The Drosophila gene Spätzle encodes the activating ligand for the Toll receptor. 
This signaling pathway is required for dorso-ventral patterning in the early 
embryo and an antifungal immune response in larvae and adults. The genome 
sequence of Drosophila shows that there are a total of eight Toll-like receptors 
and these may function in other aspects of embryonic development and innate 
immunity. Here we describe five Drosophila homologues of Spätzle (Spz2-6) found 
using an iterative searching method. All five appear to encode proteins 
containing neurotrophin-like cystine-knot domains. In addition, most retain a 
characteristic intron-exon structure shared with the prototype Spätzle gene. 
This provides evidence that the family arose by ancient gene duplication events 
and indicates that the gene products may represent activating ligands for 
corresponding Toll receptors. Expression studies show that only Spz4 is 
expressed strongly in larvae and adults and thus may be involved in an ancillary 
antifungal response mediated by Toll-5. By contrast, Spz6 shows a complex 
spatial and temporally regulated expression pattern in the late embryo. Thus the 
new Toll/Spätzle families of signaling molecules may have important roles in 
other aspects of development and immunity.