GENETIC CORRELATIONS AMONG MORPHOMETRIC TRAITS AND RATES OF GROWTH AND DIFFERENTIATION IN THE GREEN TREE FROG,HYLA CINEREA

Abstract

Abstract. -It is often proposed that the morphometric shape ofanimals often evolves as a correlated response to selection on life-history traits such as whole-body growth and differentiation rates. However, there exists little empirical information on whether selection on rates of growth or differentiation in animals could generate correlated response in morphometric shape beyond that owing to the correlation between these rates and body size. In this study genetic correlations were estimated among growth rate, differentiation rate, and body-size-adjusted head width in the green tree frog, Hyla cinerea. Head width was adjusted for size by using the residuals from log-log regressions of head width on snout-vent length. Size-adjusted head width at metamorphosis was positively genetically correlated with larval period length. Thus, size-independent shape might evolve as a correlated response to selection on a larval life-history trait. Larval growth rate was not significantly genetically correlated with size-adjusted head width. An additional morphometric trait, size-adjusted tibiofibula length, had a non normal distribution of breeding values, and so was not included in the analysis of genetic correlations (offspring from one sire had unusually short legs). This result is interesting because, although using genetic covariance matrices to predict longterm multivariate response to selection depends on the assumption that all loci follow a multivariate Gaussian distribution of allelic effects, few data are available on the distribution of breeding values for traits in wild populations. Size at metamorphosis was positively genetically correlated with larval period and larval growth rate. Quickly growing larvae that delay metamorphosis therefore emerge at a large size. The genetic correlation between larval growth rate andjuvenile (postmetamorphic) growth rate was near zero. Growth rate may therefore be an example of a fitness-related trait that is free to evolve in one stage of a complex life cycle without pleiotropic constraints on the same trait expressed in the other stage. Gould (1977) argued that morphometric shape in animals often evolves as a correlated response to selection on life-history traits such as whole-body rates ofgrowth or differentiation. This scenario is true by definition for shape variation that scales nonisometrically with overall body size (because if development rate variation causes size variation at the age or stage at which measurements are taken, then development rate is necessarily correlated with shape). It is not clear whether morphometric shape is genetically correlated with growth or differentiation rate independent ofbody size. Such a correlation would arise if the allometric relationship between shape and size is not constant, but is a function of development rate (e.g., changes in overall growth rate do not affect the growth rate of all body parts , Present address: Department of Biology, Sonoma State University, 180 I East Cotati Ave., Rohnert Park, CA 94928 USA.proportionately; ...

A Quantitative Genetic Test of Adaptive Decoupling Across Metamorphosis for Locomotor and Life-History Traits in the Pacific Tree Frog, Hyla Regilla

“…Parental effects on the seven phenotypic traits were examined in the GLM procedure with F-ratios calculated from Type III mean squares. Following Blouin (1992), block was considered a fixed effect and sire, dam, and sire ϫ dam (all nested within block) were treated as random effects. Observational variance components were estimated with the restricted maximum likelihood (REML) option in the VARCOMP procedure.…”

“…Following Blouin (1992), sires and dams were crossed in a blocked factorial breeding design. Forty-five sires were crossed with four dams, with crosses nested in two blocks (a North Carolina II design; Lynch and Walsh 1998).…”

“…At the very least, such an analysis would complement experimental and comparative developmental approaches. Only three studies have examined genetic correlations of equivalent traits across metamorphosis, and they support the prediction of the adaptive decoupling hypothesis as applied to life-history and morphological characters (growth rate : Blouin 1992;body size: Hilbish et al 1993;Phillips 1998). But in general we know very little about the genetic architecture of phenotypic traits-particularly complex performance traits that are known or suspected to be under selection-at the metamorphic boundary.…”

“…As with optimality approaches in general, these models do not consider constraints on adaptive evolution like low heritabilities, maternal effects, and genetic correlations within and between life-cycle stages. Several quantitative genetic studies of anuran larvae have demonstrated such constraints (Travis et al 1987;Newman 1988;Blouin 1992;Semlitsch 1993), and so adaptive evolution may be more complicated and constrained than is assumed by the optimality models.…”

“…One purpose for this study was to examine the correlation structure of locomotor and body-size traits across the metamorphic boundary, as discussed above. On the basis of the adaptive decoupling hypothesis and existing quantitative genetic evidence (Blouin 1992;Hilbish et al 1993;Phillips 1998), I predict that correlations between stages will be lower than those within stages. Another purpose was to estimate the amount of genetic variation for traits that are known or commonly believed to be under selection in natural populations.…”