Exercise 6.1

Quantitative Genetics and the Response to Ocean Acidification

(This exercise is based on Sunday, J.M., R. N. Crim, C. D. G. Harley, M.W. Hart. 2011. Quantifying Rates of Evolutionary Adaptation in Response to Ocean Acidification. PLoS ONE 6(8): e22881.)

(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)

INTRODUCTION

Human activity has caused atmospheric concentrations of carbon dioxide to increase from a preindustrial level of 280 parts per million to the current level of 400 parts per million. Concentrations of carbon dioxide are predicted to continue to increase throughout this century and beyond. Some of the carbon dioxide generated by human activity has gone into the ocean in the form of carbonic acid. As a result, our oceans have and will continue to become more acidic. Largely unknown is how well marine life can respond, via evolution, to the changing acidity.

Jennifer Sunday and other researchers at Simon Fraser University investigated the effects of ocean acidification on growth rates of two marine animals: the bay mussel, Mytilus trossulus, and the red sea urchin, Strongylocentrotus franciscanus. Both of these species are valuable to the fishing industry.

The researchers took a quantitative genetic approach: results they obtained from breeding designs were used to estimate the amount of additive genetic variation present in a population from each species. For the mussel, each of ten males (sires) was crossed to the same four females (dams) to yield forty families. For the sea urchin, each of ten sires was crossed to the same ten dams to generate one hundred families.

Sunday and the other researchers then measured individuals from each family during early development under two treatments: (1) current levels of carbon dioxide concentration (low CO2) and water pH, and (2) expected levels of these conditions in the year 2100 (high CO2).

QUESTIONS

Use the information in Figure 1 to answer questions 1 through 8.

Figure 1 Results for M. trossulus. Variation among offspring from different sires is shown on the left, while variation among offspring from different dams is shown on the right. Means are represented by circles, with bars showing plus and minus one standard deviation from that mean. Light bars represent values obtained under low CO2 conditions, while dark bars represent values obtained under high CO2 conditions. The dotted and solid lines represent the study means for low and high CO2 treatments, respectively. Heritabilities are shown on the far right.

 

Question 1. Comparing the means for the low and high CO2 treatments, what are the direction and magnitude (in percent) of the effect on the size of the developing mussel from raising carbon dioxide levels/ lowering pH?

 

Question 2. Did the offspring from any sire show an effect in the opposite direction from the overall trend?

 

Question 3. What is the range of variation of the means across different sires for the low CO2 treatment? (Express as a percentage of the range divided by the lowest sire mean.)

 

Question 4. Is the range of variation of the means across different sires for the high CO2 treatment comparable to that seen in the low CO2 treatment?

 

Question 5. Did the offspring from any dam show an effect in the opposite direction from the overall trend?

 

Question 6. What is the range of variation of the means across different dams for the low CO2 treatment? (Express as a percentage of the range divided by the lowest dam mean.) How does this compare with the range of variation observed among sires?

 

Question 7. The researchers were able to estimate the additive genetic variance from the variance observed among sires. Given the additive genetic variance, how would they then obtain estimates of the narrow sense heritability?

 

Question 8. What did the researchers find regarding heritability of body size in the mussel?

Use the information in Figure 2 to answer questions 9 through 12.

Figure 2 Results for S. franciscanus. Variation among offspring from different sires is shown on the left, while variation among offspring from different dams is shown on the right. Means are represented by circles, with bars showing plus and minus one standard deviation from that mean. Light bars represent values obtained under low CO2 conditions, while dark bars represent the values obtained under high CO2 conditions. The dotted and solid lines represent the study means for low and high CO2 treatments. Heritability is shown on the far right.

 

Question 9. Comparing the means for the low and high CO2 treatments, what are the direction and magnitude (in percent) of the effect on the size of the developing sea urchin from raising carbon dioxide levels/lowering pH? How does this compare with what was observed for the mussel?

 

Question 10. What is the range of variation of the means across different sires for the low CO2 treatment? (Express as a percentage of the range divided by the lowest sire mean.) How does this compare with that seen in the mussel?

 

Question 11. Do the means for dams show more variation than the means for sires?

 

Question 12. How do the heritabilities (for both carbon dioxide treatments) compare with those values obtained in the mussels?

 

Use the information in the paragraph below and in Figure 3 to answer questions 13 through 15.

 

Slower growth means that the larval urchins and mussels develop more slowly, will stay in the plankton longer, and will be exposed to predation and other agents of selection. By looking at the growth rates of the urchins and the mussels at both carbon dioxide treatments, and by using known values for mortality of mussel and urchin larvae, the authors were able to estimate a function for how fitness declines with increased development time. From this, they were able to simulate selection on these animals.

Figure 3 Phenotypic variation in planktonic duration at high CO2 before and after simulated selection for (A) M. trossulus and (B) S. franciscanus. Variation in planktonic duration was approximated from variation in size-at-day for either species under high CO2. Frequency of phenotypes before selection (dark bars) and after selection (light bars) is shown.

 

Question 13. How do the means of the populations differ from the optimum?

 

Question 14. Which species shows a greater response toward the optimum after a single generation?

 

Question 15. What factors, other than the response to selection after one generation, will determine how well a population will respond to a changing environment?