Inductive interactions regulating body patterning in planarian, revealed by analysis of expression of novel gene scarf.

Subtractive hybridization was used to search for the early difference in gene expression between anteriorly and posteriorly regenerating tissues of the same region of the planarian body. A sequence (named scarf) specific for posteriorly regenerating tissue was isolated, encoding a soluble C-type lectin consisting of two slightly different carbohydrate-recognition domains. Such an unusual bivalent structure allows attribution of the protein to a novel subfamily of C-type lectins. scarf expression in intact planarian is rather abundant and follows a characteristic pattern not linked to any known morphological structure. We performed a series of experiments using scarf as a molecular marker. Its expression was monitored during different types of regeneration by whole-mount in situ hybridization and reverse-transcription polymerase chain reaction. The obtained data suggest that scarf expression is positively regulated by anterior tissues closely adjacent to the scarf-expressing region, so that their surgical removal results in rapid scarf switch-off. In turn, tissues expressing scarf seem to inhibit its activation anteriorly. This indicates that at least some of the body patterning events in planarians are based on a system of reciprocal inductive interactions rather than on a global morphogen gradient.