XASH-3, A NOVEL XENOPUS-ACHAETE-SCUTE HOMOLOG, PROVIDES AN EARLY MARKER OF PLANAR NEURAL INDUCTION AND POSITION ALONG THE MEDIOLATERAL AXISOF THE NEURAL PLATE

We have isolated a novel Xenopus homolog of the Drosophila achaete-scu te genes, called XASH-3. XASH-3 expression is neural specific and is d etected as early as stage 11 1/2, making it one of the earliest marker s of neural induction so far described. Moreover, XASH-3 expression wi thin the neural plate is regionally restricted. Transverse bands of XA SH-3 mRNA mark discrete positions along the anteroposterior axis, whil e longitudinal bands mark a discrete position along the mediolateral a xis. This latter site of XASH-3 expression appears to demarcate the pr ospective sulcus limitans, a boundary zone that later separates the fu nctionally distinct dorsal (alar) and ventral (basal) regions of the s pinal cord. In sandwich explants lacking any underlying mesoderm, XASH -3 is expressed in longitudinal stripes located lateral to the midline . This provides the first indication that planar or midline-derived in ductive signals are sufficient to establish at least some aspects of p ositional identity along the mediolateral axis of the neural plate. By contrast, the transverse stripes of XASH-3 expression are not detecte d, suggesting that this aspect of anteroposterior neural pattern is lo st or delayed in the absence of vertically passed signals. The restric ted mediolateral expression of XASH-3 suggests that mediolateral patte rning of the neural plate is an early event, and that this regionaliza tion can be achieved in the absence of inducing signals derived from u nderlying mesoderm.