Phylogeny of nucleus medianus of the posterior tubercle in rayfinned fishes

Abstract

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.

Stalking the Everted Telencephalon: Comparisons of Forebrain Organization in Basal Ray-Finned Fishes and Teleosts

“…(As mentioned above, nucleus medianus also projects to P1v in Polypterus .) Although a few cells were retrogradely labeled in the optic tectum following nucleus medianus injections, reciprocal experiments are still needed to show definitively that the tectum projects to that nucleus in gars, as it does in Polypterus [Northcutt, 2009]. Nucleus medianus also projects relatively sparsely to Dl in gars (although it projects heavily to P2+P3 in Polypterus ), and Dl, in turn, has descending projections to the optic tectum [Northcutt, 1982].…”

“…Unfortunately, relatively little additional data are available. Northcutt [2009] has shown that calretinin-positive fibers are densely distributed in Dm and more laterally in the Dd of Northcutt and Braford [1980]. He also used carbocyanine dye injections to demonstrate reciprocal connections between nucleus medianus and Dm ( fig.…”

“…Zone P1d also projects to approximately this same deep region of P1v [Holmes and Northcutt, 2003], perhaps providing tertiary olfactory afference. Holmes and Northcutt [2003] and Northcutt [2009] reported that injections of the carbocyanine dye DiI and biotinylated dextran amine into nucleus medianus result in terminals caudally in P1v, but in a more superficial position. Because nucleus medianus receives tectal input [Holmes and Northcutt, 2003;Northcutt et al, 2004], this pathway might be relaying visual information.…”

“…They are the nucleus of the torus lateralis [Braford and Sintasath, 1999; M.R. Braford, Jr. and L. Sintasath, unpublished observations] and nucleus medianus [Holmes and Northcutt, 2003;Northcutt, 2009].…”

New perspectives in fish evolution and neurobiology

“…Wullimann (2009) examines neurogenesis in the telencephalon and compares zebrafish to mice. Northcutt (2009) then discusses the phylogeny of the posterior tubercle of the diencephalon and explores the significance of the variation found in this structure.…”