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Background  The forelimb-specific gene  tbx5  is highly conserved and essential for the development of forelimbs in zebrafish, mice, and humans. Amongst birds, a single order, Dinornithiformes, comprising the extinct wingless moa of New Zealand, are unique in having no skeletal evidence of forelimb-like structures.    Results  To determine the sequence of  tbx5  in moa, we used a range of PCR-based techniques on ancient DNA to retrieve all nine  tbx5  exons and splice sites from the giant moa,  Dinornis . Moa Tbx5 is identical to chicken Tbx5 in being able to activate the downstream promotors of  fgf10  and  ANF . In addition we show that missexpression of moa  tbx5  in the hindlimb of chicken embryos results in the formation of forelimb features, suggesting that Tbx5 was fully functional in wingless moa. An alternatively spliced exon 1 for  tbx5  that is expressed specifically in the forelimb region was shown to be almost identical between moa and ostrich, suggesting that, as well as being fully functional,  tbx5  is likely to have been expressed normally in moa since divergence from their flighted ancestors, approximately 60 mya.    Conclusions  The results suggests that, as in mice, moa  tbx5  is necessary for the induction of forelimbs, but is not sufficient for their outgrowth. Moa Tbx5 may have played an important role in the development of moa’s remnant forelimb girdle, and may be required for the formation of this structure. Our results further show that genetic changes affecting genes other than  tbx5  must be responsible for the complete loss of forelimbs in moa.

Reconstruction and in vivo analysis of the extinct tbx5 gene from ancient wingless moa (Aves: Dinornithiformes)