Comparison Zebrafish – Macrosmatic Rodent Amygdala. (A) Cladogram indicating presence of a complex amygdala ground plan and nucleus of the lateral olfactory tract (nLOT) in the last common ancestor of teleosts and mammals. Our results indicate that the last common ancestor between ray-finned fish and mammals already showed a tetrapod-like main extended amygdala ground plan (red circle). Previously, most scientists assumed that a bipartite main versus olfactory extended amygdala evolved with a vomeronasal epithelium in the last common ancestor of lungfish and tetrapods (González et al., 2010). However, molecular evidence hint toward the presence of a bipartite olfactory system already in agnathan lamprey (Chang et al., 2013). We speculate, therefore, that a bipartite and complex amygdala may evolved with the earliest vertebrates. (B) Prosomeric comparisons of the zebrafish/teleostean amygdala (left) with the situation in macrosmatic rodents (right). Both the schematized zebrafish brain cross sections and the parasagittal view on the left side indicate that the zebrafish amygdala holds several previously misinterpreted territories such as the bed nucleus of the stria terminals (BST), the medial amygdala (MeA) and its anterior (MeAa), posterior (MeAv) and dorsal division (MeAd), the posteriormedial pallial amygdala (PMPa), the integrative olfactory pallium (IOP), and the nucleus of the lateral olfactory tract (nLOT). Due to the teleostean-specific outward growing process (eversion), these territories lie on top of the telencephalon and cover the zebrafish homolog to the mammalian isocortex. In mammals (right side), we find the opposite situation. Here, the amygdaloid complex is located in the anterior ventral depth of the brain covered by the enlarged isocortex (modified after Mueller et al., 2011; Mueller, 2012). For molecular definition of pallial, subpallial, and EmT-derived (EmT-d) amygdaloid territories see Table 1. Abbreviations see table.
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