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Fig. 6. Meta-cis regulation as a mechanism that triggers vertebrate Hox
cluster consolidation. (A) Schematic of a type D cluster present in
the ancestral bilaterian animal. Hox genes (black boxes) are present in
various transcriptional orientations (arrows), dispersed over a large
fragment, which also includes foreign (non-Hox) genes (white boxes). Genes are
kept together owing to the presence of cis-acting regulatory sequences (blue
circles), which are shared between several neighboring genes. (B) A
type D/O cluster present in an ancestral chordate. Owing to the emergence of a
meta-cis regulatory sequence in a control region (CR, brown), the
genomic organization of target genes is improved as a consequence of positive
selection. Progressively, foreign genes are lost together with some large
intergenic fragments, which optimizes the meta-cis control. (C) After
genome duplication, consolidation is favored owing to the increasing
possibility to recruit novel meta-cis regulation (red, yellow, green
boxes and arrows), in particular through the intrinsic capacity of a potent
control region (CR in B) to generate regulatory novelties by `regulatory
priming' (red and dark green in C; see text). This leads to a concentration of
large-scale enhancers to form a `global control region' (GCR), which might
continue to recruit and/or evolve novel regulatory elements owing to its
intrinsic properties. As a consequence, duplicated clusters are further
consolidated.
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