During my PhD I was working on the development of the malacostracan crustacean germ band. I studied the cell lineage and gene expression of the genes engrailed and Distal-less in two species. The malacostracan crustacean show in the postnaupliar part a fascinating stereotyped cell division pattern which is not only interesting from the asthetic point of view.
This grid like pattern is unique for arthropod germ bands and enables studies of gene expression patterns on single cell resolution. I compared the development of the amphipod Orchestia cavimana with the development of the isopod Porcellio scaber. Additionally to the slightly different modes during germ band formation - amphipods have lost cells called Ectoteloblast during evolution - both crustaceans show monoramous legs in the abdomen, which have evolved convergently from plesiomorphic biramous legs. The convergent evolution is reflected on the different development of the limbs and also on the level of the expression of the gene Distal-less.
left: Amphipod Germ band with the visualization of the gene expression of distal-less (brown) and engrailed (blue). Engrailed is expressed in the posterior cell row of the developing segment. Distal-less is expressed in specific cells of the anterior cell row. right: Ectoteloblast divisions in the posterior part of the germ band of Porcellio scaber recorded with a 4D-microscope (3D timelapse). The ectoteloblasts give off smaller cells to anterior (top).