Moles have specialised morphological features, that are highly adapted for their unusual underground lifestyle. One of the most apparent of these features is their specialised forelimbs, which are larger and more robust than their hind limbs. It has been hypothesized that this may be caused by differences in timing (heterochrony) in developmental mechanisms.

In new research published in EvoDevo today, Bickelmann et al. measure the spatial and temporal distribution of SOX9 expression in the Iberian mole Talpa occidentalis. SOX9 is a transcription factor associated with chondrocyte differentiation and the development of the Os falciforme- an extra digit like structure found in both the hands and feet of talpid moles. They compare this with expression in both shrew and mouse species, which do not show these specialisations of the forelimbs.

The mole shows accelerated SOX9 expression in the forelimb in comparison to the hindlimb, whereas the shrew and the mouse show a synchronous expression of SOX9. This article not only demonstrates the presence of limb heterochrony in talpid moles, but can also identify the molecular origins of this important developmental phenomenom.

Dr Constanze Bickelmann, from the Paleontological Institute and Museum, University of Zurich, explained, “This difference in timing of expression of a gene is called transcriptional heterochrony. It is an extreme example of adaption to an ecological niche, in this case digging, which has selected for animals with bigger front paws, who were better diggers and so on.”