Nucleus rostrolateralis is a distinctive diencephalic nucleus in some ray-tinned fishes. In the osteoglossomorph Pantodon, it is a large ovoid nucleus with visual system connections. A topologically and cytoarchitectonically similar nucleus has been found in six species of non- osteoglossomorph fishes, two species of gars and four euteleosts. Of the latter, two are ostariophysans of the genus Danio, one an atherinomorph, and one a notothenioid percomorph. A variety of characteristic similarities can be found in nucleus rostrolateralis among all these species. The present study reports on these similarities in Danio and in the euteleost Xiphophorus as compared with the nucleus in Pantodon. While this nucleus has a phylogenetic distribution that might imply convergent evolution, the high degree of similarity in its features across species strongly suggests that its genetic basis may be the same despite the lack of phenotypic homology.
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