Phylogenetic changes in the connections of the lateral preglomerular nucleus in ostariophysan teleosts: a pluralistic view of brain evolution.

The connections of the lateral preglomerular nucleus were examined with the fluorescent tracer DiI in a cyprinid, a characin, a catfish and a gymnotoid species. In all of these taxa the lateral preglomerular nucleus receives inputs from the torus semicircularis and projects to the telencephalon. Cytoarchitectural and topological data further support the hypothesis that the lateral preglomerular nucleus is homologous among the species examined. A cladistic analysis of species differences, however, reveals that some connections of the lateral preglomerular nucleus have changed dramatically during the course of evolution. Some of the cell groups that are interconnected with the lateral preglomerular nucleus have increased in size, while others have decreased. Increases in the number of subdivisions of cell groups are frequently associated with size increases, but they may also be associated with size decreases. Some of the connections between cell groups have increased in strength, whereas others have decreased in strength, but these connectional changes bear no simple relationship to phylogenetic changes in the size of the cell groups. Phylogenetic increases in the number of subdivisions within a cell group may be associated with selective gains of connections, but they may be associated also with selective losses of connections or with no major changes in connections. Furthermore, several cell groups and connections appear to have evolved 'de novo', whereas others have disappeared during the course of evolution. Finally, the individual cell groups within this system of interconnected nuclei have changed largely independently of one another. These findings establish that brain evolution is not dominated by a single type of phylogenetic change.