- Open Access
A review of the correlation of tergites, sternites, and leg pairs in diplopods
© Janssen et al; licensee BioMed Central Ltd. 2006
- Received: 23 August 2005
- Accepted: 02 February 2006
- Published: 02 February 2006
In some arthropods there is a discrepancy in the number of dorsal tergites compared to the number of ventral sternites and leg pairs. The posterior tergites of the Diplopoda (millipedes) each cover two sternites and two pairs of legs. This segment arrangement is called diplosegmentation. The molecular nature of diplosegmentation is still unknown. There are even conflicting theories on the way the tergites and sternites/leg pairs should be correlated to each other. The different theories are based either on embryological analyses or on studies of the adult morphology and turned out to be not compatible with each other. We have previously used the expression patterns of segmentation genes in the pill millipede Glomeris marginata (Myriapoda: Diplopoda) to study millipede segmentation. Here we review the existing models on the alignment of tergites and leg pairs in millipedes with special emphasis on the implications the gene expression data have on the debate of tergite and leg pair assignment in millipedes. The remarkable outcome of the gene expression analysis was that (1) there is no coupling of dorsal and ventral segmentation and, importantly, that (2) the boundaries delimiting the tergites do neither correlate to the embryonic boundaries of the dorsal embryonic segments nor to the boundaries of the ventral embryonic segments. Using these new insights, we critically reinvestigated the correlation of tergites, sternites, and leg pairs in millipedes. Our model, which takes into account that the tergite boundaries are different from the dorsal embryonic segment boundaries, provides a solution of the problem of tergite to sternite/leg pair correlation in basal milipedes with non-fused exoskeletal elements and also has implications for derived species with exoskeletal rings. Moreover, lack of coupling of dorsal and ventral segmentation may also explain the discrepancy in numbers of dorsal tergites and ventral leg pairs seen in some other arthropods.
- Dorsal Side
- Lateral Plate
- Germ Band
- Segmentation Gene
- Segmental Unit
Fortunately, however, there are other millipede species, e.g. the members of Juliformia (Fig. 2C) that have their tergites and sternites fused together into rigid armour rings (for an overview see [1, 2]). Each pair of legs is jointed to a sternite that is fused to a tergite via the lateral pleurites, in this way making leg pair/tergite alignment seemingly straightforward and unambiguous. In these animals, the first tergite (collum) is free (i.e. not fused to any ventral component). The second tergite is fused with the sternites of the first pair of legs into the first cuticular ring (ring 1) (see Fig. 2C). The third and fourth tergites are fused with the second and third trunk sternites, respectively, thus forming ring 2 and 3. The anterior part of the juliform trunk, therefore, consists of a free collum, followed by three rigid rings, each of which is bearing a single pair of legs. Posterior to this something surprising happens: each tergite is fused not to a single sternite, but to a pair of sternites. The result: starting with ring 4 each ring has two pairs of legs (see Fig. 2C).
In order to distinguish between the tergites that correlate with only one leg pair and those that correlate with two leg pairs, tergites II, III, and IV are called haplotergites, whereas all following tergites are so-called diplotergites. The phenomenon that one dorsal segmental unit (one diplotergite) is correlating with two ventral segmental units (two sternites and two leg pairs) is called diplosegmentation.
As already mentioned above, in non-ring-forming species with free tergites, pleurites and sternites, the correlation of leg pairs with tergites is unclear since they are not fused into rings. However, the results from ring-forming species can be extrapolated to non-ring-forming species, res