Modelling quantitative structure-activity relationships between animal behaviour and environmental signal molecules.

Quantitative structure-activity relationships (QSARs) between the physicochemical properties of environmental signal molecules and animal behaviour have been determined. Past work has shown that oyster and barnacle larval settlement and mud crab abdominal pumping (for larval dispersal) are stimulated by small peptide cues. In all the peptides examined that were active at ecologically relevant concentrations, arginine or lysine was found at the carboxy terminus, but the amino acids found at preceding positions were highly variable. We used the multivariate partial least squares algorithm to relate composite properties for the hydrophilicity, size and charge of each amino acid and the sequence position to oyster, barnacle and crab behaviour patterns. From the information in these QSAR models, the apparent variability in amino acid sequences eliciting behavioural responses was explained in each case, and more potent peptide analogues are hypothesized on the basis of untested amino acid sequences. Remarkably, these peptide signals are all structurally related to the carboxy-terminal sequence of mammalian C5a anaphylatoxin, a potent white blood cell chemoattractant. Even more striking is the fact that these different animal species should rely on apparently similar environmental signal molecules when residing within a common habitat (southeastern US estuaries). Through the physicochemical properties of amino acids, the current QSAR models clearly differentiate between the optimal sequences for eliciting oyster, barnacle and mud crab behaviour. Thus, QSARs provide a novel and powerful method not only for relating the physicochemical properties of molecules to animal behaviour but also for differentiating responses to chemicals by individuals of different species.