What species of fish do these eggs belong to and why is the coloration so distinct?
These are unfertilized eggs from Chinook Salmon Oncorhynchus tshawytscha from the Quesnel River, British Columbia. The distinct difference in color is due to genetic polymorphisms that affect deposition of carotenoids, naturally occurring red pigments. Variation in deposition results in two distinct color “morphs”: red and white Chinook Salmon. Although both morphs incorporate red carotenoid pigments into their diet by eating animals like krill and squid, they differ in their ability to deposit the pigments into their tissues. Their eggs are different colors as the photograph shows, but they also differ in flesh color and external spawning coloration. Carotenoids are thought to be very important to salmon, because they are powerful antioxidants; therefore, it is interesting that the white phenotype continues to exist in nature.
How did you end up studying this phenomena?
I started studying Chinook Salmon for my M.Sc. with Daniel Heath (GLIER). My M.Sc. focused on understanding the potential impacts of farmed salmon escapes on wild salmon populations. Through this research, I spent a lot of time at Yellow Island Aquaculture Ltd. (YIAL), which is an organic Chinook Salmon farm in British Columbia. YIAL was a great place to do research because we could design largescale experiments and monitor Chinook Salmon over their entire life. During this time, I connected and collaborated with academic, government, and industry researchers, which led to the opportunity for my Ph.D. project. For my Ph.D., I was interested in continuing to study Chinook Salmon, but I wanted to get experience working with a wild population and answer big-picture, evolutionary questions. The opportunity to study the importance of carotenoids to salmon really excited me, and it is something that my supervisors (Daniel Heath and Trevor Pitcher, GLIER) and our collaborator (Robert Devlin, Fisheries and Oceans Canada) have been determined to understand for a long time.
What was your reaction to seeing your first white-morph Chinook Salmon?
Before I started my first field season, I was quite nervous that it might be difficult to tell red and white Chinook Salmon apart. I wasn’t sure if Chinook Salmon truly existed as these two color morphs that I had read about. But luckily, they did! Most of the Chinook Salmon that I’ve caught in the Quesnel River are either very pigmented (red) or gray (white) in external spawning color. I was very excited the first time that I saw a white Chinook Salmon in the wild. It was surreal to see this different salmon morph in person! Although white Chinook Salmon don’t have the characteristic red coloration that we know and love in Pacific salmon, they are still impressive in a different way. Some of the white Chinook Salmon seem to have a hint of blue coloration during spawning. Also, of all the Chinook Salmon that I’ve caught in the Quesnel River, the biggest ones tend to be white.
What is the proportion of white-morph individuals in a given population?
It’s thought that around 10% of all Chinook Salmon in the world are the white morph. The proportion of white-morph individuals within a river system varies across populations and ranges from 0% to 100%. In British Columbia, the Harrison River in the Lower Fraser watershed represents a population that is 100% white, whereas a lot of populations on Vancouver Island are composed of only red individuals. Chinook Salmon in the Quesnel River are 50% red and 50% white, which is why I study this population. By having equal proportions of red and white Chinook Salmon, I am able to design and carry out experiments that examine the genetic and fitness differences between the morphs.
Who are the stakeholders interested in flesh color?
Enhancing flesh color is a major goal of the aquaculture industry as consumers associate the color of a salmon fillet with its quality. Flesh color is not only an economically important trait, it is also an evolutionarily important trait. The degree of pigmentation can vary between individuals within a species (e.g., the red- and white-morph Chinook Salmon that I study), as well as among species (e.g., Sockeye Salmon O. nerka have the highest carotenoid content relative to other Pacific salmon, whereas Chum O. keta and Pink O. gorbuscha salmons have the lowest carotenoid content). This variation may reflect evolutionary adaptations related to different environmental conditions and/or life history strategies. My research is contributing to our understanding of salmon evolution, as well as benefitting the aquaculture industry.
Is flesh color heritable?
Yes! Previous work done by Ruth Withler (Fisheries and Oceans Canada) on Chinook Salmon in the Quesnel River showed that there were likely two major genes that controlled flesh color in this species. Withler’s research suggested that each flesh color gene has two alleles, an allele coding for the red morph (R allele, dominant) and an allele coding for the white morph (W allele, recessive; Withler 1986). Withler found that a Chinook Salmon needs to have at least one red allele at both of the color genes to be red (e.g., R1W1; R2W2 = red salmon).
If a Chinook Salmon only has one red allele at one of the two genes, it will be white (e.g., R1W1; W2W2 = white salmon). Determining the exact genotype of each color morph can be complex as different combinations of the alleles can produce either the red or white morph. A mating between a red and a white Chinook Salmon can produce different proportions of red and white offspring depending on the genotype of the parents.
What remains a mystery regarding pigmentation in salmon?
Why salmon evolved to use and deposit carotenoid pigments is still a mystery. My Ph.D. dissertation is providing answers to a number of questions to help solve this mystery. In populations where red and white Chinook Salmon coexist: Does pigmentation (carotenoids) provide salmon with benefits in terms of mating success? Does the maternal provisioning of carotenoids increase egg survival and offspring immune function? Are red eggs more noticeable to predators compared to white eggs? Where are the color genes located in the salmon genome (i.e., on which chromosomes)? What evolutionary mechanisms are responsible for the maintenance of the red/white color polymorphism in nature?
What parts of the red/white mystery have you unraveled so far?
So far, we have found that in the Quesnel River, red and white Chinook Salmon do not mate assortatively based on color (Lehnert et al. 2016). This finding tells us that color assortative mate choice is not the evolutionary mechanism that
Lehnert, S. J., T. E. Pitcher, R. H. Devlin, and D. D. Heath. 2016. Red and white Chinook Salmon: genetic divergence and mate choice. Molecular Ecology 25(6):1259–1274.
Withler, R. E. 1986. Genetic variation in carotenoid deposition in the ed-fleshed and white-fleshed Chinook Salmon (Oncorhynchus tshawytscha) of Quesnel River, British Columbia. Canadian Journal of Genetics and Cytology 28(4):587–594.