In the evolution of tetrapods , the hindlimb position has changed along with the vertebral formula (the number of small bones joints that form the vertebra). Tetrapods are species with four feet, but can include those without four or any feet. Tetrapods comprise all the organisms, living and extinct, that descended from the last common ancestor of amphibians, reptiles, and mammals, even if they have secondarily lost their " four feet ".
Although scientists have thoroughly studied the tetrapod anatomy, the species-specific position of the body parts of these species remains unclear. Formation and development of the hindlimbs in early life is not well understood, and solving this mystery will be a major step in evolution biology.
A team of researchers from Nagoya University in Japan have stated that the protein differentiation factor 11 (GDF11) was involved in this aspect of tetrapod evolution. This gene involved in embryonic development, plays a vital role in the eventual position of the sacral vertebrae and the hindlimb. These findings were published in July 2017 in Nature Ecology & Evolution .
Yoshiyuki Matsubara, researcher at the Division of Biological Science reported: "In laboratory mice that do not produce the GDF11 protein, we have noted that the sacral vertebrae and the hindlimbs are shifted more to the back."
The research team analyzed the expression pattern of the GDF 11 gene using chicken embryos. The relationship between the pattern and the prospective position of the spine and hindlimb at different development stages in chicken embryos was analyzed. Further, they tested whether hindlimb positioning could be altered by changing the timing of GDF11 activity in the embryos.
To confirm the hypothesis of the role of GDF11 in diversification of the hindlimb position in tetrapods , the team examined eight tetrapod species for the correlation between gene expression and hindlimb positioning. The species included in the study were African clawed frog, Chinese soft-shelled turtle, gecko ocelot, Japanese striped snake, chick, quail, emu and mouse.
Takayuki Suzuki, last author of the study stated: "Our results also suggest that species-specific hindlimb positioning may have been an effect of the change in the timing or rate of events in the gene that expresses GDF11 during embryonic development."
This conclusion explains why snakes have a long trunk – since initiation timing of Gdf 11 expression in the progressive stage is much later in snakes than in other tetrapod species. These observations explain the coupling of the sacral-hindlimb positioning in tetrapod evolution. This provides insights into the diversification of lineage-specific tetrapod hindlimb positions.