HISTORICAL BIOLOGY, 2017 https://doi.org/10.1080/08912963.2017.1337760 Heritability: the link between development and the microevolution of molar tooth form P. David Pollya and Orin B. Mockb aDepartment of Earth and Atmospheric Sciences, Indiana University, Bloomington, IN, USA; bDepartment of Anatomy, Kirksville College of Osteopathic Medicine, Kirksville, MO, USA ABSTRACT ARTICLE HISTORY The developmental gene expression, morphogenesis, and population variation in mammalian molar Received 29 May 2017 teeth has become increasingly well understood, providing a model system for synthesizing evolution Accepted 30 May 2017 and developmental genetics. In this study, we estimated additive genetic covariances in molar shape (G) KEYWORDS using parent-offspring regression in Cryptotis parva, the Least Shrew. We found that crown shape had an 2 Molars; evolution and overall h value of 0.34 (±0.08), with higher heritabilities in molar cusps than notches. We compared the development; heritability; genetic covariances to phenotypic (P) and environmental (E) covariances, and to the covariances in crown soricidae; geometric features expected from the enamel knot developmental cascade (D). We found that G and D were not morphometrics strongly correlated and that major axes of G (evolutionary lines of least resistance) are better predictors of evolutionary divergences in soricines than is D. We conclude that the enamel knot cascade does impose constraints on the evolution of molar shape, but that it is so permissive that the divergences among soricines (whose last common ancestor lived about 14 million years ago) do not fully explore its confines. Over tens of millions of years, G will be a better predictor of the major axes of evolution in molar shape than D. Introduction adult molar crown and its many cusps, ridges, and basins. A Building on foundations laid by Butler (1941), (1956), (1995), dynamic system of interactions folds the inner enamel epithelium studies of mammalian molar teeth have begun to bridge the of the tooth germ and expands its mesenchymal base (Butler gap between evolutionary developmental biology and quanti- 1956). Epithelial signaling centers called enamel knots influence tative genetics (Jernvall & Jung 2000; Tucker & Sharpe 2004; the growth of individual cusps, the formation of other knots, and Polly 2005, 2015; Kavanagh et al. 2007; Gómez-Robles & Polly cell proliferation in the epithelium and mesenchyme (Jernvall 2012; Jernvall & Thesleff 2012; Gomes Rodrigues et al. 2013; et al. 1994, 1998; Jernvall & Thesleff2000, 2012). This system Jheon et al. 2013; Harjunmaa et al. 2014; Ungar & Hlusko is shared, in its broad aspects, by species with different tooth 2016). Interest in the connections between developmental gene morphologies. Species differences in gene expression patterns are expression and quantitative models of variation, selection, and associated with their differences in molar form (Keränen et al. evolution have brought attention to genotype-phenotype maps 1998; Jernvall et al. 2000). Even though the experimental work that involve complex developmental interactions and how these from which the enamel knot cascade model was developed was processes constrain variation and influence the course of long- based on mice, histological observation (Butler 1956; Marshall & term evolution (Weiss 1993; Weiss & Fullerton 2000; Rice 2002, Butler 1966; Berkovitz 1967), empirical observations on patterns 2011; Wolf 2002; Klingenberg 2010; Salazar-Ciudad & Jernvall of variability in cusps positions (Polly 1998, 2005; Jernvall 2000), 2010; Goswami et al. 2014; Hlusko et al. 2016). In this paper, we and computer-assisted modeling of these developmental param- measure the genetic, phenotypic, and developmental covariance eters (Salazar-Ciudad & Jernvall 2002, 2010; Salazar-Ciudad structure in the tribosphenic lower molars of the Least shrew et al. 2003; Salazar-Ciudad & Marín-Riera 2013; Marin Riera (Cryptotis parva) and assess how these three population-level et al. 2015) all suggest that the enamel knot cascade model is components of variance are related to long-term phylogenetic generalizable to explain the phenotypic changes observed during divergence in molar shape. morphogenesis and evolution. These same models have empha- Mammalian teeth are a notable example of a system in which sized that development and evolution are not determined by gene the synthesis of molecular development, morphogenesis, adult expression alone, but that the changing three-dimensional topog- morphology, and evolution is well underway. We now under- raphy of the tooth bud creates morphodynamic feedback systems stand how gene products interact in the context of growing epi- that regulate developmental interactions. The cascade of interac- thelial and mesenchymal tissues to create the topography of the tions between enamel knots are expected to induce a predictable CONTACT P. David Polly [email protected] © 2017 Informa UK Limited, trading as Taylor & Francis Group P. D. POLLY AND O. B. MOCK 2 morphological differences in other shrew species to see whether phylogenetic divergences in morphology were related to genetic variances and, by extension, to developmental interactions. Materials and methods Molar shape data were collected from alcohol-preserved individuals of the Least Shrew, C. parva. These animals were from a long-term breeding colony at the Kirksville College of (A) Osteopathic Medicine (KCOM, Kirksville, Missouri, USA) that has been maintained for more than 40 years (Mock 1982). The original stocks were trapped in central Missouri in the mid 1960s buccal 3 and early 1970s. The animals included in this study were products mesial 7 of up to 30 generations of breeding, including some inbreeding. 2 No selection was applied, but indirect selective effects on molar 6 4 width have been documented in Peromyscus when wild animals were bred in laboratory conditions (Leamy & Bader 1970). The 8 1 5 colony was not maintained with the intention of studying her- 9 itability, and our selection of offspring-parent pairs was limited by the number of carcasses that had been preserved and by the amount of wear on their teeth. In most cases only individuals (B) 05 mm with unworn teeth were used, but to increase the sample size a few individuals with moderately worn teeth were included when landmarks could be reliably identified. A total of 63 single off- Figure 1. A. Schematic diagram of developmental interactions superimposed spring- single parent pairs were available for this study. on a fully mineralized tooth in functional view (as in Figure 1(B)). Enamel knots (circles) are cellular signaling centers associated with each developing cusp. The Molar shape was measured with geometric morphometrics primary knot (1) influences the position of subsequent knots (2–5) by producing (Bookstein 1991; Dryden & Mardia 1998; Zelditch et al. 2012) of diffusable molecules that inhibit knot formation in the immediate vicinity (thick nine two-dimensional Cartesian-coordinate landmarks to rep- black lines). Downstream knots have a similar effect on their own periphery. Each knot stimulates down-growth in epithelia between it and adjacent knots, resent the occlusal surface of the lower first molar (Figure 1). thus influencing the height and shape of cusps and the position of cingulae and Teeth were oriented in ‘functional view’ parallel to the angle of notches on the adult crown (thin black arrows). The knots also stimulate growth mandibular movement during phase one occlusion, an orienta- in the underlying mesenchyme, which further influences the spacing between knots (large grey arrows). Mesiodistal and buccolingual growth rates are partly tion achieved by aligning shear facets parallel to the line of sight independent. Change in any of these factors can alter placement of all the cusps. (Butler 1961). This position is the most replicable orientation B. Lower right first molar of Cryptotis parva in functional view showing the for a tribosphenic tooth, and it minimizes the effect of wear on nine landmarks used to represent crown shape. In order, they are the paraconid, preprotocristid notch, protoconid, postprotocristid notch, metaconid, cristid the apparent shape of the crown. To further minimize orienta- obliqua notch, hypoconid, entoconid, and talonid notch. The landmarks represent tion error, each specimen was photographed and landmarked those adult crown features that are directly associated with developmental factors. five times and the replicates averaged (Polly 2001, 2003, 2007). By confining our analysis to two-dimensions we risk ignoring important components of the spatial patterning of the develop- pattern of covariance between cusps and notches (Figure 1(A)). mental cascade (see discussion below), but by focusing purely on The cascade predicts which parts of the tooth crown will be more the horizontal position of cusps we avoid conflating the effects variable, which will be gained and lost, and what the patterns of of tooth wear, which modifies the apical height of cusps, with variability in molar crown shape will be (Jernvall 1995, 2000; developmental or genetic variation. Polly 1998, 2005; Salazar-Ciudad & Jernvall 2010). Shape variables were derived from the landmarks for further While we know how patterns of phenotypic shape variation analysis. The individual molar shapes were rescaled and superim- in mammalian molar crowns are associated with developmental posed using generalized least-squares Procrustes fit (Gower1975 ; processes, we neither