Disruption of continuous tooth replacement in squamates: Implications for tooth evolution and development
Authors: Brink Kirstin , Richman Joy 
Published: June 2021

The ability for animals to continuously replace their teeth, or polyphyodonty, is a character deeply nested within Vertebrata. Tooth cycling in polyphyodont vertebrates occurs in a regular pattern, passing through alternating tooth positions in waves around the mouth. Although several hypotheses have been proposed for the establishment and maintenance of this phenomenon, it is not yet known if tooth patterning is controlled through global molecular signals in the jaw environment, signalling between adjacent teeth, or local signals within each tooth family, or how tooth size differences (heterodonty) may affect the regularity of replacement waves. To investigate the mechanisms of polyphyodonty, a historical dataset of unpublished reptile dental x‐rays collected in the 1950s and 60s was analysed. Thirty species of reptiles were x‐rayed monthly for up to two years. The dataset includes animals with heterodont dentitions, such as Alligator, Varanus, and Dracaena, and iguanas with homodont dentitions (teeth are the same size around the mouth) that underwent tooth removal experiments and were tracked with bi‐monthly x‐rays for up to seven months post surgery. Surgical experiments included functional tooth extraction, tooth bud extraction, and removal of portions of the dental lamina with and without tooth extraction.Results in all animals show predictable wave replacement patterns in the dentition over long time periods. In the homodont Iguana, the timing of tooth development and tooth replacement is tightly correlated. However, in the heterodont reptiles, the developmental time is different between teeth: smaller teeth take one or two months to develop before eruption, while larger teeth take up to five months. Tooth removal experiments in Iguana show disruptions in replacement timing after surgery, but near‐complete recovery after six months when developing teeth are removed and the dental lamina is left intact. Damage to the dental lamina creates a strong phenotype in tooth replacement timing and patterning that does not recover, with critical size defects as small as one tooth position. However, tooth replacement continues anterior and posterior to the surgical site, supporting a local rather than global mechanism governing tooth replacement in adult dentitions.These results suggest that tooth replacement in the adult is independently controlled within the tooth family and not through global signalling factors traveling along the dental lamina. Tooth development time is consistent at each tooth position, and does not change after minor disruption. This has implications for the evolution of dentitions, as discrete regions of the mouth can react differently to selection pressures, generating diversity in tooth size, shape, and function.Support or Funding InformationKB is funded by a Banting Postdoctoral Fellowship, a Killam Postdoctoral Fellowship, the Michael Smith Foundation for Health Research, and the American Association of Anatomists. Access to collections was provided by the Royal Ontario Museum.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.