Annotation of Hox cluster and Hox cofactor genes in the Asian citrus psyllid, Diaphorina citri, reveals novel features

Hox genes and their cofactors are essential developmental genes that specify regional identity in animals, including insects. A particularly interesting feature of Hox genes is their conserved arrangement in clusters in the same order in which they specify identity along the anterior-posterior axis. Among insects, breaks in the cluster have been reported in a few species, but these seem to be the exception rather than the rule. We have annotated the ten Hox genes of the Asian citrus psyllid, Diaphorina citri, and determined that there is a split in its Hox cluster between the Deformed and Sex combs reduced genes. This is the first time a break at this position has been observed in an insect Hox cluster. We have also annotated the D. citri orthologs of the Hox cofactor genes homothorax, PKNOX and extradenticle. Interestingly, we found an additional copy of extradenticle in D. citri that appears to be a retrogene. Expression data and sequence conservation suggest that the extradenticle retrogene may have retained the original extradenticle function and allowed the parental extradenticle gene to diverge.