Characterization of gill-specific genes of the acorn worm Ptychodera flava

Acorn worms are hemichordate deuterostomes that have remarkable gills thoug ht to be homologous to pharyngeal ails in urochordates and cephalochordates , and pharyngeal pouches in vertebrates. In search of molecular keys to ana lyzing the origin and evolution of the anterior gut and neck region of the chordate body, the present study isolated cDNA clones for six gill-specific genes, designated PfG1 to PfG6, from Ptychodera flava using differential s creening of a cDNA library of RNA from ails. Northern blotting confirmed th at these genes were all expressed only in the gills. In situ hybridization showed that the expression of these genes is limited to the endodermally de rived columnar epithelium of the pharynx. PfG1 encodes a 42-kDa polypeptide containing sequence similar to D-domains, protein domains characteristic o f extracellular proteins. Expression of PfG1 is localized in a delimited pa ttern along the columnar epithelium of the inner all apparatus. Expression in the epibranchial ridge appears as two stripes running longitudinally in the epithelium just lateral of the midline, A stripe of expression also app ears in a slightly posterior portion on the curve of each band of columnar epithelium on the pharyngeal surface of the secondary gill bars. The five o ther all-specific genes, PfG2 to PfG6, encode a family of C-type lectin pol ypeptides that appear to be secreted proteins. PfG2 to PfG6 are also expres sed in the columnar epithelium of the epibranchial ridge as two parallel st ripes, but at the lateral margin of the ridge. One of the genes, PfG6, is a dditionally expressed in the innermost curve of the epithelium on the phary ngeal surface of each secondary gill bar. The localization of expression of PfPax1/9, a all-specific transcription factor gene, was examined and shown to also be primarily in the endodermal columnar epithelium on the pharynge al faces of the gill bars. On the secondary gill bars, where PfG1 and PfG6 are also expressed in the columnar epithelium, PfPax1/9 is expressed in the anterior and posterior portions but signal is not evident in the epitheliu m on the central, innermost curve of the gill bar. The anterior domain of P fPax1/9 expression is more extensive but overlaps the anterior domain of Pf G1 expression, whereas its posterior domain of expression is more posterior and complementary to that of PfG6. Dev Dyn 2000;217;309-319. (C) 2000 Wile y-Liss, Inc.