Quantitative Genetics of Scale Counts in the Garter Snake Thamnophis sirtalis

Abstract

This study addresses the quantitative genetic basis of phenotypic variation and covariation for a series of meristic traits in the garter snake Thamnophis sirtalis fitchi (six head scale counts: loreals, supra-and infralabials, pre-and postoculars, temporals; three body scale counts: ventrals, subcaudals, dorsal scale rows at midbody; two derived traits: umbilical scar size and position). Each trait was scored on approximately 540 offspring and their 47 dams captured in the wild while gravid. Correlations of the meristic traits with body mass at birth, dam's snout-vent length and body mass, litter size, and number of days each dam was held under laboratory conditions prior to giving birth were removed by computing residuals from multiple regression equations. Narrow-sense heritabilities (estimated by restricted maximum likelihood) of residuals were high for temporal scale counts (0.59), moderately large for ventral (0.29) and subcaudal scale counts (0.41), and low (in the range 0-0.12) for the other five traits. Probably as a consequence of the low statistical power of significance testing under restricted maximum likelihood, only the heritability for temporal scales was significantly different from zero. Phenotypic (rp = 0.25) and genetic (rg = 0.67) correlations between ventrals and subcaudals were positive and significant. Phenotypic correlations between the head scale counts were generally low; however, the genetic correlations were larger, suggesting relatively tight integration at the genetic level. Phenotypic correlations between the head and body scale counts were generally low, but several genetic correlations were large (e.g., rg =-0.59 for ventrals and infralabials, rg = 0.59 for subcaudals and supralabials). These data indicate that scale counts from different regions of the body are not evolutionarily independent characters, despite their different spatial and temporal relationships during development. Overall, genetic correlations were not strongly correlated with either phenotypic (r = 0.42) or environmental correlations (r = 0.16). EPIDERMAL scales are among the most distinctive features of living Reptilia. The functional significance of variation in reptilian scale counts is generally unclear (but see Hecht, 1952; Bennett and Licht, 1975; Regal, 1975), although numbers of ventral and subcaudal scales [which correspond directly to body and tail vertebrae (Alexander and Gans, 1966; Voris, 1975)] or their ratio may correlate with locomotor performance in garter snakes (Arnold and Bennett, 1988; Jayne and Bennett, 1989; unpubl.), and the number of head scales may affect cranial kinesis during ingestion of prey (S. J. Arnold, pers. comm.). Several early (Dunn,

MIPoD: A Hypothesis‐Testing Framework for Microevolutionary Inference from Patterns of Divergence

“…To assess the impact of the direct estimate of the G matrix on these conclusions, we repeated the above analyses by using the estimates of G for body and tail vertebral counts from the two separate T. elegans populations (coastal and inland; Arnold and Phillips 1999) and from a population of T. sirtalis (Dohm and Garland 1993;table A2). At the first level of the hierarchy, the same result as above was obtained for all direct estimates of size (Σ) of G. In all cases, the direct estimate of Σ was significantly larger than the ML estimate (P < .0001 for all), suggesting the presence of some restraining force, such as stabilizing selection, that has impeded divergence.…”

“…However, vertebral number in snakes is conspicuously variable among species, with means ranging from 100 to 300 total vertebrae. In garter snakes (Thamnophis), numbers of body and tail vertebrae are moderately heritable and buffered from temperature effects during development (Arnold 1988;Dohm and Garland 1993;Arnold and Phillips 1999;Arnold and Peterson 2002). Several studies indicate that selection favors an intermediate optimum number of vertebrae within populations of garter snakes and closely related genera (Arnold and Bennett 1984;Arnold 1988;Lindell et al 1993).…”

“…To visualize a G matrix on the scale of trait values, we plot it as an ellipse with axes scaled to 1.96 times the square root of the corresponding eigenvalue; in other words, the ellipse represents a 95% confidence ellipse around the within-taxon genetic variation. The long axis of the ellipse represents g max , the leading eigenvector of G. Additional direct estimates of G are taken individually from these two populations of T. elegans (coastal and inland, respectively; Arnold and Phillips 1999) and from a single population of Thamnophis sirtalis (Dohm and Garland 1993). Manier and Arnold (2005) estimated effective population sizes within T. elegans from microsatellite data.…”

HIERARCHICAL COMPARISON OF GENETIC VARIANCE‐COVARIANCE MATRICES. II COASTAL‐INLAND DIVERGENCE IN THE GARTER SNAKE, THAMNOPHIS ELEGANS

“…Only a few other studies have documented heritable variation in snake scalation (Beatson 1976;Arnold 1988;Dohm and Garland 1993;King 1997). The common observation in these studies and the present one of genetic correlation between different scale count traits suggests that these traits should not be treated as independent entities in studies of evolutionary process or in reconstructions of phylogeny.…”

Dominance variance: associations with selection and fitness

“…Assuming low or no common environment effects, one would not expect significant differences between FS and P0 estimates for morphological traits as dominance variance contributes little to these types of traits (h2 inflation because Vd <10 per cent). Although it is not known to what degree total nonadditive genetic effects differ in magnitude among traits (Shwartz & Herzog, 1994;Dohm & Garland, 1993), the contribution of dominance variance will vary depending on the type of trait examined (this study; Dohm & Garland, 1993). Therefore, before deciding on a particular breeding design for estimating heritabilities, it is critical to take into consideration the types of traits being measured to obtain accurate estimates.…”