Possible Hypotheses for Mechanisms of Convergent Extension in Tribolium

Novel mechanisms may underlie the cellular rearrangement during posterior segmentation in Tribolium. One hypothesis is that tissue-level forces moving cells medially, coupled with differential adhesive behaviors could create both cell mixing and elongated clones. The ectodermal tissue that elongates in Tribolium is a quite narrow tissue, bounded medially by the invaginating mesoderm and laterally by the edge of the germband. The medial ingression of mesoderm and the geometry of the tissue borders could be a “tissue-level polarizer” that directs polarized elongation of the embryo. Boundary effects have been proposed to play a role in the elongating ascidian notochord (Weliky et al., 1991) as well as more generally in migrating tissues (Vedula et al., 2013). Backes et al. (2009) argued that the physical edge of an adjacent tissue can lead to an elongated arrangement of cells, even in the absence of polarized cell behaviors. If boundaries that run along the lateral and medial edges are significant mechanical players in elongation, we might predict DV genes play a significant role. The Bmp gradient of the zebrafish gastrula guides migrating lateral cells by regulating cell-cell adhesion (von der Hardt et al., 2007). However, we have found that modeling cell movements in the Tribolium germband using forces that drive cells medially and posteriorly (mimicking those tracked by Sarrazin et al., 2012) in the absence of differential adhesion can create clones that mix and interdigitate without an active neighbor-sliding mechanism (Hester, Williams, and Nagy, unpublished).

Although not yet fully explored or characterized, it is interesting to note that the knockdown of Toll7 and TolllO in Tribolium germbands (Benton et al., 2016) not only halts intercalation but also results in distinct changes in cell shape: cells constrict globally causing a shrinking of the tissue. Cell shape change has not been a typical focus of live imaging as yet but may be significant. For example, Pechmann (2016) describes the formation of the germ disc in Parasteatoda tepidariorum arising from a combination of changes in cell shape that drive tissue movements. The loose pyramidal cells of the early blastoderm condense on one side of the egg forming a cuboidal epithelium while the extraembryonic cells spread and flatten. Condensation of the germ disc is disrupted by an actin polymerization inhibitor (cytochalasin D) but not a microtubule polymerization inhibitor (colchicine).

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