Evolutionary History of Bilaterian <i>FoxP</i> Genes: Complex Ancestral Functions and Evolutionary Changes Spanning 2R-WGD in the Vertebrate Lineage

Human and fly FoxP homologs are well-known for their roles in the development of cognitive abilities. These findings have led to the hypothesis that the ancestral function of FoxP was in the development of cognitive neural circuits. However, complex brains in human and fly evolved independently, and the similar cognitive function of FoxP in human and fly may thus be interpreted as a result of convergent evolution. In addition, the 4 gnathostome FoxP paralogs also possess diverse developmental functions unrelated to neurodevelopment, which might have been overlooked in comparative studies of invertebrate FoxP homologs. To resolve these uncertainties, we set out to improve the phylogenetic reconstruction of vertebrate FoxP homologs and broaden the taxonomic sampling of gene expression profiling to include an invertebrate chordate, ambulacrarian deuterostomes, and a spiralian protostome. Using phylogenetic analysis combined with synteny mapping, we elaborated the hypothesis that the 4 FoxP paralogs arose through the 2R-WGD events shared by all gnathostome species. Based on this evolutionary scenario, we examined the FoxP expression pattern in amphioxus development and concluded that FoxP already had complex developmental functions across all germ layers in the chordate ancestor. Moreover, in sea urchin, hemichordate, and catenulid flatworm, FoxP was expressed in the gut prominently, in addition to the anterior neurogenic ectoderm. This surprising similarity shared among these distantly related species implies that FoxP may have a significant function in gut development in addition to the neural development function in the last common ancestor of bilaterians.