A simplified explanation for the frameshift mutation that created a novel C-terminal motif in the APETALA3 gene lineage

Background: The evolution of type II MADS box genes has been extensively studied in angiosperms. One of the best-understood subfamilies is that of the Arabidopsis gene APETALA3 (AP3). Previous work has demonstrated that the ancestral paleoAP3 lineage was duplicated at some point within the basal eudicots to give rise to the paralogous TM6 and euAP3 lineages. This event was followed in euAP3 orthologs by the replacement of the C-terminal paleoAP3 motif with the derived euAP3 motif. It has been suggested that the new motif was created by an eight-nucleotide insertion that produced a translational frameshift. Results: The addition of 25 eudicot AP3 homologs to the existing dataset has allowed us to clarify the process by which the euAP3 motif evolved. Phylogenetic analysis indicates that the euAP3/ TM6 duplication maps very close to the base of the core eudicots, associated with the families Trochodendraceae and Buxaceae. We demonstrate that although the transformation of paleoAP3 into euAP3 was due to a frameshift mutation, this was the result of a single nucleotide deletion. The use of ancestral character state reconstructions has allowed us to demonstrate that the frameshift was accompanied by few other nucleotide changes. We further confirm that the sequence is evolving as coding region. Conclusion: This study demonstrates that the simplest of genetic changes can result in the remodeling of protein sequence to produce a kind of molecular 'hopeful monster.' Moreover, such a novel protein motif can become conserved almost immediately on the basis of what appears to be a rapidly generated new function. Given that the existing data on the function of such C-terminal motifs are somewhat disparate and contradictory, we have sought to synthesize previous findings within the context of the current analysis and thereby highlight specific hypotheses that require further investigation before the significance of the euAP3 frameshift event can be fully understood.