Genealogical correspondence of a forebrain centre implies an executive brain in the protostome-deuterostome bilaterian ancestor

Abstract

Orthologous genes involved in the formation of proteins associated with memory acquisition are similarly expressed in forebrain centres that exhibit similar cognitive properties. These proteins include cAMP-dependent protein kinase A catalytic subunit (PKA-Cα) and phosphorylated Ca(2+)/calmodulin-dependent protein kinase II (pCaMKII), both required for long-term memory formation which is enriched in rodent hippocampus and insect mushroom bodies, both implicated in allocentric memory and both possessing corresponding neuronal architectures. Antibodies against these proteins resolve forebrain centres, or their equivalents, having the same ground pattern of neuronal organization in species across five phyla. The ground pattern is defined by olfactory or chemosensory afferents supplying systems of parallel fibres of intrinsic neurons intersected by orthogonal domains of afferent and efferent arborizations with local interneurons providing feedback loops. The totality of shared characters implies a deep origin in the protostome-deuterostome bilaterian ancestor of elements of a learning and memory circuit. Proxies for such an ancestral taxon are simple extant bilaterians, particularly acoels that express PKA-Cα and pCaMKII in discrete anterior domains that can be properly referred to as brains.

Keywords: convergence; evolution; hippocampus; homology; mushroom body.

Figure 1.

Ground pattern organization: neuronal arrangements correspond in the lobed mushroom bodies of arthropods and annelids (a), the dome-like hemiellipsoid bodies of malacostracan crustaceans (b), and in stratified hippocampus of mammalian and non-mammalian vertebrates (c). Primary inputs (pi; e.g. olfactory) terminate distally on systems of approximately parallel fibres belonging to intrinsic neurons (in) in arthropods and the mossy fibres or equivalent in vertebrates. These are intersected by systems of afferent (aff) and outgoing efferent (eff) neurons, with local interneurons (red) defining sequential domains along the parallel fibre array.

Figure 2.

Cross-phyletic correspondence of PKA-Cα immunoreactivity. (a) In the hippocampus of the rat, Rattus norvegicus, mammalian PKA-Cα immunoreactivity (magenta) is strong in the CA fields (CA3). (b) Confocal laser scan of brain of the cockroach, Periplaneta americana, labelled with antibodies against mammalian PKA-Cα (magenta). The concentration of PKA-Cα immunoreactivity in the mushroom body (mb) and ellipsoid body (eb) of the central complex is the same as the pattern of DC0 antibody localization in the cockroach brain [2]. α-Tubulin immunoreactivity (cyan) and nucleic acid stain (green) provide structural reference (a and b). Other abbreviations: dg, dentate gyrus; cx, calyx; grc, granule cells. (c) Western blot assay of mammalian PKA-Cα antibody comparing brain tissue of R. norvegicus (rat) and P. americana reveals a band around 39 kDa, indicating cross-phyletic specificity of this antibody. Scale bars: (a) 600 µm; (b) 200 µm.

Figure 3.

Ground pattern organization: correspondence of pathways and orthogonal organization of neurons. (a,b) Congruent circuitry of the rodent hippocampus and insect mushroom body is demonstrated by pathways from olfactory bulb/antennal lobes (OB, AL) targeting, respectively, the dentate gyrus (DG) and calyces (CX), via the entorhinal cortex (EC) and lateral horn (LH). Mossy fibres, whose dendrites are located in the DG, send their parallel processes into CA fields of the hippocampus (HC). Globuli cells, whose dendrites are located in the CX, send their parallel processes in to the mushroom body lobes (MBL). (c,d.) Golgi impregnated rat CA3 (c) or turtle medial cortex (d) resolve orthogonal neuronal arrangement of parallel mossy fibre processes intersected by efferent dendrites of pyramidal cells. (e,f) Golgi-impregnated cockroach mushroom body lobes (e) or layers of Coenobita hemiellipsoid body (f) resolve corresponding orthogonal arrangements of parallel fibres.

Figure 4.

Anti-PKA-Cα and anti-pCaMKII reveal corresponding immunoreactivity of higher order brain centres across vertebrates. (a–d) Confocal laser scans of brain sections of the lamprey, P. marinus; newt, P. shermani; turtle, T. scripta elegans; and the rat Rattus norvegicus. Circumscribed volumes of the medial pallium of the lamprey (a), and rat hippocampus (d) are highly PKA-Cα immunoreactive (magenta) compared to surrounding tissue. In the newt (b), a circumscribed region of the lateral pallium shows high intensity immunoreactivity to anti-pCaMKII whereas in the turtle (c) the medial cortex is labelled. In all figures, α-tubulin immunoreactivity (cyan) and nucleic acid stain (green) provide structural references. Scale bars: (a–c) 200 µm; (d) 500 µm.

Figure 5.

Anti-DC0 and anti-pCaMKII resolve a substantial brain in the acoel S. roscoffensis. (a–d) Both antisera (DC0 in a,b; anti-pCaMKII in d,e) resolve a bilaterally symmetric region anterior to the statocyst. The origins of caudally directed nerve cords resolved by antisera against synaptotagmin (c), elav (f) and serotonin domains (see figure 6) (adapted from [66,77]) demonstrate their overlap within restricted regions of the DC0- (c) and anti-pCaMKII (f)-positive domains. Together, these relationships suggest a brain within which there are circumscribed tracts, neuropils and modulatory pathways.

Figure 6.

Brain anatomy and corresponding ground patterns with three possible relationships to acoels (brains not to scale): (a) sister group to Deuterostomia, (b) sister group to Protostomia, (c) sister group to Bilateria. In (a–c), DC0 immunoreactivity (magenta) in S. roscoffensis is shown superimposed on serotonin, elav and synaptotagmin immunoreactivity (grey). See [66,77]. In (a) regions corresponding to the hippocampus/mushroom body ground pattern (figure 1), and in (b) neuropils corresponding to mushroom bodies are shown in magenta; their globuli cells are shown in green. Data in (b) are from [3]. Magenta for the brains of S. roscoffensis implies putative ground pattern indicated by expression of elevated DC0 and pCaMKII.