In the present account, the relation between the descending, substance P (SP)-containing projection fibers from the striatum and the dopaminergic cell bodies in the midbrain tegmentum was investigated in several species of reptiles and the domestic chicken by means of antisera against SP and dopamine. It was found that in all species studied an almost complete match of the SP and the dopamine immunoreactivity occurs in the ventral tegmental area. In contrast, considerable differences were observed in the substantia nigra and the retrorubral area. Two different patterns were recognized: one pattern in which there is little overlap between the dopaminergic cells and SP fibers (lizards, turtles) and another with an extensive overlap (snakes, chickens). Similar patterns were found in mammalian species. On the basis of an out-group comparison with anamniotes, it is suggested that the pattern with little overlap represents the primitive character. The pattern with considerable overlap, found in the three classes of amniotes, should be considered an independent development from the same primitive character and should, therefore, be defined as an example of parallel homoplasy.

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