Sequence and expression of the rainbow trout winged helix/forkhead transcription factor FoxF1.

FoxF1 is a member of the winged helix/forkhead transcription factor gene family. We have cloned the cDNA encoding a rainbow trout FoxF1 homologue, and examined its developmental gene expression pattern. By 7 days postfertilization (dpf at 14 degrees C), FoxF1 is expressed throughout the alimentary tract in the mesenchymal cells adjacent to the endodermal epithelium, with intense signals on the dorsal side of the oral cavity and in the primitive stomach. As ontogeny proceeds, expression is down-regulated in the oral cavity and esophagus, but persists in the pharynx, stomach, and intestine. Hybridization signals are also detected in the developing liver, and in the mesenchyme layer around the notochord. From 18dpf onwards, dramatic changes occur in gene expression in the branchial region. As the gill filaments elongate from the branchial arches, FoxF1 begins to be expressed along the central cell cord running through each gill filament, and then switches over its rod-like expression to a repetitive pattern, alternating on either side along the proximal part of the filament. A signal is further localized to the primitive pillar cells as they form gill lamellae. In addition to illustrating the conserved FoxF1 expression pattern in the developing digestive tract and liver, the results indicate a close association of FoxF1 with the formation of the fish gills.