Zebrafish semaphorin Z1b inhibits growing motor axons in vivo

Zebrafish semaphorin Ib (sema Zlb) is a new member of the semaphorin family , related to mammalian sema Drm. It is expressed in rhombomeres three and f ive, and in the posterior half of newly formed somites which is avoided by ventrally extending motor axone. Embryos injected at the 1-2 cell stage wit h synthetic sema Zlb mRNA developed normally but many (63%) showed missing or stunted ventral motor nerves. Other axons, somites, and hindbrain rhombo meres were not affected. No abnormalities were seen in control embryos inje cted with lacZ mRNA. Sema Zlb might normally influence the midsegmental pat hway choice of the ventrally extending motor axons by contributing to a rep ulsive domain in the posterior somite. (C) 1999 Elsevier Science Ireland Lt d. All rights reserved.