NONSEQUENTIAL DEVELOPMENTAL TRAJECTORIES LEAD TO DIMORPHIC VOCAL CIRCUITRY FOR MALES WITH ALTERNATIVE REPRODUCTIVE TACTICS

Midshipman fish, Porichthys notatus, have two male reproductive morphs : type I males generate long duration advertisement calls (''hums'') t o attract females to a nest: type II males sneak-spawn and, like femal es, do not produce mate calls but generate short duration agonistic ca lls. A vocal pacemaker circuit includes: motoneurons in the caudal bra in stem and rostral spinal cord that innervate vocal/sonic muscles: pa cemaker neurons that are located ventrolateral to motoneurons and esta blish their fundamental discharge frequency; and a ventral medullary n ucleus that couples the motoneuron-pacemaker circuit bilaterally. Tran sneuronal biocytin transport identified morph-specific developmental t rajectories for the vocal circuit. Among nonreproductive, juvenile typ e 1 males, motoneuron soma size and motor nucleus volume increase most during a stage prior to sexual maturation. An additional increase in motoneuron size and nucleus volume is coupled to the greatest increase in pacemaker soma size at a stage coincident with the onset of sexual maturity; ventral medullary neurons show similar growth increments du ring both stages. Type II males (and females) mature with no or little change in cell size or motor nucleus volume. The results indicate tha t alternative mating tactics are paralleled by alternative development al trajectories for the neurons that determine tactic-specific behavio rs, in this case vocalizations. Together with aging data based on otol ith grow-th, the results support the hypothesis that alternative male morphs in midshipman fish adopt nonsequential, mutually exclusive life history tactics. (C) 1996 John Wiley & Sons, Inc.