'Ancestral' neural mechanisms of electrolocation suggest a substrate for the evolution of the jamming avoidance response.

The genus Sternopygus, believed to reflect ancestral traits of gymnotiform electric fish, is closely related to the more 'modern' genus Eigenmannia (Mago-Leccia 1978; Fink and Fink 1981). Sternopygus is the only known genus of electric fish that does not perform a jamming avoidance response (JAR) to minimize the potentially detrimental effects of signal interference between discharging neighbors (Bullock et al. 1972, 1975), and its ability to electrolocate objects is rather immune to jamming (Matsubara and Heiligenberg 1978). By studying the responses of midbrain neurons to stimulus regimes effective in eliciting the JAR in Eigenmannia, we found that Sternopygus has neurons capable of discriminating the sign of the difference frequency between interfering electric organ discharges (EODs). These 'sign-selective' neurons, which are believed to be important elements in the control of the JAR in Eigenmannia, may, therefore, fulfill a more general function in the detection of moving objects and conspecifics but could potentially be assembled for the evolution of a JAR in Sternopygus. The relative immunity to jamming in this genus may result, in part, from a stronger reliance upon the ampullary electrosensory system which operates in the DC and low-frequency range, outside the EOD spectrum of these fish.