Short and Long Germ Development
The stepwise addition of segments in relation to an extension of the germ band is called short germ development (Krause 1939; Scholtz 1992; Patel et al. 1994). In species with direct development or another mode of advanced hatching, the short germ mode of development can be explained by the embryonization of the nauplius and metanauplius stages, including the anamorphic larval development (Figures 6.1 and 6.2). However, some crustacean embryos even adapted a kind of long germ development (Scholtz 1992). In these cases, the germ disc is turned into an extended germ band, largely based on cell rearrangement. Anteroposterior segmentation is delayed with respect to germ band formation and shows a very flat gradient, i.e., the segments are formed more or less simultaneously over the entire germ band. The long germ development is correlated with direct development. Hence, it occurs only in species with an epimorphic developmental mode in which all segments are present at the hatching stage. Examples are cladocerans such a Daphnia (Schwartz 1973; Mittmann et al. 2014) and to a certain extent amphipods (Scholtz 1992) (Figure 6.5C, D).
Based on comparative and experimental data it has been suggested that two processes can be discriminated that are involved in germ band differentiation. One is formation and elongation of the germ band in an anterior direction. The other is the anteroposterior propagation of the subdivision of the germ band into serially repeated units (Dohle 1972; Scholtz 1992; Williams et al. 2012; Scholtz and Wolff 2013). Hence, the growth zone does not generate segments but just the competent cellular material, which is eventually segmented. This view is consistent with the idea that short and long germ development do not necessarily imply fundamentally different mechanisms but can be the result of a heterochronic shift between germ band elongation and subsequent segment formation (Scholtz 1992; Scholtz and Wolff 2013).
The first signs of forming segments are the invaginations of the intersegmental furrows, the limb buds, and the early ganglion anlagen (Figure 6.11). Again, the germ bands of malacostracans show the most detailed resolution of these processes. Intersegmental furrows form as transverse, slightly obliquely oriented invaginations. As mentioned earlier, they appear within the descendants of each ectoderm row (Dohle et al. 2004). The findings at the cellular level of malacostracan segmentation have been corroborated at the molecular level. Namely, the expression of segment polarity genes such as engrailed and wingless concurs with the morphological results on the formation of segmental boundaries. The engrailed gene is
FIGURE 6.11 Segment formation and expression of the segment polarity gene, engrailed in Cryptorchestia garbinii (modified after Dohle et al. 2004). (A) Differential interference contrast image of the whole mount of a germ band showing a thoracic segment during formation. The expression of engrailed is made visible with an antibody (brown nuclei). Each transverse stripe including the midline (m) marks the posterior margin of a segment. Posterior to the engrailed expression the intersegmental furrows form. The stereotyped arrangement of ectoderm cells is recognizable. (B) Drawing of the preparation of A with an analysis of the mitoses of the differential cleavages and the clonal composition. Lines connect sister cells. (C) SEM image of three segments of the embryonic thorax (th) with false-color blue staining of the cells that express engrailed. This technique demonstrates the morphogenesis of the segmental furrows and the limb buds. Lines connect sister cells. The labels mark individually identified cells, ml, midline. Compare with Figures 6.10 and 6.12.
responsible for the establishment and maintenance of a posterior segmental cell fate in Drosophila melanogaster and other arthropods such as spiders, millipedes, and crustaceans (e.g. Patel et al. 1988; Hidalgo 1998; Damen 2002; Hughes and Kaufman 2002) (Figures 6.10-6.12). Accordingly, it is expressed in transverse stripes in the posterior region of forming segments. This has also been shown for a number of crustacean representatives such as decapods, cirripedes, copepods, isopods, amphi- pods, mysids, ostracods, and branchiopods (e.g., Patel et al. 1988; Manzanares et al. 1996; Scholtz and Dohle 1996; Abzhanov and Kaufman 2004; Deutsch et al. 2004; Wolff 2009; Ikuta 2018; Hein et al. 2019). In all these species, engrailed is expressed at the posterior margin of forming segments (Figure 6.11). Yet, the knowledge of the cell lineage in the germ band of malacostracans allows for an unmatched cellular resolution of engrailed expression. It has been shown that engrailed is expressed in the anteriormost cells of the ectodermal genealogical units after the second round of mediolateral divisions just in front of the forming intersegmental furrows (Scholtz and Dohle 1996) (Figures 6.10 and 6.11). Hence, the result of the lineage analyses demonstrating that the anterior cells of a genealogical unit contribute to the posterior region of a morphological segment has been corroborated by the engrailed marker. Moreover, this result shows that the genealogical units of malacostracans can be compared to the insect parasegments, i.e., fundamental initial developmental units that subdivide the anteroposterior body axis into repeated structures that are offset compared to the morphological segments (Lawrence 1992).