Adaptations to ecological interactions.

Abstract

Three different aspects of plant adaptations to ecological interactions are examined. The first one is in the area of plant breeding systems. The adaptations investigated in this study involved interactions between male gametes competing for fertilizations on the plant Phormium tenax, an agavoid of New Zealand. In this system I discovered a new type of self incompatibility that depends on the levels of competition among self- and cross-pollinated fruits. This mechanism is parallel to cryptic self-incompatibility in which individual self-pollen grains are not as successful as cross-pollen when competing in the same pistil. The competition-dependent abscission of self-pollinations considered here, however, operates at the level of whole flowers. This form of self-incompatibility may allow a high level of outcrossing to be achieved while it assures seed set when pollinations are scarce. The second case focuses on the interaction between a plant of the genus Bursera and its herbivorous crysomelid beetle Blepharida. This Bursera produces terpenes that are stored in networks of canals that run throughout the leaves and the cortex of the stem. When damaged, there is often an abundant release of resins. Blepharida larvae have developed the behavioral adaptations to overcome the secretive canals of Bursera. Before feeding on the leaves they cut the leaf-veins, interrupting the flow of terpenes. By documenting the growth and survival costs of being on plants of different response strength I was able to show that canals can effectively decrease herbivory even against this specialized vein-cutting insect. The handling time involved in blocking the canals slows down larval growth, delays pupation and increases the risk of predation. Chapter III examines a more complex interaction among plants that produce extrafloral nectaries, ants, and homopterans. An alternative model to explain the function of extrafloral nectaries is proposed. According to this hypothesis, the function of these glands is not to attract ants for defense, but to distract them from tending homopterans by giving them a free source of sugar. Different sources of evidence that support this model are discussed.