Cell Size Predicts Morphological Complexity in the Brains of Frogs and Salamanders GERHARD ROTH*T, JENS BLANKE*, and DAVID B
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Proc. Nati. Acad. Sci. USA Vol. 91, pp. 4796-4800, May 1994 Neurobiology Cell size predicts morphological complexity in the brains of frogs and salamanders GERHARD ROTH*t, JENS BLANKE*, AND DAVID B. WAKEO *Brain Research Institute, University of Bremen, D-28334 Bremen, Federal Republic of Germany; and tMuseum of Vertebrate Zoology and Department of Integrative Biology, University of California, Berkeley, CA 94720 Communicated by Stephen Jay Gould, February 14, 1994 ABSTRACT The morphological organization of the brain the tectum opticum (3, 4), the torus semicircularis (5), and a of frogs and salamanders varies greatly in the degree to which number of diencephalic nuclei (2). it is subdivided and differentiated. Members of these taxa are The brain of salamanders long has been known to be visually oriented predators, but the morphological complexity morphologically much simpler than that of frogs and other of the visual centers in the brain varies interspecifically. We vertebrates (6, 7). It has a compact periventricular cellular give evidence that the morphological complexity of the am- layer (gray matter) and a superficial fiber layer (white mat- phibian tectum mesencephali, the main visual center, can be ter). Very few migrated nuclei can be recognized on mor- predicted from knowledge of cell size, which varies greatly phological grounds. Few to very few migrated cells are found among these taxa. Further, cell size is highiy correlated with in the superficial fiber layers ofthe mesencephalic tectum (8). genome size. Frogs with small cells have more complex mor- The tegmentum mesencephali, including the torus semicir- phologies of the tectum than do those with large cells indepen- cularis, resembles the tectum in that it has a relatively dent of body and brain size. In contrast, in salamanders compact periventricular layer (9). brain-body size relationships also are correlated with morpho- Here, we report the results ofa comparative study ofbrain logical complexity of the brain. Small salamanders with large complexity in salamanders and frogs. We concentrate on the cells have the simplest tecta, whereas large salamanders with tectum mesencephali (optic tectum). Both frogs and sala- small cells exhibit the most complex tectal morphologies. manders are predators that depend on vision, and the tectum Increases in genome, and consequently cell size, are associated is the most important visual center for localization and with a decrease in the differentiation rate of nervous tissue, identification ofprey objects. In addition, the tectum exhibits which leads to the observed differences in brain morphology. the most distinctive morphology and cytoarchitecture of any On the basis of these findings we hypothesize that important part ofthe amphibian brain. A priori, one expects the tectum features ofthe structure ofthe brain can arise independently of to present the most clear-cut influence of function on form, functional demands, from changes at a lower level of organ- if such an influence exists. We demonstrate that cell size is ismal organization this case increase in genome size, which the most likely determinant of tectal morphology in frogs. In induces simpllifcation of brain morphology. salamanders, brain size is an additional important factor. These findings show that alternatives to strict functionalism The morphological organization of the brain varies among must be considered in explaining differences in brain mor- vertebrates in the degree to which it is subdivided and phology among taxa. differentiated. Parts of brains exhibit, among other features, differences in lamination, presence of distinct nuclei, num- MATERIALS AND METHODS bers of different cell types, and degree of complexity of neuronal connectivity. There is little understanding of the Brains of22 species ofsalamanders (3 families) and 17 species processes that lead to the observed differences, although the of frogs (11 families) were used in the present study (Tables most prevalent explanations are forms offunctionalism (i.e., 1 and 2). Heads fixed in formalin were cut in 10-,um serial the observed differences are the result of environmental sections and silver-impregnated by using the Kluever- selection regarding the specific function of brain parts) and Barrera method (10). In addition, two specimens of the phylogenetic history (older lineages generally have less com- salamander genus Parvimolge stained with the Giemsa plex brains). We have examined an alternative view: that the method (11) were used. To determine brain volume, we drew simple brain morphology of salamanders is secondary, de- equidistant cross sections of the brain (30-50 sections) with rived in large part by pedomorphic evolution associated with a Zeiss camera lucida. Cell-size measurements were taken for increases in genome and cell size (1). We here argue that 50 neurons from three different tectal areas and from as many variation in morphological complexity in the brains of frogs specimens per species as were available. Information on and salamanders is based predominantly on such intrinsic genome size was obtained from the literature, and all values factors and is likely to be independent of direct selection. were converted to pg of DNA per haploid genome. The brains offrogs (Order Anura) and salamanders (Order For correlational analysis regarding cell size, brain size, Caudata) differ considerably within and among these orders and morphological complexity, we established five classes in in the degree of morphological complexity. In general, frogs frogs and six classes of morphological complexity of the have more complex brain morphology than do salamanders, tectum in salamanders (including an undifferentiated state). having morphologically distinct nuclei that often lie in mi- The classes of morphological complexity for the frog grated positions in the diencephalon, the pretectum, and the tectum are as follows (descriptions of layers are from refs. 3 mesencephalic tegmentum (2). In addition, multiple lamina- and 5). tion (an alternation of cellular and fibrous layers) is found in Class 1. Separation of layers was indistinct and not con- tinuous in mediolateral extent. Layers 7-9 were diffusely The publication costs of this article were defrayed in part by page charge arranged; there was no distinct formation of a layer 8. payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. tTo whom reprint requests should be addressed. 4796 Downloaded by guest on September 29, 2021 Neurobiology: Roth et aL Proc. Natl. Acad. Sci. USA 91 (1994) 4797 Table 1. Salamanders Table 2. Anurans Classification n GS* CSt BS* MC Family and species n GS* CSt BSt MC Family Plethodontidae Bombinatoridae Subfamily Desmognathinae Bombina orientalis 3 10.3 9.9 10.9 2 Desmognathus wrighti 4 14.1 7.7 1.2 1 Discoglossidae Desmognathus aeneus 2 7.5 1.8 1.5 Discoglossus pictus 3 5.3 8.0 15.6 3.7 Desmognathus ochrophaeus 2 14.0 7.2 2.2 2.3 Pipidae Desmognathus monticola 1 15.0 9.2 10.1 4 Xenopus laevis 1 3.0 6.2 30.9 4 Desmognathus quadramaculatus 1 15.0 9.5 12.9 5 Ranidae Subfamily Plethodontinae Rana temporaria 3 4.2 7.9 18.5 3 Tribe Hemidactyliini Mantella aurantiaca 2 4.8 6.2 5.3 4.5 Eurycea bislineata 1 20.8 8.8 3.0 1.5 Mantella cowani 2 6.8 6.5 4.2 Tribe Plethodontini Hyperoliidae Plethodon cinereus 3 22.3 8.9 3.2 3 Hyperolius quinquevittatus 2 6.4 8.8 5 Plethodon jordani 1 29.0 11.8 6.5 4 Afrixalus fornasinii 2 6.9 4.5 Tribe Bolitoglossini Hylidae Hydromantes italicus 2 76.2 15.1 12.5 0 Hyla septentrionalis 1 6.5 20.9 4 Hydromantes genei 2 15.1 16.3 0.5 Gastrotheca riobambae 2 3.7 7.3 19.2 3.5 Batrachoseps attenuatus 2 37.0 11.3 2.3 0 Bufonidae Parvimolge townsendi 3 20.3 8.1 1.4 1 Bufo bufo 2 5.8 6.4 4 Thorius narisovalis 2 25.2 10.5 1.3 0.5 Myobatrachidae Thorius pennatulus 2 9.0 0.5 0.5 Limnodynastes tasmaniensis 2 2.3 6.6 9.8 3.5 Bolitoglossa subpalmata 2 68.9 13.4 7.8 0 Arenophryne rotunda 1 19.0 10.0 5.5 1.5 Bolitoglossa dofleini 1 59.4 13.2 17.2 1 Dendrobatidae Family Ambystomatidae Dendrobates pumilio 2 7.0 5.0 3 Ambystoma opacum 2 20.5 12.2 8.1 3 Leptodactylidae Ambystoma mexicanum 2 21.9 11.0 28.1 4.3 Eleutherodactylus coqui 2 5.8 14.0 4 Family Salamandridae Sminthillus limbatus 2 5.1 1.4 3.5 Salamandrina terdigitata 1 20.5 11.0 1.8 3 Rhacophoridae Salamandra salamandra 2 33.0 10.1 19.3 3 Rhacophorus leucomystax 1 7.2 18.6 3.8 Pleurodeles waltl 1 19.7 9.7 10.9 3 GS, genome size; CS, cell size; BS, brain size; MC, morphological Triturus alpestris 1 23.7 11.0 complexity. GS, genome size; CS, cell size; BS, brain size; MC, morphological *Data are reported as pg of DNA haploid genome. complexity. tData are reported as ,um3. *Data are reported as pg of DNA haploid genome. tData are reported as mm3. tData are reported as ,m3. tData are reported as mm3. Class 5. Fiber bands extended over the entire rostrocaudal and mediolateral width ofthe tectum, separating deep cellular Class 2. Layer 6 was well separated, but layers 1-5 were layers 6 and 8 and continuing into the tegmentum; 3-10%1 of diffusely arranged, particularly in the lateral portion; layers the neurons had migrated. 7-9 were arranged as in class 1. Specimens were ranked by complexity class. In cases Class 3. Layers 1-6 were well separated from each other; where specimens of the same species ranked differently, layer 8 was separated from layers 7 and 9 and contained means were calculated.