The Morphogenetic Origin of Pattern Formation in the Drosophila Compound Eye

Gallagher, Kevin

doi:https://doi.org/10.21985/n2-w7n1-1q74

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The Morphogenetic Origin of Pattern Formation in the Drosophila Compound Eye

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Pattern formation of biological structures involves the arrangement of different types of cells in an ordered spatial configuration. Patterning is thought to involve the spatial organization of molecular pre-patterns that precede and drive subsequent cell differentiation and coinciding morphogenesis. These molecular prepatterns are often, although not exclusively, organized through Turing style reaction-diffusion processes. In this thesis, I investigate the mechanism of patterning the Drosophila eye into a precise triangular grid of photoreceptor clusters called ommatidia. Previous studies had led to a long-standing biochemical model whereby a reaction-diffusion process is templated by recently formed ommatidia to propagate a molecular prepattern across the eye epithelium. Here, I find that the templating mechanism is instead, mechanical in origin; newly born columns of ommatidia serve as a template to spatially pattern cell flows that move the cells in the epithelium into position to form each new column of ommatidia. Cell flow is generated by a pressure gradient that is caused by a narrow zone of cell dilation precisely positioned behind the growing wavefront of ommatidia. The newly formed lattice grid of ommatidia cells are immobile, deflecting and focusing the flow of other cells. Thus, the self-organization of a regular pattern of cell fates in an epithelium is mechanically driven. Loss of scabrous, a gene characterized to encode a negative inhibitor in the reaction-diffusion process believed to pattern the eye, results in a disordered lattice phenotype. I found that this mutant, in addition to creating a disordered lattice phenotype, resulted in loss of the periodic organization of cell flow in patterning wavefront, in addition to diminished cell dilation posterior to this wavefront, suggesting that scabrous may actually be a regulator of morphogenesis. My final proposed model is a hybrid reaction-diffusion / mechanical model, where Scabrous plays two crucial roles: (1) it generates cell flow by stimulating cell dilation behind the wavefront of ommatidia and (2) it signals periodic immobilization of the most nascent columns of ommatidia to generate a pattern of periodic obstructions that deflect and focus cell flow into a periodic pattern within the wavefront of patterning. The molecular and mechanical mechanism behind immobilization of ommatidia is of keen interest, as well as validation that Scabrous is, in fact, playing this dual role, which will be necessary to validate the hybrid reaction-diffusion / mechanical patterning model.

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