Myelination, the ensheathment of axons by membranes of highly specialized glial cells, has been a crucial innovation during early vertebrate evolution. It enables high nerve signal conduction velocities, while maintaining nervous system size and energy requirements at moderate levels. Consequently, myelination has been conserved in all extant gnathostome vertebrates. In a genomewide mRNA expression screen, we identified several novel neural crest and myelin-specific transcripts in the zebrafish

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... (Danio rerio). Here, we describe the characterization of two proteins, Zwilling-A and -B (ZwiA and ZwiB), which are exclusively expressed in myelinating glia of teleosts. They are structurally homologous and are translated from a common, bicistronic transcript. No similarities to sequences or domains of other proteins were detected. Analysis of phylogeny, genomic organization, and genomic syntenies suggests that the zwi gene has appeared soon after the teleost-specific genome duplication event and evolved under conservative selective pressure. We hypothesize that ZwiA and ZwiB serve important physiological functions in teleost myelin.