What Have These Case Studies Revealed?

The Growth Zone Typically Requires Mitosis But Only at Low Rates

One thing we can conclude from all three case studies, as well as previous studies in other arthropod species, is that cell division in the posterior is low. Regardless of the original size of the germband or larva, cell division occurs both in the growth zone and throughout the embryo/larva (Figure 3.7; Sarrazin et al., 2012; Auman et al., 2017; Cepeda et al., 2017; Constantinou et al., 2020). In the cockroach Periplaneta americana, Pueyo et al. (2008) point out the growth zone has a number of cells in mitosis as do the already formed segments; posterior mitosis is present but is not markedly greater than mitosis throughout the embryo (Chesebro et al., 2013). This pattern has also been observed in the velvet worm Euperipatoides rowelli (Mayer et al., 2010) and spiders (Stollewerk et al., 2003). These results are consistent with species that use teloblastic growth from the posterior. Although one row of cells arises from posterior ectoteloblasts to form the initial segmental anlage, two subsequent rounds of cell division of this initial row (intercalary divisions) occur prior to the differential medial/lateral cleavages that occur within each anlage (reviewed in Scholtz and Wolff, 2013; also see Chapter 6). Thus, for each new segment, the cell divisions outside the teloblastic growth zone account for much of the initial elongation. Our case studies are the first to quantify the requirements for cell division in non-teloblastic species, and they report that fewer than 5% of the cells are dividing in the growth zone (Auman et al., 2017; Constantinou et al., 2020). Similarly, where

Overview of cell cycling in the growth zone of three arthropods

FIGURE 3.7 Overview of cell cycling in the growth zone of three arthropods. At the top, each diagramed embryo shows the posterior region, including specified segments (red stripes) and the growth zone, partitioned into an anterior and posterior region. Expression domains of different regulators at this stage of early elongation are shown in each embryo. Below each diagram are graphical representations of cell cycle behaviors. Predicted (dotted blue lines) versus actual (solid blue lines) levels of mitosis, and cells in S-phase (solid green lines) were known. A. In Thamnocephalus, mitosis levels are low (solid blue line) relative to traditional growth zone expectations (dotted blue line). S-phase falls in discrete domains (solid green lines) that map onto Wnt4 (light green) and WntA (blue green) expression domains in the growth zone. The synchronization of cells in S-phase in the last added segment (solid green line) maps to cyclin A expression (not shown). B. In Oncopeltus, mitosis levels are low (solid blue line) relative to classical expectations (dotted blue line). Nonetheless they are regionalized within the growth zone, with more mitoses in the posterior domain in a region that corresponds to the broad domain of eve expression (purple). C. In Tribolium, mitoses are low and do not appear differentially distributed in the growth zone (solid blue line). In young germband embryos, cells in S-phase are more abundant in the posterior, in the region corresponding to the expression of cad (shaded blue), than in the mid-trunk region. In no species is the classical expectation of relatively high levels of mitosis borne out. Patterns of cell cycling revealed by S-phase do not directly map to mitoses (green versus solid blue lines). S-phase is likely not a simple index of cell division but instead carefully regulated as cells transit from the posterior growth zone to the anterior growth zone to the newly specified segment. References for data can be found in text.

it has been calculated, the estimated requirement for additional tissue in the posterior is less than expected (Nakamoto et al., 2015; Auman et al., 2017; Constantinou et al., 2020). There is, nevertheless, a demonstrated need for additional cells to account for all the segmental anlage, and cell division in a small percentage of cells in the posterior is required to meet this need.

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