Invertebrate Glia Evolution Web Page
[updated January 2018]
This figure shows the state of knowledge as of 2017 on the distribution of "glia" among the various bilaterian taxa (groups having bilateral symmetry, e.g. us but not jellyfish). The definition of "glia" depends to some extent on whom you ask. For invertebrates, it means roughly the "connective tissue of the nervous system (Bullock and Horridge 1965). In this diagram, it is based solely on morphological characteristics, primarily a cell type with a close association with neurons and/or the tendency to wrap neural structures in various ensheathments. Molecular markers that have sometimes been used to identify glial cells tend to be taxon-specific, and many are found in organisms such as sponges and cnidarians that are usually considered to lack glia (or neurons) completely. The phylogeny used is based on that of Cannon et al. (2016) supplemented with a platyhelminth tree from Egger et al. (2015). The calibration bar indicates the number of amino acid substitutions per site along each branch, the longer lines thus corresponding to a greater number of times that any given amino acid locus will have been altered in evolution. The most recent upheaval in the tree has been the evidence that the xenacoelomorphs are a basal bilaterian clade and not a clade within the platyhelminths (see e.g. Cannon et al 2016). One interpretation of the pattern in this tree is that glia-like cell types have arisen 5 times in the course of evolution, in xenacoelomorphs (very primitive flatworms), ecdysozoans(insects, crustaceans, roundworms), lophotrochozoans (e.g. earth worms, leeches, squid), platyhelminths (e.g. flatworms, tapeworms) and deuterostomes (e.g. star fish, fish, humans). However, this is highly tentative, as few organisms within many of these taxa (groupings) have been examined carefully for glia and a taxon lacking glia may still have descended from an ancestor that had glia, only to have lost it. Take this figure with a grain of salt - the next update will be different!
Hypothetical steps in glia evolution
This cartoon shows some possible pathways alnog which glia might have arisen in evolution:
(A) In the original state, as represented in the Cnidaria (jelly fish, anemones, corals and their kin), neurons (blue cells labeled "n") are either near the inner "basal" border of the outer epithelial cells of the body wall (uncolored cells labeled "e"), or sandwiched between those cells with access to the outside as sensory cells. A basement membrane (grey line, "bm") separates the epithelium and neuorns from the rest of the body.
(B) The next stage includes two possible origins: on the left an epithelial cell (yellos, "s") maintains a partial sheath around neural elements (blue "n") that descend to sub-epithelial positions; alternatively (right), supporting cells ("magenta, "t") reinforced with microfilaments or microtubules come to lie alongside neurons to provide physical support, assist in axonal bundling and perhaps participate in guidance of outgrowing axons during development.
(C) In the third stage figured, internalized neuronal elements (in blue) are accompanied by sparse, perhaps migratory, glial cells light green) that partially or fully ensheath axon bundles, with (lower schematic cross section, dark green glial cell) or without (upper section, yellow glial cell) cytoplasmic penetration between axons.
(D) In one of two possible pathways, elements of the internal nervous system (a = axons [blue]; n = neuron somata [blue]) come to be fully surrounded by sheath cells (s; yellow), poviding a sheath capable of slating neural elements form surrounding hemolymph, providing for a separate environment for the enclosed neurons.
(E) Alternatively, space between elements of the internal nervous system come to be invaded by sheet-like interstitial glial cells ("g"; dark green). Neural elements are still exposed to fluid medium outside of the nervous system, albeit still enclsd by the basement membrane
(F) Sheath cells from stage D invade the spaces between neuronal elements to generate interstitial or "neuropil" glia; alternatively interstitial glial cells from stage E expand around the outside of the neural elements to provide ensheathment, leading to the inal structure pictured here of a compact central nervous system surrounded by a glial sheath and infiltrated by tightly-associated interstitial glia.
Bullock TH, Horridge GA. (1965). Structure and Function in the Nervous System of Invertebrates Vol. I. W. H. Freeman, San Francisco. 798 pp. Vol II. ____ pp
Cannon JT, Vellutini BC, Smith III J, Ronquist F, Jondelius U, Hejnol A. (2016) "Xenacoelomorpha is the sister group to Nephrozoa" Nature 530: 89-93.
Egger B, Lapraz F, Tomiczek B, Muller S, Dessimoz C, Girstmair J, Skunca N, Rawlinson KA, Cameron CB,Beli E, Todaro MA, Gammoudi M, Norena C, Telford1 MJ (2015) "A transcriptomic-phylogenomic analysis of the evolutionary relationships of flatworms" Current Biol. 25: 1347-1353.
Artemia (brine shrimp, primitive crustacean) axons and glia
Ax = axon; n = neurite; Gl = glia.
Image by Jenn Kong
This material has been assembled and presented as a public service by Dan Hartline, Bekesy Laboratory of Neurobiology, Pacific Biosciences Research Center, University of Hawaii at Manoa (danh at hawaii.edu). Opinions expressed here are those of the author and do not represent the positon or policies of the University or any funding agency.