ulting in a similar assignment (Fig. 3A). Thus, according to the correlation scheme derived from the study of ring-forming millipedes, the millipede trunk has three "haplosegments" followed by a number of "diplosegments" (Fig. 3).
Another model for tergite-sternite-leg pair assignment based on embryology in non-ring-forming millipedes
Both models most significantly differ in the assignment and location of the tergites and the diplosegments, both of which are metameric structures. We decided to study the long-standing issue of the discrepancy in assignment of dorsal and ventral metameric units by analyzing genes controlling segmentation in the millipede Glomeris marginata . Our results show surprising differences in expression of these genes in dorsal and ventral metameric units. In our previous paper  we focused on the different mechanisms that must act during the formation of the dorsal and ventral metameric units in the embryo. In the present paper we discuss in detail the implications the lack of coupling of dorsal and ventral segmentation has for the debate on the correlation of tergites with sternites and leg pairs. We review here the evidence for an alternative model for tergite – sternite/leg pair assignment that provides a solution for the problems in the previous models.
Segmentation genes have been studied in great detail in the fruit fly Drosophila melanogaster, and as a consequence, Drosophila segmentation still forms a paradigm for segmentation. The Nobel Prize awarded D. melanogaster mutagenesis screen reported by Nüsslein-Volhard and Wieschaus  revealed several classes of mutant segmentation phenotypes. The affected genes in these mutants are the so-called segmentation genes and were shown to act in a hierarchic gene cascade to divide the early embryo into a repeating series of segmental units along the anterior posterior axis. One of the classes of segmentation genes are the segment-polarity genes that are responsible for establishing and maintaining the boundaries of the initial metameric units, 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