posted on 2016-02-02, 11:34authored byDiyan Li, Yuan Su, Jianbo Tu, Ranlei Wei, Xiaolan Fan, Huadong Yin, Huailiang Xu, Yongfang Yao, Deying Yang, Mingyao Yang
Timeless (Tim)
is considered to function as an essential circadian clock gene in Drosophila.
Putative homologues of the Drosophila timeless gene have been identified
in both mice and humans. While Drosophila contains two paralogs, timeless
and timeout, acting in clock/light entrainment and chromosome
integrity/photoreception, respectively, mammals contain only one Tim homolog.
In this paper, we study the phylogeny of the timeless/timeout family
in 48 species [including 1 protozoan (Guillardia theta), 1 nematode (Caenorhabditis
elegans), 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 timeless/timeout genes. Our
phylogeny clearly separates insect timeless and timeout lineages
and shows that non-insect animal Tim genes are homologs of insect timeout.
In this study, we explored the relatively rapidly evolving timeless lineage
that was apparently lost from most deuterostomes, including chordates, and from
Caenorhabditis elegans. In contrast, we found that the timeout protein,
often confusingly called “timeless” in the vertebrate literature, is
present throughout the available animal genomes. Selection results showed that timeout
is under weaker negative selection than timeless. Finally, our
phylogeny of timeless/timeout showed an evolutionary conservation
of the circadian clock gene timeout in Metazoa. This conservation is in
line with its multifunctionality, being essential for embryonic development and
maintenance of chromosome integrity, among others.