  To survive in ever-changing environments, living organisms need to
continuously combine the ongoing external inputs they receive, representing
present conditions, with their dynamical internal state, which includes
influences of past experiences. It is still unclear in general, however, (i)
how this happens at the molecular and cellular levels, and (ii) how the
corresponding molecular and cellular processes are integrated with the
behavioral responses of the organism. Here we address these issues by modeling
mathematically a particular behavioral paradigm in a minimal model organism,
namely chemotaxis in the nematode C. elegans. Specifically, we use a
long-standing collection of elegant experiments on salt chemotaxis in this
animal, in which the migration direction varies depending on its previous
experience. Our model integrates the molecular, cellular and organismal levels
to reproduce the experimentally observed experience-dependent behavior. The
model proposes specific molecular mechanisms for the encoding of current
conditions and past experiences in key neurons associated with this response,
predicting the behavior of various mutants associated with those molecular
circuits.
