Abstract
BackgroundThe forkhead transcription factor gene E1 (FOXE1) plays an important role in regulation of thyroid development, palate formation and hair morphogenesis in mammals. However, avian FOXE1 genes have not been characterized and as such, codon evolution of FOXE1 orthologs in a broader evolutionary context of mammals and birds is not known.ResultsIn this study we identified the avian FOXE1 gene in chicken, turkey and zebra finch, all of which consist of a single exon. Chicken and zebra finch FOXE1 are uniquely located on the sex-determining Z chromosome. In situ hybridization shows that chicken FOXE1 is specifically expressed in the developing thyroid. Its expression is initiated at the placode stage and is maintained during the stages of vesicle formation and follicle primordia. Based on this expression pattern, we propose that avian FOXE1 may be involved in regulating the evagination and morphogenesis of thyroid. Chicken FOXE1 is also expressed in growing feathers. Sequence analysis identified two microdeletions in the avian FOXE1 genes, corresponding to the loss of a transferable repression domain and an engrailed homology motif 1 (Eh1) C-terminal to the forkhead domain. The avian FOXE1 proteins exhibit a significant sequence divergence of the C-terminus compared to those of amphibian and mammalian FOXE1. The codon evolution analysis (dN/dS) of FOXE1 shows a significantly increased dN/dS ratio in the avian lineages, consistent with either a relaxed purifying selection or positive selection on a few residues in avian FOXE1 evolution. Further site specific analysis indicates that while relaxed purifying selection is likely to be a predominant cause of accelerated evolution at the 3'-region of avian FOXE1, a few residues might have evolved under positive selection.ConclusionsWe have identified three avian FOXE1 genes based on synteny and sequence similarity as well as characterized the expression pattern of the chicken FOXE1 gene during development. Our evolutionary analyses suggest that while a relaxed purifying selection is likely to be the dominant force driving accelerated evolution of avian FOXE1 genes, a few residues may have evolved adaptively. This study provides a basis for future genetic and comparative biochemical studies of FOXE1.
Highlights
The forkhead transcription factor gene E1 (FOXE1) plays an important role in regulation of thyroid development, palate formation and hair morphogenesis in mammals
FOXE1 genes in other species are listed as orthologs for this gene, the lack of synteny with other FoxE1 genes indicates that this region is not homologous to FOXE1 but with FoxE3 (Figure 1)
The expression of the chicken FOXE1 gene was confirmed with reverse Transcription (RT)-Polymerase Chain Reaction (PCR) using a chicken embryonic cDNA library
Summary
The forkhead transcription factor gene E1 (FOXE1) plays an important role in regulation of thyroid development, palate formation and hair morphogenesis in mammals. In contrast to the role of FOXE1 in placental mammals, a loss-of-function study demonstrated that zebrafish foxe is not required for the thyroid formation but is necessary for chondrogenesis during pharyngeal skeleton formation [10]. These data suggest that FOXE1 may have acquired the role in the regulation of thyroid development during the evolution of tetrapods, or may have lost this role in the fish lineage. The data supports substantial functional evolution of FOXE1 during vertebrate evolution
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
