NESTED EXPRESSION AND SEQUENTIAL DOWN-REGULATION OF THE XENOPUS CAUDAL GENES ALONG THE ANTERIOR-POSTERIOR AXIS

Expression of the Xenopus Xcad-1 and Xcad-2 genes initiates during ear ly gastrulation exhibiting a dorsoventral asymmetry in their domains o f transcription. At mid-gastrulation the ventral preference becomes st ronger and the caudal genes take up a posterior localization in their expression, which they will maintain until their downregulation along the dorsal midline. Comparison of the three Xenopus caudal genes revea led a temporal and spatial nested set of expression patterns. The tran scription of the caudal genes is sequentially downregulated with the o ne expressed most caudally (Xcad-2) being shut down first, this sequen ce is most evident along the dorsal midline. This pattern of expressio n suggests a role for the caudal genes as posterior determinants along the anteroposterior axis. In chicken, mouse, man and Xenopus three me mbers of the caudal family have been identified in the genome. Even th ough in Xenopus the Xcad-3 gene has been previously described, in orde r to obtain a better insight on the role of the caudal genes a compara tive study of all three frog genes was performed. (C) 1998 Elsevier Sc ience Ireland Ltd.