Preface to Vertebrate Brains: evolution, structures and functions.

Abstract

Living organisms have a strategy to integrate environmental information, and skillfully change physiological functions and actions so as to increase their own survival and reproductive success rates. In the vertebrate animals, the brain is playing crucial roles in the center of this strategy. Thus, vertebrates living in different environments show diversity in the brain morphology and function to achieve different survival and reproductive strategies. Indeed, if we look at the external form of the vertebrate brains, their structures are quite diverse (Fig. 1). With the remarkable development and expansion of the field of molecular neuroscience in recent years, genes important for the brain function and morphology have been elucidated one after another using experimental model organisms. However, most of the evolutionary mechanisms that generate the brain diversity remain unresolved. Although the structure and function of the brains are diverse, it is obvious that they are maintaining a lot of commonality. For example, the brains of vertebrates acquired their basic forms such as the forebrain (telencephalon and diencephalon), midbrain, and hindbrain in the early stage of evolution, and have changed the morphology and function of each region while maintaining the basic structures during evolution. Studies on the brain evolution had already been conducted by comparative anatomical approaches by scientists such as Ramon y Cajal (1995) and Johnston (1906) in the early twentieth century. Knowledge has accumulated since then, and MacLean (1990), Striedter (2005) and others have attempted to find certain patterns for the brain evolution. In spite of those efforts, the current status is still far from elucidating the whole picture of the mechanism of the brain evolution. In this special issue, we collected review and research articles from scientists performing cutting-edge research in evolutionary neuroscience. Sugahara et al. (2017) speculate the ancestral vertebrate brain architecture through comparison of the cyclostome and gnatostome brains. Yamamoto et al. (2017a) review regionalization of the anterior part of the forebrain of Osteichtyes (bony fish). The anterior part of the forebrain is subdivided into the telencephalon dorsally and the hypothalamus ventrally. They propose a new morphogenetic unit, optic recess area, between the telencephalon and the hypothalamus. Yamamoto et al. (2017bb) review descending pathways to the spinal cord in teleosts, which play crucial roles in controlling locomotion, including the rubospinal tract which originates from the nucleus ruber identified recently, and compare them with the descending pathways in mammals. Oltrabella et al. (2017) review roles of the endocannabinoid system (eCBs) in synaptic transmission, behaviors, and development in mammals, amphibians and zebrafish, and discuss evolutionary considerations. Takeuchi et al. (2017) review mating-related behaviors in the medaka fish, namely female mating preference and male mate-guarding behavior, and their control by neuropeptide neuromodulatory systems. Matsui (2017) reviews the structure and function of the dopamine system, cerebellum and nucleus ruber, commonly shared in the teleost and human brains. Hibi et al. (2017) review and discuss evolutionary mechanisms that generate the structure and neural circuit diversity of the vertebrate cerebellum. Kumamoto & Hanashima (2017) review pleiotropic roles of a forkhead box protein family gene, Foxg1, in the vertebrate brain development, and discuss its roles in emergence of the telencephalon and its repetitive use in sequential events of the telencephalic development. Nomura & Izawa (2017) focus on avian brains, and introduce developmental aspects of the brain organization and neuronal networks for specific avian behaviors. Noguchi et al. (2017) studied Sema3A expression in embryos of Chinese soft-shelled turtle, chick, ocelot gecko and mouse, and indicated that Sema3A plays a crucial role for neurolal network formation. Hatakeyama et al. (2017) introduce the guinea pig brain as a model system to study mechanisms and principles of cortical folding. We think that vertebrates acquired a basic program for building the brain on the genome at the early stage of divergence, in which the flexibility to create subsequent diversity and innovation was already pre-installed. Then a variety of brains that we are currently observed have been created upon accomplishment of numerous innovations (i.e., mutations, deletions, duplications etc.) on the genome. Recently we have been encountering dramatic advances in techniques for imaging, gene and genome engineering and large-scale sequencing, and are hoping that we will be able to open up the new field of scientific research to understand the evolutionary principle of the brain by studying model and non-model vertebrate animals.