title: Conservation and divergence of bHLH genes in the calcispongeSycon ciliatum.
abstract: BACKGROUND
  Basic Helix-Loop-Helix (bHLH) genes encode a large family of eukaryotic transcription factors, categorized into six high-order groups: pan-eukaryotic group B involved in regulation of cell cycle, metabolism, and development; holozoan-specific groups C and F involved in development and maintenance of homeostasis; and metazoan-specific groups A, D and E including well-studied genes, such asAtonal,TwistandHairy, with diverse developmental roles including control of morphogenesis and specification of neurons. Current scenarios of bHLH evolution in animals are mainly based on the bHLH gene set found in the genome of demospongeAmphimedon queenslandica.In this species, the majority of the 21 identified bHLH genes belong to group B, and the single group A gene is orthologous to several neurogenic bilaterian subfamilies, includingatonalandneurogenin.
  
  
  RESULTS
  Given recently discovered differences in developmental toolkit components between siliceous and calcareous sponges, we have carried out genome-wide analysis of bHLH genes inSycon ciliatum, an emerging calcisponge model. We identified 30 bHLH genes in this species, representing 12 individual families, including four group A families not found inAmphimedon, and two larger family groupings. Notably, the families represented inSyconare only partially overlapping with those represented inAmphimedon.Developmental expression analysis of a subset of the identified genes revealed patterns consistent with deeply conserved roles, such as specification of sensory cells byAtona-relatedand stem cells byMycgenes.
  
  
  CONCLUSIONS
  Our results demonstrate independent gene loss events in demosponges and calcisponges, implying a complex bHLH toolkit in the last common metazoan ancestor.
authors.affiliation: Sars International Centre for Marine Molecular Biology, University of Bergen, Bergen, Norway.|ARC Centre for Excellence for Coral Reef Studies, James Cook University, Townsville, QLD 4811 Australia.
authors.forename: Sofia A V
authors.initials: SAV
authors.lastname: Fortunato
authors.affiliation: Institut Jacques Monod - CNRS, Université Paris Diderot, 75005 Paris Cedex 13, France.
authors.forename: Michel
authors.initials: M
authors.lastname: Vervoort
authors.affiliation: Sars International Centre for Marine Molecular Biology, University of Bergen, Bergen, Norway.|Research School of Biology, Australian National University, Canberra, Australia.
authors.forename: Marcin
authors.initials: M
authors.lastname: Adamski
authors.affiliation: Sars International Centre for Marine Molecular Biology, University of Bergen, Bergen, Norway.|Research School of Biology, Australian National University, Canberra, Australia.
authors.forename: Maja
authors.identifier: 0000-0002-4223-8450
authors.initials: M
authors.lastname: Adamska
keywords: Developmental regulatory genes; Evolution; Sponges; Sycon ciliatum; Transcription factors; bHLH