A ROLE FOR CYTOPLASMIC DETERMINANTS IN MESODERM PATTERNING - CELL-AUTONOMOUS ACTIVATION OF THE GOOSECOID AND XWNT-8 GENES ALONG THE DORSOVENTRAL AXIS OF EARLY XENOPUS-EMBRYOS

Although an induction event is required for the formation of mesoderm in Xenopus embryos, it is not clear that this induction is wholly suff icient to give rise to a correctly patterned mesodermal layer. We have studied the expression of the two genes, goosecoid and Xwnt-8, in Xen opus gastrulae in which cell-cell communication, and therefore mesoder m induction, has been prevented by frequent cell dispersion. Although neither the early pan-mesodermal marker Xbra nor the muscle-specific a -actin gene were activated under these conditions, goosecoid and Xwnt- 8 were activated in cells of dorsal and ventrolateral origin respectiv ely, thus correctly reflecting their distribution during normal develo pment. We also show that the spatial pattern of expression of these tw o genes along the animal-vegetal axis is similar in normal and in diss ociated early gastrulae: goosecoid is mainly expressed in future mesod erm while the domain of expression of Xwnt-8 spans the mesoderm-endode rm boundary. These results show that, during the blastula and early ga strula stages, gene activation can be controlled cell-autonomously alo ng both the animal-vegetal and dorsoventral embryo axes. This suggests that the inheritance of localised maternal cytoplasmic determinants i s a key event for the patterning of mesoderm. We present a modified mo del of mesoderm formation in which the different mesoderm cell types a re produced as a result of cooperation between induction-dependent and induction-independent immediate-early genes.