Gustation is an essential sensory modality for food selection, reproduction and avoiding toxic substances. In cockroaches, sugars such as glucose, fructose, sucrose, maltose and maltotriose act as phagostimulants, and nutritional requirement studies indicate that cockroaches develop optimally on diets with more than 50% carbohydrate. In 1993, Jules Silverman discovered a field population of glucose-averse German cockroaches that rejected D-glucose and consumed less of a glucose-supplemented diet in all its life stages; however, other sugars (e.g., D-fructose, D-mannose, sucrose and maltose), stimulated feeding in these glucose-averse cockroaches. Glucose-averse cockroaches develop slower and have lower survival than wild-type cockroaches, suggesting that they would be at a selective disadvantage when naturally foraging, especially for glucose-containing foods. Conversely, the glucose-averse trait would turn out to be a significant advantage in avoiding insecticide baits that contain glucose. Because hemocoelic injections of glucose have no apparent detrimental effects on weight and mortality of the glucose-averse cockroaches, it appears that glucose does not act as a toxic substance in the glucose-averse strain. Rather, it appears that glucose inhibits feeding through the chemosensory system.
As a lead to electrophysiological and molecular genetics studies to investigate the mechanisms that underlie glucose-aversion, our behavioral studies, in collaboration with Jules Silverman, examine the hypothesis that the four major chemosensory appendages function as first-order peripheral detectors in food preference, but that not all sensory appendages have the same chemospecificity for reception of phagostimulants and deterrents. We found that the paraglossae alone are sufficient for maximum sensitivity to both phagostimulants and deterrents, including glucose as a deterrent in the glucose-averse strain. In addition to the paraglossae, the labial palps are more important than the maxillary palps in the reception of deterrents (caffeine in both strains and glucose in the glucose-averse strain). The maxillary palps, on the other hand, play a more important role in the reception of phagostimulants (fructose in both strains and glucose in the wild-type strain). Our results thus far suggest that distinct inputs from the chemosensory system mediate opposite feeding responses to glucose in the wild-type and glucose-averse strains.
On-going studies examine the elctrophysiological responses of single gustatory units on the paraglossae, and gene expression patterns of gustatory receptors of the paraglossae.
Blanton J. Whitmire Endowment