QUANTITATIVE GENETICS OF SCALE COUNTS IN THE GARTER SNAKE THAMNOPHIS-SIRTALIS

This study addresses the quantitative genetic basis of phenotypic vari ation and covariation for a series of meristic traits in the garter sn ake Thamnophis sirtalis fitchi (six head scale counts: loreals, supra- and infralabials, pre- and postoculars, temporals; three body scale c ounts: ventrals, subcaudals, dorsal scale rows at midbody; two derived traits: umbilical scar size and position). Each trait was scored on a pproximately 540 offspring and their 47 dams captured in the wild whil e gravid. Correlations of the meristic traits with body mass at birth, dam's snout-vent length and body mass, litter size, and number of day s 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 likelih ood) of residuals were high for temporal scale counts (0.59), moderate ly 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 conseq uence of the low statistical power of significance testing under restr icted maximum likelihood, only the heritability for temporal scales wa s significantly different from zero. Phenotypic (r(p) = 0.25) and gene tic (r(g) = 0.67) correlations between ventrals and subcaudals were po sitive and significant. Phenotypic correlations between the head scale counts were generally low; however, the genetic correlations were lar ger, suggesting relatively tight integration at the genetic level. Phe notypic correlations between the head and body scale counts were gener ally low, but several genetic correlations were large (e.g., r(g) = 0. 59 for ventrals and infralabials, r(g) = 0.59 for subcaudals and supra labials). These data indicate that scale counts from different regions of the body are not evolutionarily independent characters, despite th eir different spatial and temporal relationships during development. O verall, genetic correlations were not strongly correlated with either phenotypic (r = 0.42) or environmental correlations (r = 0.16).