I’m Looking Over a White-Striped Clover by Evarts, Krufka, and Wilson

Part II—Unlucky Clover

Some variants of white clover produce cyanide (CN), which is a powerful poison. Two gene products are required to produce active cyanide. One gene encodes an inactive cyanide-sugar complex that is stored in the plant cell’s cytoplasm. The other gene encodes an enzyme that cleaves the sugar to activate the cyanide. This enzyme is stored in the cell wall. In general, striped clover contains cyanide; plain clover does not.

In consistent freezing temperatures, plant cell membranes (surrounding organelles and the cell itself) can burst. This is why the parts of plants above ground die back in colder climates. Root cells, however, are less likely to burst because they are underground and often store sugars, which protect the cell from freezing (just like antifreeze). This allows perennial plants to survive and grow again in the spring. Like the damage caused by freezing, herbivores can also damage plant cells. In the process of eating a leaf, herbivores destroy the membranes and organelles of the cells that make up the leaf.

Exercise 2

Why do you think the two gene products are stored in different parts of the cell?
Suggest at least two ways these products might come together to make active CN in nature.
Suggest a reason that clover may produce cyanide. That is, what advantage does a plant gain by producing cyanide? Also suggest a possible disadvantage of producing cyanide. Or might there be no advantage?
It takes energy for an organism to produce a particular structure such as a stripe on a clover leaf that is otherwise plain. Why might cyanide-producing clover produce striped leaves?
To explore this idea a little further, consider the following results of the hypothetical experiments shown below. In each situation, snails that have been taken from a wild habitat where both types of clover are present were put in a Petri dish containing varying types of clover. How would you interpret each result?

Table 2
Clover presented to snails	Snail response	Interpretation
plain leaves	eaten
striped leaves and plain leaves	plain leaves eaten
striped leaves	not eaten
plain leaves painted with white stripe	not eaten

Exercise 3

To understand why cyanide producing/striped clover is found at a higher frequency in North Carolina than in Minnesota, you must consider the “fitness” of each variant in the different habitats available in the two states. Fitness is determined by the ability of an organism to survive, grow, and reproduce in a particular habitat. You have probably heard the term “survival of the fittest,” but if an organism is not able to also grow and reproduce, it will not be able to pass any of its alleles (genetic information) on to its offspring. An organism that has high fitness does well in its habitat and passes those favorable alleles onto its offspring when it reproduces.

Go back and review the habitats you described in Exercise 1 and think about the factors that would be important for plant fitness. Then list the ecological differences between North Carolina and Minnesota that might affect the fitness of each variant. In other words which factors might increase plant growth, survival, and reproduction in each habitat, and which factors might inhibit them?

Go to Part III—“Investigating Clover Distribution”

Originally published at http://www.sciencecases.org/clover/clover2.asp