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Fig. 3. Cluster types in various animal species. A recent animal phylogeny
with deuterostomes shown in orange, protostomes in green (lophotrochozoans)
and yellow (ecdyzozoans), and ctenophorans, cniderians and poriferans
arbitrarily shown in blue, as outgroups of bilateria. Cluster type is
indicated to the right. Amongst the deuterostomes, cephalochordates are still
positioned closer to vertebrates than are tunicates (urochordates), contrary
to the proposal from Delsuc et al. (Delsuc
et al., 2006). This uncertainty does not change the nature of the
argument regarding the evolution of chordate Hox clusters (see text). Whereas
type A, S and D clusters are found throughout these large groups, genuine type
O clusters are only described in vertebrates (see text for a discussion of the
type D/O cluster found in cephalochordates).
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