The supernumerary cheek tooth in tabby/EDA mice—a reminiscence of the premolar in mouse ancestors

SummaryObjective: rnumerary cheek tooth occurs mesially to the first molar in tabby/EDA (Ta) mice affected by hypohidrotic ectodermal dysplasia. The supernumerary tooth (S) has been hypothetically homologized to the premolar, which has disappeared during mouse evolution. : ypothesis was tested using available morphological data on the lower cheek teeth in wild type (WT) and Ta mice. s: esence of S is accompanied by a reduction in the mesial portion of the M1 in mutant mice. 3D reconstructions suggest that the S in Ta homo/hemizygous embryos originates from a split off the mesial portion of the first molar (M1) cap. In WT embryos, two vestigial tooth primordia are transiently distinct in front of the M1. The distal vestige has the form of a wide bud and participates during the development of the mesial portion of the M1. This bud has been homologized with the vestigial primordium of the fourth premolar of mouse ancestors. The premolar disappearance coincided with a mesial lengthening of the M1 during mouse evolution. The incorporation of the distal premolar vestige into the mesial part of the M1 in WT embryos can be regarded as a repetition of the premolar disappearance during evolution. sion: netic and phylogenetic data support that the S in Ta mice arises due to the segregation of the distal premolar vestige from the molar dentition and thus represents an evolutionary throwback (atavism).