Fig. 1

Hox gene data places rotifers and chaetognaths in Gnathifera within Spiralia. a Phylogenetic tree depicting the relationship of MedPost genes to PG8 and posterior class Hox genes. Tree topology is from Bayesian analysis. Bayesian posterior probabilities based on 400,000 trees from 40,000,000 generations and ML support values from 1000 iterations are shown above branches. Single values represent Bayesian posterior probabilities only. Asterisks denote ML support below 50%. b Alignment of ten amino acids of the carboxy flanking region to the homeodomain of PG6 genes. Sequences highlighted with yellow contain the new signature found in rotifers and chaetognaths. Blue highlighting marks the lox5-parapeptide of lophotrochozoan genes. Neither is found in Ecdysozoa, Ambulacraria, Chordata, or Xenacoelomorpha. c Summary of representative characteristics of the Hox cluster within different metazoan taxa. The tree to the left represents bilaterian phylogeny with Cnidaria as an outgroup. Boxes in the middle depict Hox gene contingents (color coded according to the assignment of the Hox genes to the different paralogous groups) isolated from representative species. The right hand column summarizes characteristic Hox gene evolution and duplication events along with presence of special Hox signatures resulting in Hox genes characterizing the respective groups