Experimental Studies on Slug-Plant Interactions: III. Differences in the Acceptability of Individual Plants of Trifolium Repens to Slugs and Snails

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Abstract
Differences in the acceptability of individual plants of T. repens (white clover) to 4 spp. of slug and snail were investigated. Trials with mollusks, both in the field and in the laboratory, showed that cyanogenesis markedly reduced but did not wholly prevent damage to the clover leaves. In areas of high density of active mollusks the cyanogenic morphs were over-represented, while in areas of low density of active mollusks, the acyanogenic morphs were over-represented. Slugs fed on monotonous diets of lettuce grew considerably faster than did those feeding on monotonous diets of either cyanogenic or acyanogenic clover leaves. Slugs fed on cyanogenic leaves made slightly smaller live-weight gains, or lost weight faster, than did those fed on acyanogenic leaves. Homogenates of the digestive tract of slugs produced cyanogenic reactions with those clover leaves that possessed cyanoglucosides but lacked hydrolyzing enzymes, but grazing trials with plants that contained the glucoside but not the enzyme showed that both the enzyme and the glucoside must be present in the clover for the animal to be deterred from feeding. The information obtained suggests a possible co-evolutionary pathway of the mollusk-clover interaction. Co-evolutionary interpretations may be inadequate if they do not include evidence that both the plant and the animal have undergone reciprocating selection. If the clover-mollusk system is co-evolved, it is surprising that the genes controlling production of the enzyme and of the glucoside assort independently.