Fig. 4

With CCS, different segmentation mechanisms can evolve (transient signal, CCS). a This individual uses cell–cell signalling at the boundaries of an emerging segment to split the segments into two. The segmentation gene and the division gene are maintained only in the two boundary cells of this primordial segment, because they receive different inputs from their neighbours. The division gene then generates a new burst of divisions in that cell, expanding the single cell into a new segment. This mechanism piqued our interest because it superficially resembles the splitting of the odd stripes in Tribolium [42]. It provides an alternative to Turing-like wave splitting in growing media [34], which uses diffusive signalling over longer distances, while this segment-splitting mechanism uses only direct CCS. Another difference is that in the Turing mechanism, the wave splitting results from growth, while here segment-splitting directs divisions. Although cell divisions are thought to play a minor role in the axis extension of Tribolium, tissue-wide divisions have been observed that could support the segment-splitting mechanism we find here [22]. In Tribolium, however, segment addition happens sequentially, while segment splitting here occurs in simultaneously generated segments. Furthermore, the mechanism in Tribolium is distinctly asymmetric: the secondary stripe that splits off is considerably narrower than the primary stripe. It thus remains an open question which mechanism causes segment doubling in Tribolium: Turing-like, the mechanism described here, or an as yet unidentified mechanism. b This individual uses signalling cues emanating from the static head to stimulate divisions in the cell adjacent to the head. The graph depicts the gene expression oscillations that occur in this cell, which subsequently pattern the tissue. In the networks, the division gene is circled in magenta, the segmentation gene in blue and the signalling genes in yellow