In Drosophila melanogaster, the bicoid, or Bcd, gene triggers the construction of the anterior portion of the organism during development. Cyclorrhapha, a taxa of organisms containing D. melanogaster, is individual in their possession of the gene. The lack of bicoid causes the posterior portion of the organism to duplicate itself in what would have been the position of the head. Cyclorrhapha possess a paralog of a duplication of the HOX3 gene named zen– creating Bcd and zen. The bicoid portion of this duplication appears to be from an evolutionary hand, while zen seems to be ancestral and passed down.

To get to the root of the evolution of bcd, the modern gene was removed from D. melanogaster embryos and replaced with the according ancestral genes, AncZB and AncBD. It was first found that the embryos possessing the two ancestral genes failed to produce a head and created two tails at each end of the organism. From here, historically accurate mutations were added–these mutations being the addition of lysine residue located at position 50 and glutamine residue also at 50. In conjunction with one another, these additions created a functioning head akin to the production of modern bicoid. From this, it was gathered that these genetic predecessors, plus other primordial mutations, were the cause of the evolution of the modern bcd gene.

Citation:
Qinwen Liu, Pinar Onal, Rhea R Datta, Julia M Rogers, Urs Schmidt-Ott, Martha L Bulyk, Stephen Small, Joseph W Thornton. Ancient mechanisms for the evolution of the bicoid homeodomain’s function in fly development. eLife, 2018; 7 DOI: 10.7554/eLife.34594 https://elifesciences.org/articles/34594#fig1s1