Limb Bone Loading in Salamanders During Terrestrial

1042 ICVM-8 ABSTRACTS I. Plenary Lectures are also cases of syngeny, the opposite condition being allogeny, which essentially equals convergence. Likewise, the concept of field homology Comparative Vertebrate Neuroanatomy: Diversity in Brain Evolution has been bolstered by illumination of developmental events. Functional Across the Taxon characteristics, including neuronal physiology and correlated behavioral Ann B. Butler; Krasnow Institute for Advanced Study and Department of outcomes, are not appropriate criteria for any kind of homology, how- Psychology, George Mason University, Fairfax VA, USA (abbutler@ ever, but rather indicate similar function, i.e., analogy. gmu.edu) Comparison of the telencephalic pallium (and some other elaborated The extensive data now available on brain anatomy in all of the major dorsal neural derivatives) across Group II vertebrates implies that both vertebrate radiations—cyclostomes, cartilaginous fishes, ray-finned shared patterning genes for elaboration plus unique gene expression pat- fishes, and tetrapods—are of interest not only for understanding the terns and/or timing differences contribute to substantial diversity of neural diverse lines of brain evolution but also for their more general implica- architectures, and controversy persists over possible homological relation- tions, including theoretical aspects of how and what homology is and ships of some pallial areas, particularly between mammals and sauropsids. bioethical issues ranging from animal welfare considerations to how Both the pallium—specifically mammalian neocortex—and dorsal thala- consciousness evolved and what its neural substrate consists of. To mus have been hypothesized to be involved in the generation of con- address these issues, the breadth of diversity in brain evolution will be sciousness, including the higher-order consciousness of humans. However, surveyed first. From comparative studies, models of the earliest verte- while much of the research on cognitive abilities has focused on mam- brate brains and how brain evolution has proceeded within each radia- mals, and on humans in particular, recent studies on birds have revealed tion can be constructed. Common features of vertebrate brains will be cognitive abilities in some species of a surprisingly high degree. Since considered first, including postulated features in the earliest vertebrates, higher-order consciousness is correlated with high levels of cognitive abil- and then some of the specializations that have evolved in various ities in humans, it is parsimonious to hypothesize that this correlation groups. These findings and hypotheses of homology are based on cladis- holds for other vertebrate species as well, with the important caveat that tic analyses of data mostly derived from extant species, since most neu- absence of cognitive ability does not guarantee absence of consciousness. ral features are not preserved in the fossil record. The methodology will Nonetheless, the strategy of comparing the behavioral abilities of birds be considered in the context of the current views of homology and the and mammals, including humans, and comparing their neural circuitry implications of new findings of shared gene-expression patterns that are and related features may contribute to elucidating the neural basis of con- more widely based than all instances of consistent phenotypic expression sciousness. Similarities do not include some specific architectural features of particular traits. Finally, the findings of different degrees of elabora- of mammalian neocortex, which birds lack, but do include circuitry that tion and expansion of various brain regions in different vertebrate radia- operates by inhibition of inhibition mechanisms, allowing for the genera- tions has profound bioethical implications, including the questions of tion of synchronous activity by pallial neurons. how consciousness—from basic sensory awareness to very high levels— is generated by neurons and in which groups of animals it is present. The comparative method may have the highest potential for yielding Understanding Muscle Performance: The Structural and Molecular insight into these tantalizing questions. Common features of the brain Basis of Function across all vertebrates (the term vertebrates used here as synonymous to Hans Hoppeler; Department of Anatomy, University of Bern, Baltzerstrasse 2, the term craniates, i.e., to include cyclostomes as monophyletic and as CH-3000 Bern 7, Switzerland ([email protected]) the outgroup to jawed vertebrates) include the three major brain divi- When animals move they make use of a multitude of specific design fea- sions of forebrain, midbrain, and hindbrain. The forebrain consists of a tures of their muscular system which characterize muscle on different lev- telencephalon rostrally, which includes a dorsal portion, the pallium, and els of structural organization as well as with regard to particular functional a ventral portion, the subpallium, and the diencephalon caudally with its properties of muscle tissue components. On the macroscopic level it is four main divisions, one of which is the dorsal thalamus, which relays found that parallel fibered muscles and pinnate muscles perform differently ascending sensory inputs to the telencephalon. The midbrain roof is with regard to speed of contraction, excursion and force per muscle cross- involved in spatially mapping sensory inputs, while the ventral part of sectional area. Looking at the microscopic composition of muscle cells, we the midbrain and the hindbrain are the site of multiple cranial nerve sen- find that muscles that are active most of the time such as the heart and the sory and motor nuclei, as well as the reticular formation, which has diaphragm contain many more mitochondria and capillaries than locomotor diverse motor and integrative functions, and the cerebellum, which is muscles that are used only occasionally. Speed of contraction of muscle is involved in motor control. In contradiction to the now out-dated but per- further regulated by particular characteristics of the myosin molecule and sistent notion of a Scala Naturae, brain evolution has proceeded inde- associated proteins which come in several molecular configurations. Of pendently within each major radiation. Two groups across the radiations particular interest is the malleability of skeletal muscle tissue. Massive can be recognized based on the degree of brain elaboration, i.e., neuro- changes in muscle structure and function can be induced rapidly using spe- nal proliferation and migration and thus the degree of enlargement and cific exercise regimes. The advent of appropriate molecular tools has complexity of brain structure. Group I consists of those species with rel- enabled us to begin to study the mechanisms of muscle plasticity. Muscle atively little elaboration and includes lampreys, some sharks, non-teleost cells rely on external mechanical, metabolic, neuronal and metabolic sig- ray-finned fishes, and amphibians (as well as lungfishes and crossoptery- nals which are sensed and transduced over multiple pathways to the mus- gians). Group II, whose members have relatively elaborated brains, cle genome. In exercise many of these sensory and stimuli dependent sig- include hagfishes, other sharks and skates and rays, teleosts, and naling cascades are activated, the individual characteristic of the stress amniotes. Thus, it is clear that brain elaboration has occurred independ- leading to a specific response of a network of signaling pathways. Signal- ently at least four times across the vertebrate spectrum. Further, different ing typically results in the transcription of multiple early genes among parts of the brain are differentially elaborated in various groups, from those of the well known fos and jun family as well as many other tran- the huge cerebellum of mormyrid fishes, to the taste lobes in the hind- scription factors. These bind to the promoter regions of downstream genes brain of some other teleosts, to the large telencephalic pallium of Group initiating the structural response of muscle tissue. While signaling is a mat- II cartilaginous fishes and, independently, in mammals and birds. In ter of minutes, early genes are activated over hours leading to expressional evaluating homologies, multiple neural criteria are used, including rela- modifications of structure genes that can then be effective over days. The tive topographic (or topologic) position of a given neuronal cell group, multiplicity of the signaling pathway and of the early gene activation leads its afferent and efferent connections, morphological and histochemical to a bewildering number of possible genomic responses. The response is similarities of the neurons, and embryological origin. These criteria are tailored by ‘‘structural’’ genes having promoter regions capable of recog- usually used to assess historical, or phylogenetic homology, noting the nizing a host of activators and depressors. The current molecular techni- distribution of the neural character across a phylogeny generated from ques are in principle capable of dealing with the task of unraveling the non-neural traits. Recent examples of neural characters with multiple and minute similarities but with sporadic, non-congruent phylogenetic occurrence, which might be regarded as distant parallelism, and also cases of reversal, might now be considered to be syngeny, or genera- Published online 23 October 2007 in tively homologous, i.e., based on the inheritance of their patterning Wiley InterScience

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