Both genes and the environment contribute to most phenotypes. This complicates studies of local adaptation: local adaptation occurs when genotypes found in the "home" environment tend to outperform genotypes from an "away" environment. However, because the environment also influences most phenotypes, any phenotypic differences observed among populations cannot be attributed to genetic differences, without first controlling for environmental differences. In this experiment, you will test for differences among tide pools in the heat tolerance of the copepod Tigriopus californicus,
both before and after controlling for environmental effects on this trait.PART 1: Initial thermal tolerance measurements
In the field:
from the two shallowest/sunniest and two deepest/ most shaded pools where you can find them in the splash zone of the intertidal. For each pool collect 3 half-full 1 L containers, each containing at least ~100 individuals. Place a datalogger at the bottom of each pool, set to record hourly temperature measurements.
In the lab:
1) Measure heat tolerance by placing indivudual female Tigriopus
in pcr tubes half filled with filtered seawater. Test nine females per pool per temperature (1 hour exposure) at a range of temperatures extending from the 'all alive' to 'all dead' temperature, at 0.2˚C intervals. I suggest 34.8, 35.0, 35.2, 35.4 and 35.6˚C. But you should test the upper and lower temperatures first to see if you need to extend the range. Allow 24 hours recovery before assessing survival. Record number alive and dead in this spreadsheet,
then use the R-scripts found in Data Analysis and Tools,
to test for differences in heat tolerance among pools and to estimate LT50 temperatures.
2) Maintain each container of copepods at 19˚C for 3 weeks, feeding twice per week (Dr K will show you how much)PART 2: Thermal tolerance after heat shock vs. common garden acclimation