THE CHICK BRACHYURY GENE - DEVELOPMENTAL EXPRESSION PATTERN AND RESPONSE TO AXIAL INDUCTION BY LOCALIZED ACTIVIN

The mouse Brachyury gene (T) is required in notochord differentiation and posterior mesoderm formation during axial development. We have iso lated the chick homologue of T(Ch-T) and determined its putative prote in sequence and expression pattern during embryogenesis. Ch-T is expre ssed in the epiblast close to and within the primitive streak, in earl y migrating mesoderm and in the notochord. In later stages Ch-T expres sion is found in the tail bud and in the entire notochord. The notocho rd expression ceases in an anterior-posterior wave when the formation of the body anlage is completed. This pattern is consistent with those reported for the expression of the mouse T gene and the T homologues of Xenopus laevis and zebrafish, suggesting that the mechanisms of emb ryonic pattern formation are highly conserved in all vertebrates. The N-terminal half of Ch-T shows a very high degree of sequence identity with the corresponding region of mouse T which has DNA-binding activit y, and with the N-terminal half of Xenopus (Xbra) and zebrafish (Ntl) T protein. Finally, we have analyzed the effects of activin A on Ch-T induction and axis formation. Localized activin A treatment of prestre ak blastoderms results in ectopic Ch-T expression that correlates with formation of second primitive streaks or with repositioning of the si te of single streak origin (Cooke et al., 1994). These results strengt hen the previous evidence that Brachyury activation is an early respon se to axis-inducing signals in vivo. (C) 1995 Academic Press, Inc.