How Do Posterior Cells in Tribolium Converge and Extend?
Following clonal fates over long time periods reveals additional data about cell rearrangements in Tribolium. By following small clones of cells marked with a caged dye at the blastoderm stage, we found that clones marked in the posterior blastoderm elongate disproportionately compared with clones from more anterior regions of the blastoderm (Nakamoto et al., 2015). Posterior clones undergo extensive rearrangement and form narrow, elongated clones that extend through four or five segments. By contrast, anterior clones extend through only one or two segments. This switch in clone distribution occurs at the boundary between addition of thoracic and abdominal segments, but is not due to cell division since the doubling of anterior and posterior clones was not significantly different. Interestingly, the clones rarely mixed with nonlabeled cells (although they sometimes did), even in the posterior when they undergo extreme elongation to form very narrow clones (Figure 3.9A, B). In contrast, cells in the mesodermal layer undergo clear intercalary movements (Figure 3.9C, D). Despite extensive cell rearrangement, posterior clones do not show evidence of arising from repeated rounds of the kind of neighbor-sliding intercalation described in Drosophila (as shown in Figure 3.8A). At present, we do not know how the descendants of the clones maintain connection. The same kinds of contiguous, elongated
FIGURE 3.9 Blastoderm clones scored after completion of elongation. All panels show clones of roughly 20 cells marked with a photoactivated dye in the late blastoderm stage between 50-60% egg length. The fate of the clones was scored in fully elongated embryos stained with the 4D9 antibody that detects the Engrailed/Invected proteins. Because DV position cannot be determined on the blastoderm, some clones mark ectodermal precursors (A; B, higher magnification of clone in A), while others mark mesodermal precursors (C; D, higher magnification of clone in C). The ectodermal clones form remarkably elongated clusters of cells that are only occasionally interrupted by nonlabeled cells. The mesodermal clones behave quite differently and distribute across several segments, restricted to non-Engrailed/ Invected expressing cells.
clones were found by Benton (2018, e.g. his Supplemental Figures 3 and 4). In some cases, a region of nonactivated cells breaks the integrity of the clone, however, never in the regular interdigitating fashion seen in the Drosophila germband. In sum, all studies indicate that cells undergo some kind of convergent extension to elongate, but the mechanism remains elusive.