Abstract
Mushroom bodies are known from annelids and arthropods and were formerly assumed to argue for a close relationship of these two taxa. Since molecular phylogenies univocally show that both taxa belong to two different clades in the bilaterian tree, similarity must either result from convergent evolution or from transformation of an ancestral mushroom body. Any morphological differences in the ultrastructure and composition of mushroom bodies could thus indicate convergent evolution that results from similar functional constraints. We here study the ultrastructure of the mushroom bodies, the glomerular neuropil, glia-cells and the general anatomy of the nervous system in Sthenelais boa. The neuropil of the mushroom bodies is composed of densely packed, small diameter neurites that lack individual or clusterwise glia enwrapping. Neurites of other regions of the brain are much more prominent, are enwrapped by glia-cell processes and thus can be discriminated from the neuropil of the mushroom bodies. The same applies to the respective neuronal somata. The glomerular neuropil of insects and annelids is a region of higher synaptic activity that result in a spheroid appearance of these structures. However, while these structures are sharply delimited from the surrounding neuropil of the brain by glia enwrapping in insects, this is not the case in Sthenelais boa. Although superficially similar, there are anatomical differences in the arrangement of glia-cells in the mushroom bodies and the glomerular neuropil between insects and annelids. Hence, we suppose that the observed differences rather evolved convergently to solve similar functional constrains than by transforming an ancestral mushroom body design.
Highlights
Mushroom bodies are paired brain centers described for arthropods and annelids (Strausfeld et al 2009; Wolff and Strausfeld 2015)
In order to fill this gap of knowledge we studied the mushroom bodies in Sthenelais boa (Johnston 1833) (Sigalionidae), a vagile, carnivorous and presumably predatory species
The brain is composed of a central neuropil, which is embedded in a massive layer of glia-cell processes and somata showing an orange coloration in Azan staining (Figs. 2, 3, 4)
Summary
Mushroom bodies are paired brain centers described for arthropods and annelids (Strausfeld et al 2009; Wolff and Strausfeld 2015). Already mentioned for Arthropods in 1850 (Dujardin 1850), similar structures were described nearly 40 years later in species of the annelid taxon Polynoidae (Haller 1889). Due to their similar anatomy, these higher brain centers were historically supposed to be one character, among others, to substantiate annelid and arthropod. Due to the high number of neurons the somata form a hemispherical “helmet” of staggered perikarya that reminds on the hood of a mushroom. In insects, these cells are called Kenyon-cells, in Annelida globulicells. Detailed studies on the function of mushroom bodies of polychaetes are lacking, but recently their relation to chemosensory function was shown in the annelid Platynereis dumerilii (Audouin and Milne Edwards 1833) (Chartier et al 2018)
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