanin still had far higher survival rates than those lacking gadusol. And by carefully removing the transparent shell of the egg using forceps, the researchers discovered it provided some UV protection, but the eggshell wasn't nearly as effective as gadusol, suggesting that gadusol is the primary, most effective sunscreen in early fish development.
However, natural sunscreens are not unique to zebrafish, so the researchers looked at the genomes of 136 fish species – including many species whose lifestyles protect them from sun damage – to see whether these fish had lost the genes involved in gadusol production. They concluded that gadusol production had been repeatedly lost in several species that live in habitats unpenetrated by sunlight such as caves and the deep-sea, as well as in fish that have live babies such as the greater pipefish and fish that use electroreception to navigate murky waters such as the electric eel. Fish moms make a natural sunscreen for their young that is very effective at preventing DNA damage, but evidently if you don't use it, you may lose it.
