Models for the evolution of Posterior and Central Hox genes in bilaterians. A. Posterior Hox genes. The predicted PGs for each phylogenetic group are indicated with colors in the tables. Inside these table, the names of the genes are indicated when HoxPred predictions differ from their current annotation. The possible emergence of individual PGs are indicated on the schematic tree with vertical bars (only the PG content is considered, not the actual number of genes belonging to each PG, i.e. lineage-specific duplication and losses of individual genes are not indicated). Given that both protostomes and deuterostomes have PG9 predictions, it seems that a Hox9 gene was already present in Urbilateria. PG10 would have emerged in deuterostomes, in the lineage leading to chordates. After the divergence of cephalochordates, the lineage leading to urochordates and vertebrates would have acquired PG12. PG14 appeared in vertebrates. With respect to PG11, this group could have emerged either before or after the split between urochordates and vertebrates. Considering that both Ciona intestinalis and Oikopleura dioica have disintegrated clusters and likely miss PGs, we cannot exclude a possible loss of PG11 in urochordates. The emergence of PG13 is uncertain due to the prediction of the amphioxus Hox15 gene as PG13. It could either be early in the chordate lineage, or in the last common ancestor of urochordates and vertebrates. B. Central Hox genes. The possible emergence and loss of individual PGs are indicated on the schematic tree with vertical bars and crosses, respectively. Four Central PGs were present in Urbilateria (PG4, PG5, PG6 and PG7). PG6 and PG7 would have been independently lost within deuterostomes. PG8 emerged in vertebrates.