Pea aphids, Acyrthosiphon pisum, can be red or green. Weirdly, red aphids make carotenoids (red pigments) with genes that jumped from a fungus into the aphid genome some time during recent evolutionary history. What’s more, some red aphids start out red and then change to green later in life.

Observation suggested that color changers were infected with a bacterium, Rickettsiella. To test whether Rickettsiella was the cause of color change, Tsuchida et al. (2010) experimentally injected the bacterium into a sample of red aphids. The data below are color measurements of genetically identical and bacteria-free red aphids that were either uninjected (original), injected successfully with Rickettsiella (infected), or injected but the bacterium failed to establish (uninfected). Color was measured as hue angle in degrees; for these data, small angles indicate red, whereas larger angles represent green.

a. Show the data in a graph. What trend is suggested?

b. By eye, describe how the data might depart from the assumptions of ANOVA.

c. The data were analyzed using a Kruskal–Wallis test. What are the null and alternative hypotheses for this test?

d. The results of the test were as follows: H=21.1. What is the conclusion?

e. H is not calculated directly from the original measurements. What does it use instead?

f. Under what assumption are we able to use the results of the Kruskal–Wallis test to conclude that the means differ among the three groups? Is this assumption met here?

Transcribed Image Text:

Rickettsiella- free strain Rickettsiella- transfected strain 0 day old 6 day old 11 day old