Ontogeny of Seriality and Segmentation

Generally, we can distinguish two different modes of ontogenetic segment formation, which can be described as segmentation by addition and by subdivision. In the first case, the segments are formed through a successive addition of segment anlagen by means of a posterior extension of the embryo or the larva (Figure 1.4). In this process, two morphogenetic processes interact closely. First, cells must be

Sequential addition of segments during the ernbryogenesis of A

FIGURE 1.4 Sequential addition of segments during the ernbryogenesis of A. a crayfish and B. a chicken (modified after Scholtz 2017). The germ band of the crayfish can be seen in a ventral view; that of the chicken in a dorsal view. The anterior end with the head anlage is at the top of each picture. The yolk has been removed in both embryos and the cells have been dyed. The anterior segments are further developed than those more posterior. At the posterior end new segments are being added (arrows).

proliferated that supply the material for the segmentation process from a posterior budding zone and elongate the germ band (Scholtz and Wolff 2013; Auman et al. 2017; see Chapter 6). Second, the differentiation into segments takes place sequentially from anterior to posterior (Williams et al. 2012; Scholtz and Wolff 2013). This means that the anterior segments are formed earlier than the posterior ones, both molecularly and morphologically (Figure 1.4).

In the other mode the segments form by means of a subdivision of the already elongated embryo, and the actual segmentation process takes place almost simultaneously (Figure 1.5). This process has been examined in most detail in Drosophila melanogaster, the common fruit fly. In this case, the embryo is gradually divided into smaller and smaller compartments by means of a cascade of expressions of different gene classes (Gilbert 2003). After the maternal genes have determined the longitudinal axis and the terminal regions through gradients, the gap genes

Synchronous segment formation as exemplified by the fruit fly embryo

FIGURE 1.5 Synchronous segment formation as exemplified by the fruit fly embryo (modified after Scholtz 2017). The earliest stage of embryonic development is at the top. As the embryo develops, it becomes more and more subdivided by means of different gene classes (see text). The actual segmentation takes place in parallel to the diversification of the body regions. Here the head anlage is always facing left (Bottom drawing by Fabian Scholtz).

divide the body into an anterior, middle, and posterior region. These activate the pair-rule genes, which each mark every second of the future segments. Finally, the expression of the segment polarity genes leads to the determination and differentiation of the segmental boundaries. The last two gene groups are each expressed in transverse stripes in the germ bands. Parallel to this, the segments along the longitudinal body axis are specified differently through the Hox genes (Gilbert 2003).

Sequential segment formation can be found in most annelids, arthropods, and chordates. As compared with synchronous segmentation, this is viewed as the evo- lutionarily more ancestral mode. Some of the genes identified in the segmentation of Drosophila display similar functions and patterns of expression, also in organisms with sequential segmentation. This pertains in particular for segment polarity genes and the homeotic genes. In many cases, mixed forms occur in which some of the anterior segments appear synchronously and the more posterior ones are then formed sequentially (Scholtz and Wolff 2013). Regarding the details of these processes, the three major segmented animal groups differ substantially from one another. This pertains to the gene, cell, germ layer, and morphogenesis levels. For example, segmentation in annelids and arthropods begins on what will become the ventral side and then extends to the dorsal side, whereas the segmentation in chordates begins dorsally and later proceeds ventrally (Figure 1.4). Furthermore, in malacostracan crustaceans and in clitellate annelids specialized stem cells and stereotyped cell-lineages are involved in the segmentation process, which do not occur in other arthropods, in other annelids nor in chordates (Dohle 1999; Dohle et al. 2004; see Chapters 6 and 7).

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