This week, I’m going to discuss a cool paper that came out of Dolph Schluter’s lab in 2008. The paper zooms in on a particularly interesting part of stickleback evolution, the transition between an ancestral marine form that breeds in fresh water to a population that lives in freshwater year-round.
Usually, (and this is one of the “color-coded for your convenience” things that make stickleback a fantastic model system) you can get a good idea where a stickleback is from by looking at its armor plates. Stickleback from marine habitats tend to have a full complement of plates, whereas sticklebacks from freshwater habitats will have few to no plates:
The authors sorted through hundreds of marine stickleback to find fish that had intermediate numbers of plates, which signified that they were heterozygotes for the gene that governs plate number, Eda. These fish were placed in experimental ponds and allowed to breed. Because the fish were heterozygotes for Eda, they produced offspring with high, medium, and low plates, which gave the authors a chance to observe if natural selection favored the low-plated form in freshwater.
In each pond, the frequency of the low allele increased over time, and in a similar way. There was a slight dip when fish were very young, but then frequency increased until the fish reached breeding condition. Interestingly, fish carrying the low allele grew faster and reached breeding condition sooner than fish carrying the high allele, probably because building armor plates takes energy that could be spent on growing more quickly.
The story is more complicated than that, though – not only is there a period early in life where the high allele appears to be favored, but there is also a point where fish with intermediate plates have the highest fitness, which is difficult to explain. The authors raise the possibility that the Eda gene that controls plates in stickleback may affect other traits (pleiotropy). Either way, it looks like even the most well-understood stickleback phenotype has more to tell us.
Barrett, R., Rogers, S., & Schluter, D. (2008). Natural Selection on a Major Armor Gene in Threespine Stickleback Science, 322 (5899), 255-257 DOI: 10.1126/science.1159978