The Anterior Growth Zone Functions As a Transition Zone in Segmental Specification

Both Oncopeltus and Thamnocephalus show different cell cycle behaviors in the anterior versus posterior growth zone, in particular, the anterior appears devoid of cycling (Figure 3.7A, B). Interestingly, in a comparison of cell death to normal mitosis during knockdown of pair-rule genes in an intermediate germband dermestid beetle, Xiang et al. (2017) used pH3 staining to reveal a region of mitosis in the posterior germband separated from the rest of the germband by a region lacking mitosis.

The dermestid, like other species examined, also had cell division throughout the germband. In Oncopeltus and Thamnocephalus, the differential cell cycling behaviors that subdivide the growth zone map to expression domains of the segmentation genes. In Thamnocephalus, WntA is expressed in the anterior growth zone where no DNA replication is observed and could function to inhibit S-phase in the anterior growth zone. Similarly, both WntA and caudal have boundaries coincident with the activation of a synchronized S-phase. However, the gene expression domains in the posterior growth zone of both Oncopeltus and Thamnocephalus cover the entire posterior part of the growth zone and do not map directly to the populations of cells in M- or S-phase in the growth zone. This is distinct from the well-characterized segmentation gene expression and mitotic domains in Drosophila (Foe, 1989; Edgar and O'Farrell, 1989; Bianchi-Frias et al., 2004).

Cell Division Is Highly Regulated and Regionalized in Both the Growth Zone and Trunk

Although cell proliferation is not abundant in the growth zone of many species, it is required and it can be highly regulated either spatially or temporally (Figure 3.7; Auman et ah, 2017; Sarrazin et ah, 2012; Constantinou et ah, 2020). We and others have observed a temporal pulse of cell division in the Tribolium embryo. In vertebrates, recent studies have shown that a precisely regulated pattern of proliferation, arrest, and proliferation is necessary during elongation for proper differentiation of somites (Bouldin et ah, 2014). Similarly, the posterior cells in Thamnocephalus undergo a highly regulated cell cycle as cells transit from the posterior to the anterior growth zone and finally become specified as a new segment. We predict that dynamic temporal regulation of cell cycling will be found in many arthropods.

Rosenberg et ah (2014) were the first to document segmentally iterated mitotic domains that appear progressively in the developing trunk segments of hymenopteran Nasonia: growth appears to be fueled by segmentally iterated mitotic domains that appear in an anteroposterior progression (Rosenberg et ah, 2014). These domains correlate with the iterated bands of Nasonia eve expression. The synchronized band of cells undergoing S-phase in the most recently specified Thamnocephalus trunk segments also correlate with segmentation gene expression (Constantinou et ah, 2020). This provides support for the hypothesis of Rosenberg et ah (2014) that use of coordinated cell cycle domains is a strategy that seems to have evolved multiple times. This apparent coordination of mitotic or replication domains and segmentation gene expression is likely an ancestral feature of arthropod development.

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