Control of gastrula cell motility by the Goosecoid/Mix.1/ Siamois network: basic patterns and paradoxical effects.

In the vegetal half of the Xenopus gastrula, cell populations differ with respect to migration on fibronectin substratum. We show that the paired-class homeodomain transcription factors Goosecoid (Gsc), Mix.1, and Siamois (Sia) are involved in the modulation of migration velocity and cell polarity. Mix.1 is expressed in the whole vegetal half and serves as a competence factor that is necessary, but not sufficient, for rapid cell migration and polarization. In the head mesoderm, Gsc and Sia are coexpressed with Mix.1, promoting rapid cell migration and polarization. Ectopic expression of Gsc and Sia in both vegetal and ventral regions often generates paradoxical effects; if a factor activates a certain motility trait in one region, it inhibits it in the other. Migration velocity and cell polarity are regulated independently. Fast and efficiently migrating multipolar cells and slow-moving polarized cells can be obtained by ectopic expression of these transcription factors in different combinations.