MRC National Institute for Medical Research, London
Science for Health
How limbs evolved
14 December 2009
The limbs have been one of the most successful innovations in vertebrates and are used in numerous fundamental activities, including locomotion, feeding and breeding. Understanding their development and evolution is therefore a matter of interest. The genes Tbx5 and Tbx4 encode two closely related transcription factors that are the earliest factors required to initiate forelimb and hind limb outgrowth, respectively. Amphioxus, the closest living invertebrate relative of the vertebrates, lacks paired appendages and possesses a single Tbx4/5 gene, whereas all vertebrates with paired appendages possess distinct Tbx4 and Tbx5 genes (that are the earliest factors required to initiate limb bud outgrowth). Gene duplication events generate genes with the potential to acquire functions, and two rounds of genome duplication took place during vertebrate evolution. Amphioxus diverged from other chordates before these duplication events and can be used to deduce the functions of ancestral genes, present in single copy in amphioxus, compared to the functions of their duplicated vertebrate orthologues.
Malcolm Logan (pictured), from NIMR's Division of Developmental Biology, has worked with colleagues to investigate whether the duplication of the ancestral Tbx4/5 gene, represented by the single gene AmphiTbx4/5 present in the amphioxus, to give rise to two distinct Tbx4 and Tbx5 genes, was instrumental in the acquisition of paired fins/limbs during vertebrate evolution. He used transgenic and knock-out-rescue strategies in the mouse to examine the limb-forming potential of the AmphiTbx4/5 gene, as well as the ability of the genomic region where this gene is embedded to drive gene expression in the pre-limb bud lateral plate mesoderm. This work shows that changes at the level of the regulation of vertebrate Tbx genes, rather than changes at the level of the coding region, may have been instrumental in the origin of vertebrate limbs during evolution.
Lateral plate mesoderm expression
Lateral plate mesoderm expression driven by the murine Tbx5 and Tbx4 promoter region and expression of AmphiTbx4/5. (A and B) b-galactosidase activity staining of an m5–10 (A) and m4–10 (B) e9.5 transgenic embryo.
This study provides an explanation for how limbs evolved in primitive vertebrate species. We have demonstrated that a gene present in a limb-less (common) ancestor of all vertebrates has the capacity to form a limb in the mouse. The critical step that allowed for the evolution of limbs in vertebrates was the acquisition of regulatory elements that enabled this gene to be activated in the appropriate regions of the embryo flank where it can execute its limb-inducing activity.
Malcolm Logan
Original article
The research findings are published in full in:
Carolina Minguillona, Jeremy J. Gibson-Brown, and Malcolm P. Logan (2009)
Tbx4/5 gene duplication and the origin of vertebrate paired appendages
Proceedings of the National Academy of Sciences of the USA, Epub ahead of print. PubMed abstract.
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