Figure 6

Amphimedon queenslandica bHLHs that are associated with more than one bHLH family. We took, as an example, the case of the ARNT and Bmal families. In Amphimedon queenslandica, there is a single bHLH that clusters with both families as an outgroup to them (Figure 3). There are two main scenarios that can explain this situation. (A) The duplication that gives rise to the two families occurred after the divergence between demosponges and the other metazoans. No duplication occurred in Amphimedon queenslandica, which displays a single gene as in the ancestral situation. (B) The duplication that gives rise to the two families occurred before the divergence between demosponges and the other metazoans. Amphimedon queenslandica displays a single gene because one of the duplicates was lost. The remaining gene became quite divergent in such a way that it cannot be confidently related to either of the two families.