Review
Forebrain evolution in bony fishes

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Abstract

The bony fishes consist of ray-finned fishes and lobe-finned fishes. In ray-finned fishes, the forebrain forms a morphocline from the cladistian bichirs through teleosts regarding the number and increasing complexity of pallial connections. The nuclei of the posterior tubercle parallel this increase in complexity, but the dorsal thalamic nuclei do not. The primary targets of the dorsal thalamic nuclei are the subpallial nuclei, whereas the primary targets of the posterior tubercle are various pallial divisions. Primitively, nucleus medianus is the primary projection nucleus of the posterior tubercle. It is either reduced or lost in teleosts, and its role is taken over by the preglomerular complex, which appears to develop from proliferative zones in both the thalamic alar plate and the posterior tubercle. Although there are numerous hodological data for the pallium in ray-finned fishes, there is no consensus regarding its homologies with other vertebrates. In contrast to ray-finned fishes, very few experimental data exist for lobe-finned fishes. The coelacanth, Latimeria, is extremely rare, and lungfishes are the best source for new experimental data. At this point, there are sufficient data to suggest that lungfishes are characterized by a pallium that is divided into four components, separate dorsal and ventral striatopallidal systems, and an amygdala that consists of anterior, central, lateral, and medial nuclei. The data suggest that telencephalic organization in lungfishes is far more similar to that in amphibians than was previously suspected.

Introduction

The bony fishes represent approximately half of all living vertebrates and constitute more than 30,000 living species. These fishes form two great clades: the actinopterygians or ray-finned fishes, which include most bony fishes, and the sarcopterygians or lobe-finned fishes, which are restricted to only seven or eight living species (Fig. 1). Given the large number of species and their long phylogenetic history, it is not surprising that the brains of bony fishes exhibit a remarkable range of variation [57], [58], [64], [106], [112]. Much of that variation occurs in the forebrain. In this review, evolutionary trends in forebrain variation will be summarized first for the ray-finned fishes and subsequently for the lobe-finned fishes.

Section snippets

Ray-finned fishes

The forebrains of ray-finned fishes form a morphocline of increasing complexity [71], [72] from bichirs (cladistians), through sturgeons and paddlefishes (chondrosteans), to the neopterygians, which comprise the gars (ginglymodes), bowfins (halecomorphs), and the teleosts, which represent the largest group of living vertebrates. Due to extensive research in several laboratories, there is considerable experimental data on forebrain organization in one cladistian (Polypterus), two chondrosteans (

Lobe-finned fishes

There are only two groups of living lobe-finned fishes: the dipnoans, or lungfishes, and the actinistians, or coelacanths (Fig. 1). The living lungfishes comprise the African genus Protopterus, the Australian genus Neoceratodus, and the South American genus Lepidosiren. In turn, Protopterus is represented by four species, whereas Neoceratodus and Lepidosiren are each monotypic. Today, coelacanths are represented by a single genus, Latimeria, which occurs over a large but spotty range along the

Conclusions

Although the last few years have seen considerable information generated on the organization of the forebrain in ray-finned fishes, particularly the organization of area dorsalis or the pallium of the telencephalon, far more data are needed on subpallial organization in these fishes. Also, there is a paucity of data on forebrain organization in the bowfins and gars, which reflect a holostean grade of organization in the evolution of the forebrain in ray-finned fishes. Further attention to

Acknowledgements

I thank Jo Griffith for assistance with the illustrations, Susan Commerford for literature retrieval and word processing, and Mary Sue Northcutt for assistance with numerous phases of the research. It is a pleasure to dedicate this review to Rudolf Nieuwenhuys, a consummate comparative neuroanatomist and scholar, whose pioneering studies on the organization of the forebrain of bony fishes established a body of knowledge that all subsequent comparative neuroanatomists have built upon. This

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