A genome-wide survey of bHLH transcription factors in the Placozoan Trichoplax adhaerens reveals the ancient repertoire of this gene family in metazoan

• 32 bHLH genes were identified in the Placozoan, Trichoplax adhaerens.
• They were categorized into 16 or 17 orthologous families.
• Fer1 and ASCb seem to be most ancient tissue-specific Group A genes in metazoa.

Basic helix–loop–helix (bHLH) transcription factors play significant roles in multiple biological processes in metazoan cells. To address the evolutionary history of this gene family, comprehensive and detailed characterization in basal metazoans is essential. Here I report a genome-wide survey of bHLH genes in the Placozoan, Trichoplax adhaerens. The present survey revealed ancient origins of two orthologous families, 48-related-1/Fer1 and ASCb, which both belong to high-order Group A. Group A factors are mainly involved in neural and mesodermal differentiation. I also identified novel members of a Group E orthologous family previously thought to be unique to Homo sapiens. These were discovered in Trichoplax, Saccoglossus kowalevskii, Euperipatoides kanangrensis, and Crassostrea gigas, but apparently are not found in Drosophila melanogaster, Caenorhabditis elegans, or Nematostella vectensis. Furthermore, as reported previously, many unclassified Group A members were observed in Trichoplax. The present study provides important information to infer the ancestral state of bHLH components in the Metazoa.