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Clustered Phylogenetic Distribution of Nucleus rostrolateralis among Ray-Finned Fishes

Nucleus rostrolateralis, which was named for its location in the rostrolateral part of the diencephalon of neopterygian fishes, has been identified in a variety of species based on position, cytoarchitecture, hodology, and/or histochemistry. The phylogenetic distribution of the nucleus is highly sporadic, however. Due to this distribution, nucleus rostrolateralis cannot be regarded as phylogenetically homologous, but it might be an example of syngeny, or generative homology, which applies to characters that have the same genetic and/or developmental basis inherited from a common ancestor, whether or not the character itself has a phylogenetic distribution congruent with a monophyletic taxon – i.e., in general terms, an example of either phylogenetic homology or parallelism. To test whether the nucleus occurs in closely related taxonomic clusters, as might be expected for a character with a shared generative basis, a number of species of cyprinids and atherinomorphs (both teleost taxa) were examined for its presence. Many of the species examined appear to lack the nucleus, but a clustered occurrence of it was found within both taxa. Within cyprinids, nucleus rostrolateralis occurs in all three members examined of the Subfamily Rasborinae. Within atherinomorphs, it occurs in both members examined of the Tribe Poeciliini (of the Subfamily Poeciliinae, Family Poeciliidae) and in the one member examined of the Family Anablepidae. The clustered occurrence of nucleus rostrolateralis supports the hypothesis that it is an example of syngeny. Its postulated shared generative basis appears to derive from the common ancestor of the entire neopterygian radiation despite the rare occurrence of the character itself.