Claude Desplan (New York University, New York, USA) reported on an apparent example of convergent evolution in the patterning of the anterior-posterior axis in insects. His group compared anterior-posterior patterning in Tribolium (a beetle), a so-called short germ-band insect that forms the segments sequentially, and in Drosophila and Nasonia (a parasitic wasp), both of which are long germ-band insects (forming the segments simultaneously). Desplan argued that an ancestral mode of anterior-posterior patterning acting in short germ-band insects involved the anteriorly acting genes otx and hunchback (hb), which counteracted the action of caudal at the posterior end. The developing Drosophila embryo has departed considerably from this scenario by establishing an anterior morphogenetic center that uses a gradient of maternally supplied bicoid (bcd) mRNA, which establishes the correct sequence of anterior-to-posterior identities all at once. This is possible because Drosophila is a long germ-band insect with simultaneous segment formation. However, bicoid is a phylogenetically young gene that evolved late and only in flies. It does not exist in other long-germ band insects, not even in the mosquito Anopheles, a close relative of flies. How then do other long germ-band insects establish their anterior-posterior axis?
Desplan reported that in order to answer this question Jeremy Lynch in his lab investigated the role of hb and otx in anterior axis specification in Nasonia, a long germ-band hymenopteran whose lineage departed from that of the flies 220 million years ago (Mya). Interestingly, otx mRNA in Nasonia is localized at both the anterior and posterior poles of the egg. Inactivation of the otx gene by parental RNA interference (RNAi) produces headless embryos, indicating that otx has a bicoid-like role in patterning the anterior of Nasonia. RNAi of otx also causes significant disruptions in posterior segmentation.
In the beetle Tribolium, hb is not required for the development of the anterior-most head segments, but in Nasonia, in contrast, a zygotic mutation in hb (generated by Mary Anne Pultz, Western Washington University, Bellingham, USA) deletes almost the entire anterior end of the animal. This indicates that in Nasonia, hb functions as a major player, and that the otx-hb interaction in anterior patterning in Nasonia is as strong as, or stronger than, the analogous interaction of bcd and hb in the fly. Desplan proposed that, like Drosophila, Nasonia has independently evolved an anterior morphogenetic center by localizing otx at the anterior (and the posterior) end of the embryo to take advantage of, or to make possible, its long germ-band mode of development.
Michalis Averof (Institute of Molecular Biology and Biotechnology (IMBB), Crete, Greece) presented an analysis of the posterior patterning gene caudal (cad) in crustaceans and a comparison of its function across different phyla. In the brine shrimp Artemia, cad expression specifically localizes to the posterior growth zone. Moreover, Averof reported that Tijana Copf in his lab had shown that cad inactivation by RNAi in the larva can abolish all trunk segments, depending on the time of the interfering RNA injection. At the molecular level cad inactivation leads to severe perturbation of expression of the early segmentation genes engrailed and even-skipped. Averof's group, together with Reinhard Schröder's group at the University of Tübingen (Germany), found that interference with cad function similarly abolishes trunk segment formation in the beetle Tribolium. This implies that, in the most recent common ancestor of crustaceans and insects, segments were already being formed from a posterior growth zone under the control of cad.
As Averof pointed out, this role of cad also extends beyond the protostomes (the wider phylogenetic branch to which insects belong). In vertebrates, which are deuterostomes, cad orthologs (cdx1-cdx4) are expressed in the caudal presomitic mesoderm, where they are required for the segmental generation of the somites and the specification of somite identity. Mutations in the vertebrate cdx genes also compromise the self-renewing capacity of the presomitic mesoderm. Therefore, it appears that the function of cad in the growth zone and in posterior segmentation is ancestral for bilaterians (animals with bilateral symmetry, including both insects and vertebrates).
Averof also reported work of Tassos Pavlopoulos in his group who has succeeded in establishing transgenesis in the amphipod crustacean Parhyale hawaiiensis by injecting DNA into one-cell and two-cell embryos. He used a vector containing the minos transposable element from Drosophila hydei (developed by C. Savakis, Institute of Molecular Biology and Biotechnology, Crete, Greece). This vector appears to have the potential to function in a wide range of phyla.