Elle and associates investigated the cost to Datura wrightii of producing sticky leaf trichomes.

Elle and Associates Investigated the Cost to Datura wrightii of Producing Sticky Leaf Trichomes

The jimsonweed plant (Datura wrightii) grows in arid regions of the southwestern U.S. and Mexico. This plant produces leaves whose epidermal surfaces feature hair-shaped trichomes. Some D. wrightii plants produce leaf trichomes that secrete sticky sugar solutions at their surfaces and are thus known as glandular trichomes; such plants are described as “sticky” plants. In contrast, other D. wrightii plants produce similarly shaped, nonglandular leaf trichomes that do not secrete sugars. The leaves of these plants are smooth rather than sticky, and they are described as “velvety.” In D. wrightii, trichome phenotype is controlled by variation in a single gene, and sticky is dominant over velvety. If the phenotypes were of equal advantage in nature, sticky and velvety plants should occur in a 1:1 proportion. However, the actual frequency of sticky plants in nature ranges from 0% to 93%, depending on the environment. This variation and other data suggest that sticky trichomes deter certain herbivores, thereby providing a growth advantage. However, sticky trichomes may be excessively costly in terms of resources when those herbivores are not present, because sugar secretion uses up photosynthetic products.

Evolutionary ecologists Elizabeth Elle, Nicole van Dam, and Daniel Hare of Simon Fraser University devised an experiment to determine the relative cost of producing sticky trichomes, which is shown in Figure 33.14. The team hypothesized that sticky plants might produce fewer viable seeds than velvety plants, because plant photosynthetic products are diverted from reproduction. The investigators grew sticky and velvety D. wrightii plants together, protecting some from insect herbivores but not protecting others. Then the investigators compared the numbers of seeds capable of germinating that were produced by sticky and velvety plants. (In this experiment, seed production was a measure of Darwinian fitness.) The team found that velvety plants sustained more insect damage than sticky plants, verifying the protective function of glandular trichomes. When insect herbivores were present, there was no significant fitness difference between sticky and velvety plants. However, when herbivores were absent, though both types of plants flowered similarly, the sticky plants produced 45% fewer viable seeds than did the velvety plants. These data supported the initial hypothesis that sticky trichomes were more costly than smooth trichomes. This and other studies suggest that the investment required to produce sticky trichomes may pay off by reducing the destructive effects of insects.