The morphological organization of the brain of frogs and salamanders varies greatly in the degree to which it is subdivided and differentiated. Members of these taxa are visually oriented predators, but the morphological complexity of the visual centers in the brain varies interspecifically. We give evidence that the morphological complexity of the amphibian tectum mesencephali, the main visual center, can be predicted from knowledge of cell size, which varies greatly among these taxa. Further, cell size is highiy correlated with genome size. Frogs with small cells have more complex morphologies of the tectum than do those with large cells independent of body and brain size. In contrast, in salamanders brain-body size relationships also are correlated with morphological complexity of the brain. Small salamanders with large cells have the simplest tecta, whereas large salamanders with small cells exhibit the most complex tectal morphologies. Increases in genome, and consequently cell size, are associated with a decrease in the differentiation rate of nervous tissue, which leads to the observed differences in brain morphology. On the basis of these findings we hypothesize that important features of the structure of the brain can arise independently of functional demands, from changes at a lower level of organismal organization this case increase in genome size, which induces simpllifcation of brain morphology.The morphological organization of the brain varies among vertebrates in the degree to which it is subdivided and differentiated. Parts of brains exhibit, among other features, differences in lamination, presence of distinct nuclei, numbers of different cell types, and degree of complexity of neuronal connectivity. There is little understanding of the processes that lead to the observed differences, although the most prevalent explanations are forms of functionalism (i.e., the observed differences are the result of environmental selection regarding the specific function of brain parts) and phylogenetic history (older lineages generally have less complex brains). We have examined an alternative view: that the simple brain morphology of salamanders is secondary, de-rived in large part by pedomorphic evolution associated with increases in genome and cell size (1). We here argue that variation in morphological complexity in the brains of frogs and salamanders is based predominantly on such intrinsic factors and is likely to be independent of direct selection.The brains of frogs (Order Anura) and salamanders (Order Caudata) differ considerably within and among these orders in the degree of morphological complexity. In general, frogs have more complex brain morphology than do salamanders, having morphologically distinct nuclei that often lie in migrated positions in the diencephalon, the pretectum, and the mesencephalic tegmentum (2). In addition, multiple lamination (an alternation of cellular and fibrous layers) is found in the tectum opticum (3, 4), the torus semicircularis (5), and a number of diencephal...

In six miniaturized salamanders of the family Plethodontidae, including one of the smallest tetrapod vertebrates, Thorius pennatulus, the anatomical consequences of miniaturization for the brain were investigated. We determined (1) absolute and relative size of the brain, major parts of the brain, the tectum and tectal gray matter, (2) nerve cell size and density, and (3) the number of cells within the visual and visuomotor centers (thalamus, tectum/praetectum and tegmentum). No common compensatory strategy for the brain among the miniaturized salamanders was found. Except for the smallest species, T. pennatulus, only some of the expected compensatory processes (increase in relative size of the brain, relative size of visual centers, relative amount of gray matter or relative density of cell packing density) are found in any species, and these occur in different combinations and degrees. The most decisive factor for maximizing cell number was cell size. Miniaturized species with small cells also have many visual cells, regardless of the other factors. In contrast, the minimum number of visual neurons is found in miniaturized salamanders with large cells. It is concluded that the neuroanatomical traits investigated exert different degrees of resistance to adaptive compensatory processes. Cell size seems to be the most resistant parameter and is strictly dependent on genome size.