It's no secret that who we are has a lot to do with our parents (genes) and how we were raised (environment). Add on a layer of biological gender, and you're sure to have a unique individual: it's fascinating to consider that much of what defines us is determined long before we're born. That's because environment and genes can interact to influence critical processes such as brain development, and this shapes individuals through neuroplasticity. In humans, for example, if a mother is extremely stressed during pregnancy, her baby is more likely to suffer from mental health issues as an adult, and the nature of these effects are often sex-specific. But does the same hold true for egg-laying animals, such as fish, where the eggs develop outside of the mother's ‘environment’? David Metzger and Patricia Schulte, from the University of British Columbia in Canada, asked this question in their recent study published in Proceedings of the Royal Society B. They wanted to know what effect a mother fish's experiences have on her offspring's brains, and whether these effects are different in sons and daughters.
Metzger began his experiments by stressing threespine sticklebacks. Over a 2-week period, he chased and air-exposed these fish for 1 min every day, which increased their stress hormone levels. He also left some fish undisturbed during these 2 weeks for his unstressed group. Metzger then made several half-sibling crosses, where he used sperm from the same unstressed father fish to fertilize eggs from one stressed and one unstressed mother fish. These half-siblings differed only in their mother's genes and her experiences. Metzger spent a full year carefully raising the offspring under identical conditions before checking to see how their brains differed.
Using a technique called RNA-seq, Metzger identified and quantified essentially all of the genes being expressed in the brains of the offspring. He compared the gene expression data of male and female half-siblings against the backdrop of ‘Mom’ and found that maternal stress had a big effect on neuroplasticity. Even more striking, Metzger also found that sons and daughters of stressed mothers tended to have opposite changes in gene expression, resulting in profoundly different neural phenotypes. For example, the expression of genes involved in cellular metabolism and protein synthesis increased in daughters and decreased in sons of stressed mothers; whereas the expression of genes for neural development and synapse formation increased in sons and decreased in daughters. And don't forget that the main factor driving these changes – maternal stress – happened while these fish were merely un-spawned eggs, and a whole year had passed since then. This study by Metzger and Schulte offers compelling evidence that even in egg-laying animals, environmental factors that occur at the earliest stages of development have a dramatic and unique effect on sons and daughters. So neuroplasticity might just be one word made up of six syllables, but it adds up to some really exciting science!