Developmental variability channels mouse molar evolution

Abstract

Do developmental systems preferentially produce certain types of variation that orient phenotypic evolution along preferred directions? At different scales, from the intra-population to the interspecific, the murine first upper molar shows repeated anterior elongation. Using a novel quantitative approach to compare the development of two mouse strains with short or long molars, we identified temporal, spatial and functional differences in tooth signaling center activity, that arise from differential tuning of the activation-inhibition mechanisms underlying tooth patterning. By tracing their fate, we could explain why only the upper first molar reacts via elongation of its anterior part. Despite a lack of genetic variation, individuals of the elongated strain varied in tooth length and the temporal dynamics of their signaling centers, highlighting the intrinsic instability of the upper molar developmental system. Collectively, these results reveal the variational properties of murine molar development that drive morphological evolution along a line of least resistance.

Segmental Series and Size: Clade-Wide Investigation of Molar Proportions Reveals a Major Evolutionary Allometry in the Dentition of Placental Mammals

“…Our study shows that variation in upper molar proportions across placentals deviates from the line predicted by the IC model, and maybe even slightly more than for lower molars (e.g., Kavanagh et al, 2007;Bernal et al, 2013). Antagonist molars may partly differ in their development (e.g., Hayden et al, 2020) and probably experience different spatial constraints on the palate compared to the narrow lower jaw (Renvoisé et al, 2017). Whether these factors could cause differences in proportional relationships among the upper and lower molar rows remains to be evaluated.…”

Timing of Mouse Molar Formation Is Independent of Jaw Length Including Retromolar Space

“…Our results for the onset times and durations of molar development align well with past reports based on other inbred wild-type laboratory mouse strains, e.g., ICR/Jcl [ 32 ] and CD1 [ 19 , 21 ] (reviewed in [ 19 ]). Differences among our collective findings likely reflect technical variation in data collection methodologies, as well as biological variation within and among genetic backgrounds and breeding colonies [ 34 ]. Our results also agree with past work [ 19 ] reporting that different molar types (e.g., M1, M3) develop through homologous stages at comparable tempos and durations.…”

Deciphering the morphological variation and its ontogenetic dynamics in the Late Devonian conodont Icriodus alternatus

“…Regarding Icriodontan elements, this would correspond to relatively poorly constrained initial stages of development, when the earliest triads are added to the blade, converging towards the typical phenotype of the species, before that continuous growth and remodeling generate extensive morphological variation when late triads are added. Indeed, surprisingly high phenotypic variation has been found early during embryonic tooth development (Hayden et al 2020) despite canalized adult phenotypes. Such early variation may be favored because the tooth-like structures are not yet functional.…”