Science Friday: What is introgression?

For this edition of Science Friday, we continue to define terms used in studies that evaluate hatchery steelhead (and often other species of salmon and trout). Last week we defined fitness and discussed how it is measured and why it is important to understanding the biology of steelhead. This week we take another step into the deeper end of the pool and focus on introgression.


What is introgression, how is it measured and why is it important? It is a term that can be easily misconstrued, yet is critical to understanding genetic interactions between wild and hatchery steelhead.



From a biological perspective, introgression is the transfer of genetic information – such as genes or alleles – from one population to another. Genes are the basic units of heredity that determine characteristics of individuals, some of which are easily seen, including eye and hair color in humans. Think of them like little pieces of luggage that carry around important information about our physiological, mental and behavioral development. Alleles are simply variants of the same gene, which helps explain why some people have blue eyes and others have brown eyes.


A fundamental question for those tasked with managing and monitoring steelhead is: What is the level of genetic interaction between hatchery and wild steelhead in a given population? How much hatchery “baggage” is found in wild fish? Scientists accomplish this by collecting DNA from numerous individuals, often over several years. They use specific genetic markers that distinguish the hatchery and wild steelhead. Introgression is measured by identifying and enumerating the number of wild individuals that contained some hatchery genes or alleles. Wild populations with a high frequency of hatchery genes are considered to have a high level of genetic introgression, and vice-versa when no or few hatchery genes are found.


Measuring introgression is valuable for answering a number of questions.  In some cases steelhead managers may need to understand the degree to which hatchery fish are contributing to the abundance and distribution of wild populations. In other situations they might want to know whether high levels of introgression are correlated with changes in fitness of the wild population.


While introgression is important, it does not necessarily tell us about all of the potential genetic effects of mating interactions between hatchery and wild fish. Why? Because a proportion of hatchery x wild offspring don’t survive, which is probably somewhat common since hatchery steelhead tend to have lower fitness than wild steelhead. For example, some genetic studies sample adults. However, if the offspring don’t survive to adulthood it is impossible to measure introgression because the genes are not being successfully passed along. This can result in erosion of genetic diversity.



Hence, just because introgression levels are low for adults does not mean there have not been genetic effects, which is another reason that scientists will often — though not always — sample fish across a variety of life stages, from young-of-the-year to smolts to adults. This type of information will be crucial to understanding how interbreeding between hatchery and wild steelhead influences survival at different life stages and the extent to which that genetic information is passed on to the next generation of wild fish.