Evidence for the derivation of the Drosophila fushi tarazu gene from a Hoxgene orthologous to lophotrochozoan Lox5

The DNA-binding homeobox motif was first identified in several Drosophila h omeotic genes but also in fushi tarazu, a gene found in the Hox cluster yet involved in segmentation, not anteroposterior patterning [1]. Homeotic tra nsformations are not seen in insect ftz mutants, and insect ftz genes do no t have Hox-like expression except within the nervous system [2,3]. Insect f tz homeobox sequences link them to the Antp-class genes and Tribolium and S chistocerca orthologs have Antp-class YPWM motifs aminoterminal to the home obox [2,3]. Orthologs of ftz cloned from a centipede and an onychophoran [4 ] show that it predates the emergence of the arthropods, but the inability to pinpoint non-arthropodan orthologs suggested that ftz is the product of a Hox gene duplication in the arthropod ancestor [4,5]. I have cloned ftz o rthologs from a mite and a tardigrade, arthropod outgroups of the insects [ 6]. Mite ftz is expressed in a Hox-like pattern, confirming its ancestral r ole in anteroposterior patterning. Phylogenetic analyses indicate that arth ropod ftz genes are orthologous to the Lox5 genes of lophotrochozoans (a gr oup that includes molluscs) [7] and, possibly, with the Mab-5 genes of nema todes and Hox6 genes of deuterostomes and would therefore have been present in the triploblast ancestor.