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Fig. 4. The dilemma of bilaterian Hox cluster evolution. Two possible models
can account for the restriction of type O clusters to vertebrates. (A)
In the `vertebro-centrist' view, the ancestral bilaterian animal had a type O
cluster, which was maintained all the way through to the vertebrate lineage,
as illustrated in blue. All other bilaterian animals are derived from this
ancestral form, first through a type D cluster (red) and, subsequently, with
type S or A clusters. Although this view provides the easiest mechanistic
explanation (as it progresses towards disorganization only), it implies that
vertebrates are `direct descendants' of this ancestor, which is at odds with
our modern view of animal evolution. (B) In the `consolidation model',
the ancestral cluster was of type D, as illustrated in red, which implies that
a Hox cluster, at some point during the evolution of deuterostomes, underwent
a phase of consolidation (that is, an increased organization), as suggested by
the blue color on the top right. Although this view fits better with our
current understanding of animal phylogeny, it requires some conceptual tools
to explain how such an `organization' can occur and be selected for.
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