Evolutionary conservation of the circadian gene timeout in Metazoa

<i>Timeless </i>(<i>Tim</i>) is considered to function as an essential circadian clock gene in <i>Drosophila</i>. Putative homologues of the <i>Drosophila timeless </i>gene have been identified in both mice and humans. While <i>Drosophila </i>contains two paralogs, <i>timeless </i>and <i>timeout</i>, acting in clock/light entrainment and chromosome integrity/photoreception, respectively, mammals contain only one <i>Tim </i>homolog. In this paper, we study the phylogeny of the <i>timeless</i>/<i>timeout </i>family in 48 species [including 1 protozoan (<i>Guillardia theta</i>), 1 nematode (<i>Caenorhabditis elegans</i>), 8 arthropods and 38 chordates], for which whole genome data are available by using MEGA (Molecular Evolutionary Genetics Analysis). Phylogenetic Analysis by Maximum Likelihood (PAML) was used to analyze the selective pressure acting on metazoan <i>timeless</i>/<i>timeout </i>genes. Our phylogeny clearly separates insect <i>timeless </i>and <i>timeout </i>lineages and shows that non-insect animal <i>Tim </i>genes are homologs of insect <i>timeout</i>. In this study, we explored the relatively rapidly evolving <i>timeless </i>lineage that was apparently lost from most deuterostomes, including chordates, and from <i>Caenorhabditis elegans</i>. In contrast, we found that the <i>timeout </i>protein, often confusingly called “<i>timeless</i>” in the vertebrate literature, is present throughout the available animal genomes. Selection results showed that <i>timeout </i>is under weaker negative selection than <i>timeless</i>. Finally, our phylogeny of <i>timeless</i>/<i>timeout </i>showed an evolutionary conservation of the circadian clock gene <i>timeout </i>in Metazoa. This conservation is in line with its multifunctionality, being essential for embryonic development and maintenance of chromosome integrity, among others.