LOCALIZED ENHANCEMENT AND REPRESSION OF THE ACTIVITY OF THE TGF-BETA FAMILY MEMBER, DECAPENTAPLEGIC, IS NECESSARY FOR DORSAL-VENTRAL PATTERN-FORMATION IN THE DROSOPHILA EMBRYO

Seven zygotically active genes are required for normal patterning of t he dorsal 40% of the Drosophila embryo. Among these genes, decapentapl egic (dpp) has the strongest mutant phenotype: in the absence of dpp, all cells in the dorsal and dorsolateral regions of the embryo adopt f ates characteristic of more ventrally derived cells (Irish and Gelbart (1987) Genes Dev. 1, 868-879). Here we describe the phenotypes caused by alleles of another of this set of genes, tolloid, and show that to lloid is required for dorsal, but not dorsolateral, pattern. Extrageni c suppressors of tolloid mutations were isolated that proved to be mut ations that elevate dpp activity. We studied the relationship between tolloid and dpp by analyzing the phenotypes of tolloid embryos with el evated numbers of the dpp gene and found that doubling the dpp+ gene d osage completely suppressed weak tolloid mutants and partially suppres sed the phenotypes of tolloid null mutants. We conclude that the funct ion of tolloid is to increase dpp activity. We also examined the effec t of doubling dpp+ gene dosage on the phenotypes caused by other mutat ions affecting dorsal development. Like tolloid, the phenotypes of mut ant embryos lacking shrew gene function were suppressed by elevated dp p, indicating that shrew also acts upstream of dpp to increase dpp act ivity. In contrast, increasing the number of copies of the dpp gene en hanced the short gastrulation (sog) mutant phenotype, causing ventrola teral cells to adopt dorsal fates. This indicates that sog gene produc t normally blocks dpp activity ventrally. We propose that the tolloid, shrew and sog genes are required to generate a gradient of dpp activi ty, which directly specifies the pattern of the dorsal 40% of the embr yo.