the parasegments. These genes also define the A-P polarity within each metameric unit.
The best-studied segment-polarity gene is the engrailed gene. The engrailed gene is expressed in segmentally reiterated stripes in the embryo. In Drosophila, another segment-polarity gene, the wingless gene, is active in cells just anteriorly adjacent to the stripe of engrailed expressing cells. The engrailed and wingless expressing cells are mutually exclusive and define an important boundary between them, the parasegmental boundary. Genetic and molecular studies have shown that the parasegments are fundamental units in the design of the early Drosophila embryo [8–11].
The analysis of Drosophila segment-polarity gene homologs in other arthropods, like more primitive insects, crustaceans, centipedes, and spiders, revealed that the role of these genes in defining the parasegmental boundary represents a conserved function among arthropods (e.g. [12, 13]). Also in these arthropods wingless/Wnt genes are active in cells anteriorly adjacent to the engrailed expressing cells. Detailed analyses, e.g. in the spider Cupiennius salei and the centipede Lithobius atkinsoni, demonstrated that the border between the engrailed and wingless/Wnt expressing cells also defines the important boundary of the parasegment, similar as in insects [12, 13]. Also in crustaceans engrailed expression is associated with units comparable to parasegments . The expression of these genes thus forms an excellent marker for the formation of the segmental units in arthropods.
On the dorsal side the gene expression patterns are significantly different. As described above, development of the presumptive dorsal tissue in Glomeris starts later than the ventral tissue, and is forming the so-called lateral plates [4, 6] (Fig 4). The anterior portion of the germ band up to the 4th ventral embryonic segment of the trunk initially comprises ventral tissue only. At stage 3 the development of the dorsal tissue commences (Fig 4AB) and engrailed expression is activated de novo 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 b