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Secreted morphogens induce distinct cellular responses in a concentration-dependent manner and act directly at a distance 1-7. The existence of morphogens during mesoderm induction and patterning in vertebrates has been highly controversial, and it remains unknown whether endogenous mesoderm inducers act directly as morphogens 8-10, function locally 9 or act through relay mechanisms 11-12. Here we test the morphogen properties of Cyclops and Squint-two Nodal-related transforming growth factor-[beta] signals required for mesoderm formation and patterning in zebrafish 13-16. Whereas different levels of both Squint and Cyclops can induce different downstream genes 14,17-19, we find that only Squint can function directly at a distance. These results indicate that Squint acts as a secreted morphogen that does not require a relay mechanism.

(C) 2001 Nature Publishing Group