The central class is data that is replicated in the routing models of [25] and Figure 12 to denote a connection between the respective models.
The central Class B Mup gene cluster formed through a number of sequential duplications from one of the Class A Mups.
It is composed by three regions, the class I and class II regions flanking the central class III region.
Posterior to the bilaterally restricted patches of ClantHox expression, similar domains of the central class Hox gene (ClcentHox) are expressed in the acoel embryo.
As cnidarians, the sister group to the Bilateria, have no central class Hox gene, acoels are the pivotal group for reconstructing the origin and expansion of the central class genes and the possible roles of Hox genes in the evolution of bilaterian complexity.
Our analysis suggests that a true anterior and posterior gene has been present in the bilaterian-cnidarian ancestor and that the central class genes are a novelty of the Bilateria.
As in previous phylogenetic analyses of acoel Hox genes [ 5, 45, 64], the anterior class Hox orthologs of acoels groups as the sister to all remaining Hox1/ labial genes, the central class Hox gene branches with Hox4/5 and the posterior Hox with the posterior class genes Hox9-14.
Together with the gene complement in acoels, the results favour a two-gene model of the evolution of the Hox genes from a proto-Hox cluster followed by several duplication events that gave rise to the central class and expansion within each class to give rise to the expanded Nephrozoan complement [ 13].
The central class ortholog of acoels contains a PBX-motif that is close to the homeodomain (2 amino acids), as it is in the bilaterian orthologs of Hox6/ 7 (5-8 amino acids), but its C-motif has greater similarity to the bilaterian Hox4/5 orthologs.
This chapter describes the requirements for our new architecture, justifies the design decisions, sketches out central classes, and highlights a few of the common tasks that the new software can perform.
