Phylogeny of nucleus medianus of the posterior tubercle in ray-finned fishes

The brains of ray-finned fishes form a morphocline of increasing complexity, from cladistians through teleosts. This is particularly apparent in the posterior tubercle of the diencephalon. In cladistians, the posterior tubercle consists of a periventricular nucleus and a migrated nucleus medianus that is fused across the midline. In more advanced ray-finned fishes, such as gars and bowfins, the posterior tubercle comprises numerous additional migrated nuclei, termed the preglomerular complex, in addition to a more well developed nucleus medianus. In teleosts, the most derived ray-finned fishes, there is an even more elaborate preglomerular complex, but there is no recognizable nucleus medianus. In an attempt to explain the variation in the posterior tubercle of the diencephalon in ray-finned fishes, the immunohistochemistry and connections of nucleus medianus were examined in cladistians, gars and bowfins. In each of these taxa, nucleus medianus exhibits large numbers of calretinin-positive neurons and has ascending projections that terminate in several divisions of the pallium. Although teleosts, such as goldfish, also exhibit numerous cell groups in the posterior tubercle that are rich in calretinin, none of these cell groups has connections that are comparable to those of nucleus medianus in non-teleost ray-finned fishes. It is possible, therefore, that nucleus medianus was lost with the origin of teleosts.