Transcriptional heterochrony in talpid mole autopods

The temporal and spatial distribution of SOX9 expression, which is an early marker of chondrification, is analyzed in autopods of the fossorial Iberian mole Talpa occidentalis, as well as in shrew (Cryptotis parva) and mouse (Mus musculus) for comparison.SOX9 expression is advanced in the forelimb compared to the hind limb in the talpid mole. In contrast, in the shrew and the mouse, which do not show fossorial specializations in their autopods, it is synchronous. We provide evidence that transcriptional heterochrony affects the development of talpid autopods, an example of developmental penetrance. We discuss our data in the light of earlier reported pattern heterochrony and later morphological variation in talpid limbs.Transcriptional heterochrony in SOX9 expression is found in talpid autopods, which is likely to account for pattern heterochrony in chondral limb development as well as size variation in adult fore- and hind limbs.Talpid moles (Talpidae, Lipothyphla sensu[1]) show a great number of morphological peculiarities in their postcranial skeleton which can be interpreted as being related to their specialized locomotor behavior. Among other modifications, the forelimbs of fossorial talpid moles are enlarged and more robust than the hind limbs (Figure？1A, B). The manus is broad and strong and its palm faces outward (Figure？1A) [2]. Serving for further enlargement of the autopodial area, fossorial talpid moles also bear an extra digit-like structure (’Os falciforme’) in both hands and feet (Figure？1A, B) [3]. The molecular evolution and development of these accessory sesamoid bones were recently investigated in the fossorial Iberian mole, Talpa occidentalis, by an analysis of expression patterns of SOX9,Fgf8 and Msx2 in mole autopodia [4]. Analysis of the timing of SOX9 expression showed that the ‘Os falciforme’ develops later than the true digits and extends into the digital area in spatial relationship with a Msx2 expressing domain [4]. However, such extreme