PARENT SESSION
Oral Session #18: Predator-Prey Interactions: Plants and animals. Presiding: B. Goodwin.
Monday, August 6, 2001. 1:00 PM to 5:00 PM. Hall of Ideas I.

One insect and two invertases: the mechanisms and function of sink strength in galls.

Rehill, Brian¹, Schultz, Jack¹, ¹

ABSTRACT- The ability to induce a sink in host plant tissue is crucial for the success of insect gall formers, however the mechanisms by which sink strength is enhanced remain unknown. We demonstrated that soluble and cell wall invertase activities are elevated in galls made by the aphid Hormaphis hamamelidis on witch hazel leaves, and studied the characteristics and fitness impacts of sink induction at different times in the aphid's life cycle. Initially, when the gall is forming and undergoing its most rapid growth, the gall-forming female aphid ('fundatrix') induces high levels of soluble acid invertase activity, an enzyme that is important for sink strength and growth. Aphid mortality eliminates invertase enhancement. By the time the gall has reached 85% of full size and the first parthenogentically reproduced offspring appear in the gall, levels of cell wall invertase activity (important for carbon partitioning and sink strength) are greater in the gall than those in surrounding leaf tissue. At the time of peak offspring abundance in the gall, the total amount of cell wall invertase activity of the gall accounts for 17.5% of the variance in fecundity (F_1,24 = 4.88; p = 0.0374). Also, the level of cell wall invertase activity per unit fresh weight of occupied galls is ca. 70% greater than that of leaves, but is not related to the number of offspring in the gall, consistent with induction by the fundatrix, not the offspring. Gall volume, likely to be a result of the fundatrix' ability to create a strong sink by inducing high soluble invertase activities early in the life of the gall, accounts for 67% of the variation in fecundity. Also, the positive effect of cell wall invertase activity on fecundity is influenced by gall volume. Overall, it appears that the ability to induce higher levels of invertases and thus form stronger sinks plays a role in reproductive success throughout the life of the fundatrix, first via soluble invertases in gall creation, then later via cell wall invertases in supporting offspring. Further, there appears to be a 'rich get richer' scenario; fundatrices that initially form stronger sinks and therefore larger galls can benefit more from a later increase in sink strength.

KEY WORDS: gall, sink, invertase, aphid