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Pattern Formation: how to produce anthocyanin spots

Anthocyanin spots

Figure 1. The Reaction-Diffusion (RD) model of anthocyanin spot formation. The flower images show natural variation of anthocyanin spots in the nectar guides (e.g., M. lewisii has fine spots and M. guttatus has large spots), and M. lewisii transgenic lines or EMS mutants with altered patterns. On the right is a screen capture of a SimRDj simulation (Kondo & Miura, 2010) based on the RD model.

Many organisms exhibit interesting pigmentation patterns (e.g., zebra stripes, leopard spots). Formation of such periodic patterns in biological objects is often explained by the reaction-diffusion model (Turing, 1952) or various modified versions of this model (Meinhardt, 1982; Meinhardt & Gierer, 2000; Kondo & Miura, 2010). The essence of these reaction-diffusion based models is an interacting network that contains a local autocatalytic feedback loop and a long-range inhibitory feedback loop (Meinhardt & Gierer, 2000). Computer simulations using this model can generate patterns that are breathtakingly similar to those found in real organisms. However, empirical demonstration that biological patterns are actually generated by this model is scarce.

M. lewisii flowers have fine anthocyanin spots in the nectar guides (Figure 1). We have identified an anthocyanin activator, NEGAN, which is self-activating and is specifically expressed in the nectar guide, and a repressor, RED TONGUE, that inhibits NEGAN activity. Loss of function red tongue mutant produces so much more anthocyanins in the nectar guides that the “spots” are expanded into large patches (Figure 1), indicating the critical role of this repressor in the normal spot formation.   Furthermore, NEGAN activates RED TONGUE gene expression. So far these genetic data seem to fit the RD model precisely. At the moment we are testing the other requirement of the model, that is, the repressor “diffuses” into neighboring cells. Ultimately we would like to test the model by fine-tuning the parameters via transgenic manipulations to “engineer” different patterns and to explain the natural variation of the spotting pattern among species.