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Fig. 4. Evolving a feed-forward regulatory network from a single input or a simple
cascade regulatory network. (A) In the feed-forward network, factor A directly
regulates each gene: sequential activation is achieved by requiring both A and
B to express gene C; and both A and C to express gene D. (B) In a single-input
network, factor A directly regulates the three targets B, C and D and does not
have temporal patterning. (C) The simple cascade accomplishes sequential gene
activation with only gene B directly activated by A. It is easy to see how
generating a new single-input network might generate a selectable phenotype
that could evolve feed-forward regulation. Evolving a cascade motif would
require a selective advantage for each stage, but once it had evolved it could
be invaded by factors such as Myod to convert it into a feed-forward
network.
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