Figure 3

Reconstructed evolutionary history of Hox cluster evolution within the sarcopterygian vertebrates. Colored squares indicate Hox genes that have been identified; white squares are Hox genes that have not yet been sequenced but probably are present in the cluster(s). Solid lines connecting gene boxes indicate physical genomic linkage. Genomic sequences flanking HoxC3 gene in both opossum and platypus are still missing thus we consider the presence/absence of HoxC3 in marsupial and monotreme mammals is unknown yet (indicated by question marks). The gene inventory of the Hox clusters in the hypothetical ancestors of major evolutionary lineages are inferred based on parsimony principles, shown in open boxes above branches. Secondary losses of Hox genes are indicated in orange boxes along branches. A currently accepted phylogenetic tree is shown on the left, with divergence times (taken from [51]) shown beside nodes. Note that we tentatively favor a turtle-archosaur relationship based on HoxC3 variation among different amniote groups (see text for detailed discussion).