IntroductionDuring vertebrate development, the central nervous system (CNS) has stereotyped neuronal tracts (scaffolds) that include longitudinal and commissural axonal bundles, such as the medial longitudinal fascicle or the posterior commissure (PC). As these early tracts appear to guide later-developing neurons, they are thought to provide the basic framework of vertebrate neuronal circuitry. The proper construction of these neuronal circuits is thought to be a crucial step for eliciting coordinated behaviors, as these circuits transmit sensory information to the integrative center, which produces motor commands for the effective apparatus. However, the developmental plan underlying some commissures and the evolutionary transitions they have undergone remain to be elucidated. Little is known about the role of axon guidance molecules in the elicitation of early-hatched larval behavior as well.ResultsHere, we report the developmentally regulated expression pattern of axon-guidance molecules Slit2 ligand and Robo2 receptor in Xenopus laevis and show that treatment of X. laevis larvae with a slit2- or robo2-morpholino resulted in abnormal swimming behavior. We also observed an abnormal morphology of the PC, which is part of the early axonal scaffold.ConclusionOur present findings suggest that expression patterns of Slit2 and Robo2 are conserved in tetrapods, and that their signaling contributes to the construction of the PC in Xenopus. Given that the PC also includes several types of neurons stemming from various parts of the CNS, it may represent a candidate prerequisite neuronal tract in the construction of subsequent complex neuronal circuits that trigger coordinated behavior.

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With the exception of that from the olfactory system, the vertebrate sensory information is relayed by the dorsal thalamus (dTh) to be carried to the telencephalon via the thalamo-telencephalic tract. Although the trajectory of the tract from the dTh to the basal telencephalon seems to be highly conserved among amniotes, the axonal terminals vary in each group. In mammals, thalamic axons project onto the neocortex, whereas they project onto the dorsal pallium and the dorsal ventricular ridge (DVR) in reptiles and birds. To ascertain the evolutionary development of the thalamo-telencephalic connection in amniotes, we focused on reptiles. Using the Chinese soft-shelled turtle (Pelodiscus sinensis), we studied the developmental course of the thalamic axons projecting onto the DVR. We found, during the developmental period when the thalamo-DVR connection forms, that transcripts of axon guidance molecules, including EphA4 and Slit2, were expressed in the diencephalon, similar to the mouse embryo. These results suggest that the basic mechanisms responsible for the formation of the thalamo-telencephalic tract are shared across amniote lineages. Conversely, there was a characteristic difference in the expression patterns of Slit2, Netrin1, and EphrinA5 in the telencephalon between synapsid (mammalian) and diapsid (reptilian and avian) lineages. This indicates that changes in the expression domains of axon guidance molecules may modify the thalamic axon projection and lead to the diversity of neuronal circuits in amniotes.Key words: amniote, axon guidance molecule, dorsal thalamus, telencephalon, turtle. IntroductionAll extant amniotes are classified into synapsids or diapsids, based on anatomical traits. Synapsids, whose skulls possess a single foramen (temporal fenestra) behind the orbit, diverged from ancestral reptiles in the Carboniferous period (~300 Ma). Living mammals originated from a certain group of cynodontia, a clade in the synapsids that arose in the late Permian period (O'leary et al. 2013;Ruta et al. 2013). Through evolution, synapsids generated various lineages and prospered worldwide. The origin and diversification of extant mammals are thought to have occurred in the Mesozoic era (Bininda-Emonds et al. 2007;Luo 2007;Rowe et al. 2008). Diapsids, on the other hand, represent a lineage with two temporal fenestrae in the skull, including all living reptiles and birds and inhabit various environments on earth. Dinosaurs originated from diapsids in the Triassic period, evolved various forms, and reigned on the terrestrial environment for approximately 170 million years. Numerous studies have shown that birds evolved from theropoda, a group of saurischian dinosaurs (Ostrom 1976;Padian & Chiappe 1998).Since the mammals and birds have diverged from their common ancestor for a long period, the brains of animals in each lineage are thought to have developed independently. The most obvious differentiation between the mammalian and avian brain can be observed in their telencephalon. Notably, the mammalian telenceph...