A 4D-microscopic analysis of the germ band in the isopod crustacean Porcellio scaber (Malacostraca, Peracarida)-developmental and phylogenetic implications.

Malacostracan crustaceans have evolved a conserved stereotyped cell division pattern in the post-naupliar germ band. This cleavage pattern is unique in arthropods investigated so far, and allows a combined analysis of gene expression and cell lineage during segmentation and organ development at the level of individual cells. To investigate the cell lineage in the germ band of the isopod Porcellio scaber, we used a 4D-microscopy system, which enables us to analyse every cell event in the living embryo. The study was combined with the analysis of the expression of the gene engrailed (en) at different stages of germ band formation. Our findings confirm the results of earlier investigations of the cell division pattern in the posterior part of the isopod germ band. Furthermore, we can show that in the anterior region, in contrast to the posterior part, cleavage directions are variable and cell sorting takes place-similar to other arthropod germ bands. Additionally, the gene expression pattern of en in this region is not as regular as in the post-naupliar germ band, and only later becomes regulated into its characteristic stripe pattern. The comparison of the cell lineage of P. scaber with that of other malacostracan crustaceans shows an enhancement in the velocity of cell divisions relative to the arrangement of these cells in rows in the isopod germ band. The striking similarity of the formation of the genealogical units in the anterior part suggests a sister group relationship between the peracarid taxa Tanaidacea and Isopoda.