Development of locomotor behavior in wild type and spastic (sp/sp) axolotls, Ambystoma mexicanum.

The homozygous recessive spastic mutant found in the Mexican axolotl shows violent coiling and thrashing behavior when subjected to strong tactile or electrical stimulation. In order to establish the time of onset of the first behavioral manifestation of the spastic gene, an etiological analysis of the ontogeny of swimming behavior in mutants and wild type siblings was undertaken. The locomotor patterns shown by embryos in response to an electrical stimulus were analyzed quantitatively from the embryonic early flexure stage through the larval early feeding stage. Spastic larvae failed to show dorsal-up swimming frequencies equal to those of sibling controls from day 12 (Harrison stage 40) of development indicating a lack of equilibrium. Both spastics and their siblings showed "sinusoid swimming" and "coiling" behavior in response to an aversive stimulus through day 18 (Harrison state 46, early feeding stage) of development. From day 18, wild type siblings abruptly decreased "coiling" behavior and showed strong "escape swimming" in response to an intense stimulus. Spastics never developed "escape swimming" patterns but retained a mixture of "sinusoid swimming" and "coiling" patterns characteristic of pre-feeding stage larvae.