AN AXIAL DOMAIN OF HOM HOX GENE-EXPRESSION IS FORMED BY MORPHOGENETICALIGNMENT OF INDEPENDENTLY SPECIFIED CELL LINEAGES IN THE LEECH HELOBDELLA/

The homeobox gene Lox2, a member of the HOM/Hox gene class, is express ed in a restricted domain along the anteroposterior (A-P) body axis of the leech Helobdella. The segmental tissues of the leech embryo arise from the parallel merger of five distinct and bilaterally paired cell lineages generated by embryonic stem cells or teloblasts. Injection o f cell lineage tracers coupled with anti-LOX2 immunochemistry reveals that all five teloblast lineages generate central nervous system neuro ns that express the LOX2 protein, and that each lineage expresses LOX2 within a similar domain of body segments. Some lineally identified ne urons display anti-LOX2 immunoreactivity over the entire expression do main, but the OM7 neuron has a distinctively high level of LOX2 expres sion, which is restricted to the seventh midbody ganglion. To ascertai n the role of positional information in the axial patterning of LOX2 e xpression, we performed focal cell ablations that displaced one or ano ther of the teloblast lineages out of segmental register with the othe r axial tissues. Such displacements brought about a corresponding shif t in the LOX2 expression of the perturbed lineage, and had little or n o effect on the LOX2 expression of the other, unperturbed lineages. Th is result indicates that the axial domain of LOX2 expression is not sp ecified by positional cues acting coordinately across the various telo blast lineages, nor would it seem that the expression domain is imprin ted from one lineage to the others. Rather, the different teloblast li neages acquire their axial patterns independently, and secondarily bri ng these patterns into alignment along the A-P axis through a process of morphogenetic assembly.