Biomedical subjects

D A Roff

Publications and source records attributed to D A Roff.

At least 19 recordsLinked to original sources

PubMed2006

The quantitative genetics of sexual dimorphism: assessing the importance of sex-linkage.

Sexual dimorphism (SD) is a defining feature of gonochorous animals and dioecious plants, but the evolution of SD from an initially monomorphic genome presents a conundrum. Theory predicts that the evolution of SD will be facilitated if genes with sex-specific fitness effects occur on sex chromosomes. We review this theory and show that it generates three testable predictions. For organisms with an XX/XY chromosomal system of sex determination: (1) SD should be associated with X-linked effects; (2) X-linked effects should show strong directional dominance for sexually dimorphic traits favored in males but expressed in both sexes; and (3) SD should be associated with a reduction in the between-sex additive genetic covariance and correlation. A literature review reveals that empirical evaluations of the association between sex-linkage and SD have lagged behind theory. Tests for the presence of sex-linked effects have been plagued by the need to make simplifying assumptions, such as the absence of dominance or maternal effects, that greatly weaken their discriminatory power. Further, most have used comparisons between species or populations, whereas the correct level of analysis is within populations. To overcome these problems, we derive a novel pedigree design that permits separate estimation of X-linked, dominance and maternal effects. We suggest that the data from such a design would be most appropriately analyzed using the animal model. This novel protocol will allow quantitative evaluation of the above predictions, and hence should spur progress in understanding the role of sex-linkage in the evolution of SD.

Animals↗

PubMed2006

Analysis of the importance of genotypic variation, metabolic rate, morphology, sex and development time on immune function in the cricket, Gryllus firmus.

Immune defence is hypothesized to be a trait that bears significant fitness costs as well as benefits in that mounting a defence depreciates the value of other life-history traits. Thus the cost of mounting an immune response could affect the evolution of both the immune system and correlated life history traits. In this study we examined, by means of a diallel cross of four inbred lines, the genetic basis of two measures of immune function, metabolic rate and several traits in the sand cricket, Gryllus firmus. We specifically addressed the following questions: (1) is immune function determined primarily by genetic constitution or correlations with phenotypic traits that could reduce the effectiveness of the immune response; (2) do the two measures of immune function covary; (3) What are the contributions of additive, nonadditive and maternal effects to the immune function? As estimates of immune function, we used lytic activity and encapsulation rate. We found that inbred crickets were smaller than individuals from the crossed lines and took longer to develop. However, inbred lines did not differ from the crossed lines in immune function nor metabolic rates, suggesting that increased homozygosity has little or no effect on these traits in G. firmus. We found that both immune parameters showed significant genetic variation but no consistent relationships with the other phenotypic traits (metabolic rate, head width, body mass, development time and activity). There was significant additive genetic variation only in encapsulation rate, but, with the exception of the activity measure, significant nonadditive and reciprocal variances were found in all traits. Metabolic rate of crickets was heritable, but there was neither phenotypic nor genetic association between metabolic rate and the two parameters of immune function. Further, there was no correlation between these two measures. Females showed a higher encapsulation response than males, but there was no sex differences in lytic activity. Our study indicates that genetic variation in immune parameters can be a very significant contributor to phenotypic variation in immune function.

Animals↗

PubMed2006

The importance of growth and mortality costs in the evolution of the optimal life history.

A central assumption of life history theory is that the evolution of the component traits is determined in part by trade-offs between these traits. Whereas the existence of such trade-offs has been well demonstrated, the relative importance of these remains unclear. In this paper we use optimality theory to test the hypothesis that the trade-off between present and future fecundity induced by the costs of continued growth is a sufficient explanation for the optimal age at first reproduction, alpha, and the optimal allocation to reproduction, G, in 38 populations of perch and Arctic char. This hypothesis is rejected for both traits and we conclude that this trade-off, by itself, is an insufficient explanation for the observed values of alpha and G. Similarly, a fitness function that assumes a mortality cost to reproduction but no growth cost cannot account for the observed values of alpha. In contrast, under the assumption that fitness is maximized, the observed life histories can be accounted for by the joint action of trade-offs between growth and reproductive allocation and between mortality and reproductive allocation (Individual Juvenile Mortality model). Although the ability of the growth/mortality model to fit the data does not prove that this is the mechanism driving the evolution of the optimal age at first reproduction and allocation to reproduction, the fit does demonstrate that the hypothesis is consistent with the data and hence cannot at this time be rejected. We also examine two simpler versions of this model, one in which adult mortality is a constant proportion of juvenile mortality [Proportional Juvenile Mortality (PJM) model] and one in which the proportionality is constant within but not necessarily between species [Specific Juvenile Mortality (SSJM) model]. We find that the PJM model is unacceptable but that the SSJM model produces fits suggesting that, within the two species studied, juvenile mortality is proportional to adult mortality but the value differs between the two species.

Animals↗

PubMed2006

A quantitative analysis of the mechanism that controls body size in Manduca sexta.

BACKGROUND: Body size is controlled by mechanisms that terminate growth when the individual reaches a species-specific size. In insects, it is a pulse of ecdysone at the end of larval life that causes the larva to stop feeding and growing and initiate metamorphosis. Body size is a quantitative trait, so it is important that the problem of control of body size be analyzed quantitatively. The processes that control the timing of ecdysone secretion in larvae of the moth Manduca sexta are sufficiently well understood that they can be described in a rigorous manner. RESULTS: We develop a quantitative description of the empirical data on body size determination that accurately predicts body size for diverse genetic strains. We show that body size is fully determined by three fundamental parameters: the growth rate, the critical weight (which signals the initiation of juvenile hormone breakdown), and the interval between the critical weight and the secretion of ecdysone. All three parameters are easily measured and differ between genetic strains and environmental conditions. The mathematical description we develop can be used to explain how variables such as growth rate, nutrition, and temperature affect body size. CONCLUSION: Our analysis shows that there is no single locus of control of body size, but that body size is a system property that depends on interactions among the underlying determinants of the three fundamental parameters. A deeper mechanistic understanding of body size will be obtained by research aimed at uncovering the molecular mechanisms that give these three parameters their particular quantitative values.

Animals↗

PubMed2005

The evolution of the phenotypic covariance matrix: evidence for selection and drift in Melanoplus.

Phenotypic variation in trait means is a common observation for geographically separated populations. Such variation is typically retained under common garden conditions, indicating that there has been evolutionary change in the populations, as a result of selection and/or drift. Much less frequently studied is variation in the phenotypic covariance matrix (hereafter, P matrix), although this is an important component of evolutionary change. In this paper, we examine variation in the phenotypic means and P matrices in two species of grasshopper, Melanoplus sanguinipes and M. devastator. Using the P matrices estimated for 14 populations of M. sanguinipes and three populations of M. devastator we find that (1) significant differences between the sexes can be attributed to scaling effects; (2) there is no significant difference between the two species; (3) there are highly significant differences among populations that cannot be accounted for by scaling effects; (4) these differences are a consequence of statistically significant patterns of covariation with geographic and environmental factors, phenotypic variances and covariances increasing with increased temperature but decreasing with increased latitude and altitude. This covariation suggests that selection has been important in the evolution of the P matrix in these populations Finally, we find a significant positive correlation between the average difference between matrices and the genetic distance between the populations, indicating that drift has caused some of the variation in the P matrices.

Animals↗

PubMed2005

The evolution of fledging age in songbirds.

In birds with altricial young an important stage in the life history is the age at fledging. In this paper we use an approach proven successful in the prediction of the optimal age at maturity in fish and reptiles to predict the optimal age of fledging in passerines. Integrating the effects of growth on future fecundity and survival leads to the prediction that the optimal age at fledging is given by a function that comprises survival to maturity, the exponent of the fecundity-body size relationship and nestling growth. Growth is described by the logistic equation with parameters, A, K and t(i). Assuming that the transitional mortality curve can be approximated by the nestling mortality, M(n), the optimal fledging age, t(f), is given by a simple formula involving the three growth parameters, nestling mortality (M(n)) and the exponent (d) of the fecundity-body size relationship. Predictions of this equation underestimate the true values by 11-16%, which is expected as a consequence of the transitional mortality function approximation. A transitional mortality function in which mortality is approximately 0.3-0.4 of nesting mortality (i.e. mortality declines rapidly after fledging) produces predictions which, on average, equal the observed values. Data are presented showing that mortality does indeed decline rapidly upon fledging.

Age Factors↗

PubMed2004

Evolutionary genetics of lifespan and mortality rates in two populations of the seed beetle, Callosobruchus maculatus.

The age at which individuals die varies substantially within and between species, but we still have little understanding of why there is such variation in life expectancy. We examined sex-specific and genetic variation in adult lifespan and the shape of mortality curves both within and between two populations of the seed beetle, Callosobruchus maculatus, that differ in a suite of life history characters associated with adaptation to different host species. Mean adult lifespan and the shape of the logistic mortality curves differed substantially between males and females (males had lower initial mortality rates, but a faster increase in the rate of mortality with increasing age) and between populations (they differed in the rate of increase in mortality with age). Larger individuals lived longer than smaller individuals, both because they had lower initial mortality rates and a slower increase in the rate of mortality with increasing age. However, differences in body size were not adequate to explain the differences in mortality between the sexes or populations. Both lifespan and mortality rates were genetically variable within populations and genetic variance/covariance matrices for lifespan differed between the populations and sexes. This study thus demonstrated substantial genetic variation in lifespan and mortality rates within and between populations of C. maculatus.

Animals↗

PubMed2004

Geographic variation in the G matrices of wild populations of the barn swallow.

In this paper, we present an analysis of genetic variation in three wild populations of the barn swallow, Hirundo rustica. We estimated the P, E, and G matrices for six linear morphological measurements and tested for variation among populations using the Flury hierarchical method and the jackknife followed by MANOVA method. Because of nonpositive-definite matrices, we had to employ 'bending' to analyse the G and E matrices with the Flury method. Both statistical methods agree in finding that the P and G matrices are significantly different but comparison between the analysis of the P matrices and pairwise analyses of the P, E, and G matrices suggests caution in interpreting the Flury results concerning differences in matrix structure. The significant variation among the populations in the G matrices appears to be due in large measure to the most geographically distant population.

Animals↗

PubMed2004

Extra-nuclear effects on growth and development in the sand cricket Gryllus firmus.

Although largely ignored until recently, parental effects on the phenotypes of their offspring are both ubiquitous in nature and of a potentially great importance to evolution. Our study examines the presence of extra-nuclear (maternal and paternal) effects in growth traits, development time and adult size in the sand cricket Gryllus firmus using a diallel cross of inbred lines. Sex linkage was shown to be nonsignificant for development time but the other traits could not be tested. We assume that they are nonsignificant but use the term 'reciprocal' effects to include this effect. We show that reciprocal effects are present in the growth traits and development time, where they account for 10-30% of the phenotypic variance. They are not present in adult size as indexed by head width. We demonstrate that reciprocal effects are due, at least in part, to maternal effects by an analysis of the positive correlation between egg size, a maternal trait, and the growth traits. The growth rate traits show no significant decline with age either with respect to extra-nuclear contributions to variance or difference between phenotypic means of reciprocal pairs. This study demonstrates that extra-nuclear effects are important contributors to the phenotypic variation in life history traits of G. firmus.

Age Factors↗

PubMed2004

The effect of temperature and wing morphology on quantitative genetic variation in the cricket Gryllus firmus, with an appendix examining the statistical properties of the Jackknife-MANOVA method of matrix comparison.

We investigated the effect of temperature and wing morphology on the quantitative genetic variances and covariances of five size-related traits in the sand cricket, Gryllus firmus. Micropterous and macropterous crickets were reared in the laboratory at 24, 28 and 32 degrees C. Quantitative genetic parameters were estimated using a nested full-sib family design, and (co)variance matrices were compared using the T method, Flury hierarchy and Jackknife-manova method. The results revealed that the mean phenotypic value of each trait varied significantly among temperatures and wing morphs, but temperature reaction norms were not similar across all traits. Micropterous individuals were always smaller than macropterous individuals while expressing more phenotypic variation, a finding discussed in terms of canalization and life-history trade-offs. We observed little variation between the matrices of among-family (co)variation corresponding to each combination of temperature and wing morphology, with only one matrix of six differing in structure from the others. The implications of this result are discussed with respect to the prediction of evolutionary trajectories.

Analysis of Variance↗

PubMed2003

Phenotypic plasticity and the evolution of trade-offs: the quantitative genetics of resource allocation in the wing dimorphic cricket, Gryllus firmus.

In the wing dimorphic sand cricket, Gryllus firmus, there is a pronounced trade-off between flight capability and fecundity. This trade-off is found both between morphs and within the macropterous morph, in which fecundity is negatively correlated with the mass of the principle flight muscles, the dorso-longitudinal muscles (DLM). In this paper, we examine how this trade-off is affected by a reduction in food and its genetic basis. We find that the relative fitness of the two wing morphs is not changed although both fecundity and DLM mass are decreased. A quantitative genetic analysis shows that the trade-off function is genetically variable but that most of the variation occurs in the intercept rather than the slope of the function. Analysis further indicates a very high genetic correlation between environments (food ration) supporting the hypothesis of a strong functional constraint between reproduction and flight capability.

Analysis of Variance↗

PubMed2003

The evolution of trade-offs: geographic variation in call duration and flight ability in the sand cricket, Gryllus firmus.

Quantitative genetic theory assumes that trade-offs are best represented by bivariate normal distributions. This theory predicts that selection will shift the trade-off function itself and not just move the mean trait values along a fixed trade-off line, as is generally assumed in optimality models. As a consequence, quantitative genetic theory predicts that the trade-off function will vary among populations in which at least one of the component traits itself varies. This prediction is tested using the trade-off between call duration and flight capability, as indexed by the mass of the dorsolateral flight muscles, in the macropterous morph of the sand cricket. We use four different populations of crickets that vary in the proportion of macropterous males (Lab = 33%, Florida = 29%, Bermuda = 72%, South Carolina = 80%). We find, as predicted, that there is significant variation in the intercept of the trade-off function but not the slope, supporting the hypothesis that trade-off functions are better represented as bivariate normal distributions rather than single lines. We also test the prediction from a quantitative genetical model of the evolution of wing dimorphism that the mean call duration of macropterous males will increase with the percentage of macropterous males in the population. This prediction is also supported. Finally, we estimate the probability of a macropterous male attracting a female, P, as a function of the relative time spent calling (P = time spent calling by macropterous male/(total time spent calling by both micropterous and macropterous male). We find that in the Lab and Florida populations the probability of a female selecting the macropterous male is equal to P, indicating that preference is due simply to relative call duration. But in the Bermuda and South Carolina populations the probability of a female selecting a macropterous male is less than P, indicating a preference for the micropterous male even after differences in call duration are accounted for.

Animals↗

PubMed2002

The common quantitative genetic basis of wing morphology and diapause occurrence in the cricket Gryllus veletis.

A covariation between wing morphology and diapause occurrence has been observed in many insect species, but the genetic basis of this covariation has never been established. This study measures the heritability of, and genetic correlation between, these two ecologically important threshold traits in the cricket Gryllus veletis. A total of 81 full-sib families were reared in the laboratory to estimate these parameters. A comparison of laboratory and field samples showed that these two traits are highly plastic. The heritability of wing morphology was 0.25 (0.09), the heritability of diapause occurrence was 0.77 (0.11) and the genetic correlation between them was 0.61 (0.19). These estimates did not differ between males and females. The significance of these quantitative genetic parameters is discussed with reference to the monomorphism of natural populations of G. veletis for diapause occurrence and with reference to the trade-off between the ability to disperse by flight and the ability to diapause found in at least one closely related species. A survey of the literature reveals that genetic correlations between diapause occurrence or wing morphology and various other traits are common in insects, suggesting that these two traits are often genetically integrated in insect life-histories.

Animals↗

PubMed2001

The threshold model as a general purpose normalizing transformation.

The estimation of heritabilities and genetic correlations is based on the assumption that the trait distributions are normal. When the distributions are not normal it is advisable to transform the data to produce normality. However, it is possible that no suitable transformation can be found. The purpose of the present paper is to point out that the threshold model of quantitative genetics can be used as a generalized transformation. To utilize this method it is only necessary to divide the data at the median (approximately) and code the two halves as 0 and 1. Estimates can then be made using algorithms outlined herein. A simulation study shows that the threshold transformation gave unbiased estimates of the heritability and genetic correlation in all cases. The 95% confidence limits correctly included the true heritability value in the required 95% of cases, while the estimated confidence region for the genetic correlation was also correct provided that the geometric mean heritability was greater than approximately 0.15, a restriction that applied also to the normally distributed data. Confidence intervals estimated from the non-normal data were consistently too small. The method is illustrated using data on the proportion of diapausing eggs produced by the cricket, Allonemobius socius.

Animals↗

PubMed2001

Estimating genetic correlations in natural populations in the absence of pedigree information: accuracy and precision of the Lynch method.

Usually, genetic correlations are estimated from breeding designs in the laboratory or greenhouse. However, estimates of the genetic correlation for natural populations are lacking, mostly because pedigrees of wild individuals are rarely known. Recently Lynch (1999) proposed a formula to estimate the genetic correlation in the absence of data on pedigree. This method has been shown to be particularly accurate provided a large sample size and a minimum (20%) proportion of relatives. Lynch (1999) proposed the use of the bootstrap to estimate standard errors associated with genetic correlations, but did not test the reliability of such a method. We tested the bootstrap and showed the jackknife can provide valid estimates of the genetic correlation calculated with the Lynch formula. The occurrence of undefined estimates, combined with the high number of replicates involved in the bootstrap, means there is a high probability of obtaining a biased upward, incomplete bootstrap, even when there is a high fraction of related pairs in a sample. It is easier to obtain complete jackknife estimates for which all the pseudovalues have been defined. We therefore recommend the use of the jackknife to estimate the genetic correlation with the Lynch formula. Provided data can be collected for more than two individuals at each location, we propose a group sampling method that produces low standard errors associated with the jackknife, even when there is a low fraction of relatives in a sample.

Analysis of Variance↗

PubMed2001

The evolution of trade-offs: effects of inbreeding on fecundity relationships in the cricket Gryllus firmus.

The evolution of traits is modulated by their interrelationships with each other, particularly when those relationships result in a fitness trade-off. In this paper we explore the consequences of genetic architecture on functional relationships between traits. Specifically, we address the consequences of inbreeding on these relationships. We show that the linear regression between two traits will not be affected if there is no dominance genetic variance in either trait, whereas the intercept but not the slope of the regression will change if there is dominance genetic variance in one trait only. We test the latter hypothesis using fecundity relationships in the cricket Gryllus firmus. Data from pedigree analysis and an inbreeding experiment show that there is significant dominance genetic variance in fecundity, but not head width (an index of body size) or dorsal longitudinal muscle (DLM) mass. Fecundity increases with head width, but decreases with DLM mass. As predicted, the intercepts of the regressions of fecundity on these two morphological traits decrease with inbreeding, but there is little or no change in slope. Gryllus firmus is wing dimorphic, with the macropterous (LW) morph having a lower fecundity than the micropterous (SW) morph. We hypothesize that the difference in fecundity arises primarily because of a competition for resources in the LW females between DLM maintenance (i.e., mass) and egg production. As a consequence, we predict that the fecundity within each morph should decline linearly with the inbreeding coefficient at the same rate in both morphs. The result of this will be a change in the relative fitness of the two morphs, that of the SW morph increasing with inbreeding. This prediction is supported. These results indicate that trade-offs will evolve and such changes will affect evolutionary trajectories by altering the pattern of relationships among fitness components.

Animals↗

PubMed2000

A quantitative genetic analysis of phenotypic plasticity of diapause induction in the cricket Allonemobius socius.

Although numerous studies have indicated that diapause is heritable and phenotypically plastic, none of them has examined the quantitative genetic basis of this plasticity. In this paper we report such an analysis for egg diapause in the cricket Allonemobius socius, the induction of which appears to be largely determined by the mother. We analysed the quantitative genetic basis of the phenotypically plastic response of female A. socius to age and environmental conditions. We measured the production of diapause eggs on four occasions over a 16-day period, and in two environments; one mimicking an 'early' period of the year and another mimicking a 'late' period. We analysed genetic variation in phenotypic plasticity using the character-state approach. Diapause proportion was heritable (h2 ranged from 0.17 to 0.49, being larger in the 'early' environment), and the genetic correlation between ages in proportion of diapausing eggs was close to 1 but showed a decrease with increased difference between ages. There were significant genetic correlations between environments for all ages. Because of the reduction in genetic correlation as the difference in ages increases, selection will be more effective at changing the overall shape of the reaction norm than causing local changes. Furthermore, the high genetic correlations may constrain the evolution of the reaction norm. When the two environments are converted into the estimated days in the year the two reaction norms form approximately a single curve as predicted from previous theoretical analysis of the optimal reaction norm.

Analysis of Variance↗

PubMed1999

Inbreeding depression in the wild.

Despite its practical application in conservation biology and evolutionary theory, the cost of inbreeding in natural populations of plants and animals remains to a large degree unknown. In this review we have gathered estimates of inbreeding depression (delta) from the literature for wild species monitored in the field. We have also corrected estimates of delta by dividing by F (coefficient of inbreeding), to take into account the influence that the variation in F will have on delta. Our data set includes seven bird species, nine mammal species, four species of poikilotherms (snakes, fish and snails) and 15 plant species. In total we obtained 169 estimates of inbreeding depression for 137 traits; 81 of those estimates included estimates of F. We compared our mammalian data (limited to those traits related to juvenile mortality) to the estimates for captive zoo species published by Ralls et al. (1988) to determine if, as predicted from the literature, natural estimates of inbreeding depression are higher than captive estimates. The mean delta +/- SE (significantly different from zero and not corrected for F ) for homeotherms was 0.509 +/- 0.081; for poikilotherms, 0.201 +/- 0.039; and for plants, 0.331 +/- 0.038. Levels of inbreeding depression this high in magnitude will be biologically important under natural conditions. When we limited our data set to mortality traits for mammals and corrected for F=0.25 (as is the case for the Ralls et al. data set), we found a significant difference between the two data sets; wild estimates had a substantially higher mean cost of inbreeding at F = 0.25: 2.155 (captive species: 0.314). Of the 169 estimates of delta, 90 were significantly different from zero, indicating that inbred wild species measured under natural conditions frequently exhibit moderate to high levels of inbreeding depression in fitness traits.

Animals↗