Krox-20 patterns the hindbrain through both cell autonomous and non cell-autonomous mechanisms

The Krox-20 gene encodes a zinc finger transcription factor, which has been shown previously, by targeted inactivation in the mouse, to be required fo r the development of rhombomeres (r) 3 and 5 in the segmented embryonic hin dbrain. In the present work, Krox-20 was expressed ectopically in the devel oping chick hindbrain by use of electroporation. We demonstrate that Krox-2 0 expression is sufficient to confer odd-numbered rhombomere characteristic s to r2, r4, and r6 cells, presumably in a cell-autonomous manner. Therefor e, Krox-20, appears as the major determinant of odd-numbered identity withi n the hindbrain. In addition, we provide evidence for the existence of a no n cell-autonomous autoactivation mechanism allowing recruitment of Krox-20- positive cells from even-numbered territories by neighboring Krox-20-expres sing cells. On the basis of these observations, we propose that Krox-20 reg ulates multiple, intertwined steps in segmental patterning: Initial activat ion of Krox-20 in a few cells leads to the segregation, homogenization, and possibly expansion of territories to which Krox-20 in addition confers an odd-numbered identity.