 in this tissue (Fig. 6A,B asterisks). These dorsal engrailed stripes, however, have no connection with the ventral engrailed stripes and in contrast to the ventral engrailed stripes are not associated with the morphologically visible grooves. Furthermore, the dorsal engrailed stripes are not aligned with the ventral engrailed stripes, but are shifted in relation to them. More posterior the dorsal tissue forms simultaneously with the ventral tissue. Here, the engrailed gene is expressed in each ventral embryonic segment, while the gene is only expressed in the dorsal tissue corresponding to every other ventral embryonic segment . On the dorsal side there are thus reiterated stripes of engrailed expression (Fig. 6), but they do not correspond in a one-to-one fashion to the engrailed stripes on the ventral side, they are not associated with a morphological boundary, and, surprisingly, they are not associated with wingless expression (Fig. 7) (see also ). The lack of wingless expression in the dorsal tissue implies that there cannot be an interface of engrailed and wingless expressing cells to define a boundary, like on the ventral side. This argument is strengthened by the lack of cubitus interruptus gene expression anterior to the dorsal stripes of engrailed expression . Cubitus interruptus is the activator of wingless gene expression. These data imply that the genetic interactions must be different in the dorsal and ventral tissue of Glomeris. In order to acknowledge this fact we regard the dorsal metameric units, the lateral plates , as segmental units and in order to distinguish them from the ventral embryonic segments we designate them as: dorsal embryonic segments
There thus appears to be a lack of coupling of dorsal and ventral segmentation as is obvious from several pieces of evidence. (1) The anterior dorsal tissue forms later than the ventral one and (2) the engrailed stripes appear de novo in this anterior dorsal tissue. (3) The dorsal engrailed stripes are not located near the morphological boundaries between the metameric units and (4) the dorsal metameric units differ in number from the ventral ones. (5) But most significantly, the genetic interactions must be different as wingless expression is missing in dorsal tissue. Dorsal and ventral segmentation thus are not coupled in Glomeris.
Our data show that the tergite borders are defined independently from the embryonic segment borders (both ventral and dorsal) and are established by a distinct genetic mechanism that uses engrailed but not wingless signalling. Thus, in terms of genetic mechanisms involved, the segmentation of the dorsal exoskeleton appears to be different from the segmentation processes leading to the formation of the ventral and dorsal embryonic segments. On the other hand, the formation of the tergites seems to build on the prior establishment of the dorsal embryonic segments. Tergite borders invaginate roughly in the middle of each dorsal embryonic segment and the tergites themselves are formed by the posterior half of one dorsal embryonic segment and the anterior half of the following one. This process bears some resemblance to resegmentation in vertebrates where the primary metameric units, the somites, are divided in the middle and the final metameric units, the vertebral bones, are formed from the posterior half of one somite and the anterior half of the next one (reviewed in ). The two processes in Glomeris and vertebrates certainly are not homologous, but might nevertheless share some developmental mechanisms by convergent evolution. These new insights now allow a modified model for tergite – sternite/leg pair correlations in basal milipedes with non-fused exoskeletal elements, like Glomeris (see Fig. 9A). But our data also have ramifications to the ring-forming process in derived millipedes. According to our results, the tergites are shifted in relation to the sternites on the ventral side of the animals. In order to fuse into a ring, a tergite therefore has to "choose" between two alternatives: for example tergite III may fuse either