Myelin Phylogeny and Evolution
The four general taxa in which myelin occurs are widely separated phylogenetically:
Among the deuterostomes, vertebrates alone are myelinated (other deuterostomes include the echinoderms such as starfish and sea urchins, and protochordates such as tunicates and salps). If we take the lack of myelin in hagfish and lampreys as primitive, the earliest myelin shows up in the sharks. The situation vis a vis myelin is unknown in the many extinct intervening taxa, as the rocks are mute on this issue for soft tissue. It might be that the sharks, having perfected myelin sufficiently upon their divergence from the placoderm line, may have used this edge to stave off extinction where the placoderms could not. It is tempting to reason that the major selective advantage afforded by myelin was a key factor in the success of sharks and all later lines of vertebrates (Zalc & Colman 2000). However, there are many earlier divergences of extinct groups where myelin could have arisen.
The currently available annelid phylogeny is confusing as far as putative myelination patterns go. "Putative" because only the oligochaetes have been examined with modern electron microscopy, so other identifications of myelin must be considered tentative pending electronmicroscopic confirmation. If the situation inferred from light microscopic studies is correct (Nicol 1948), it would appear that annelids may have evolved myelin independently as many as three times.
The malacostracan crustaceans are similarly puzzling for similar reasons. Few of the diverse groups have been surveyed with electron microscopy. The clearest cases are the pelagic or quasipelagic decapod shrimp, but except for one unconfirmed report, the major benthic decapods lack myelin. Reports from light microscopy of myelin in two more primitive pelagic malacostracans, a euphausiid and a mysid, remain to be confirmed, but if confirmed, it would suggest that myelin might have been lost by species returning to benthic life. The enhanced predator pressures in pelagic life may promote the evolution of myelin.
The clearest split in a crustacean line occurs in the calanoid copepods. Two superfamilies have uniformly lacked myelin, and two others have uniformly possessed it. The current phylogeny of the group suggests that the latter groups are the more recently evolved. These, too, operate most freely in photic zones of the open ocean where they have little opportunity to hide from predators.
Reports of myelin among several phylogenetically diverse groups are shown on the phyletic tree below. Not all of these reports have been confirmed in the electron microscope yet (asterisks in the figure below; examination of a bamboo worm has failed to turn up myelin*).
* Hartline, D.K. and J.H. Kong (2009) "Axonal sheaths in two reportedly myelinated
polychaete nervous systems: Asychis elongata and Capitella sp. I.
Bull. Mt. Desert Is. Biol. Lab. 48: 86-87
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 position or policies of the University or any funding agency.
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