Siamois functions in the early blastula to induce Spemann's organiser

In Xenopus, the Spemann organiser is defined as a dorsal territory in the early gastrula that initiates development of the embryonic axis. It has been shown that the early zygotic transcription factor Siamois is essential for Spemann's organiser formation. By the onset of gastrulation, the organiser is patterned into a vegetal head organiser, which induces anterior structures upon transplantation, and a more animal trunk organiser, which induces a posterior neuraxis. However, it is unclear when these distinct organiser domains are initially specified. To shed light on this question, we analysed the temporal activity of Siamois, as this factor induces both head and trunk development, when ectopically expressed via mRNA injection. In this study, we expressed Siamois ectopically at different time points and analysed the extent of axial development. Using a hormone-inducible version of Siamois, we found evidence for a tight window of competence during which ventral cells can respond to Siamois by commencing both the head and the trunk genetic programmes. The competence to form Spemann's organiser was lost 2 h before gastrulation, although partial axis formation could still occur following delayed activation of Siamois. We demonstrate that this late response to Siamois involves a new role for this gene, which can indirectly repress ventral gene expression, in the absence of known organiser genes.