PRDL-A, A GENE MARKER FOR HYDRA APICAL DIFFERENTIATION-RELATED TO TRIPLOBLASTIC PAIRED-LIKE HEAD-SPECIFIC GENES

Two homeobox genes, prdl-a and prdl-b, which were isolated from a Hydr a a vulgaris cDNA library, encode paired-like class homeodomains highl y related to that of the aristaless-related genes. In adult polyps, pr dl-b is a marker for synchronously dividing nematoblasts while prdl-a displays an expression restricted to the the nerve cell lineage of the head region. During budding and apical regeneration, an early and tra nsient prdl-a expression was observed in endodermal cells of the stump at a time when the head organizer is established. When apical regener ation was delayed upon concomittant budding, prdl-a expression was fou nd to be altered in the stump, Furthermore, a specific anti-prdl-a pro tein immunoserum revealed that prdl-a was overexpressed in adult polyp s of the Chlorohydra viridissima multiheaded mutant, with an expressio n domain extending below the tentacle ring towards the body column. Ac cordingly, prdl-a DNA-binding activity was enhanced in nuclear extract s from this mutant. These results suggest that prdl-a responds to apic al forming signals and might thus be involved in apical specification, When a marine hydrozoan (Podocorynae carnea) was used, the anti-prdl- a antibody showed crossreactivity with cells located around the oral r egion, indicating that prdl-a function is shared by other cnidaria. Th e ancestral role for prdl-a-related genes in the molecular definition of the head (or oral-surrounding region) is discussed.