GENETICS OF PHYSIOLOGICAL AND BEHAVIORAL RESISTANCE TO HOST FURANOCOUMARINS IN THE PARSNIP WEBWORM

Depressaria pastinacella, the parsnip webworm, feeds almost exclusively on the flowers and fruits of Pastinaca sativa, the wild parsnip. Resistance to webworms in wild parsnip populations is largely attributable to genetically based variation in furanocoumarin chemistry; by differentially reducing fruit set among chemical phenotypes, parsnip webworms may act as selective agents on wild parsnip populations. To determine whether wild parsnip chemistry can act as a selective agent on webworm populations, it is necessary to establish that resistance mechanisms in the webworm to furanocoumarins are genetically based. In this study, we estimated the amount of genetic variation in behavioral and physiological responses of webworms to parsnip furanocoumarins. Virtually no variation was found among webworm families for feeding preferences for diets varying as much as fourfold in furanocoumarin content. Nor was significant variation found for mean furanocoumarin intake over the assay period, except in one case, in which maternal effects may account for differences among families. In contrast, substantial familial variation existed for cytochrome P450–mediated metabolism of bergapten and xanthotoxin, two host furanocoumarins. The presence of additive genetic variation in metabolism, and the absence of such variation in discriminative feeding behavior, suggests that adaptation to changes in furanocoumarin chemistry, resulting either from changes in the distribution of chemical phenotypes in parsnip populations or from shifts to new chemically different host plants, is likely to be facilitated by physiological rather than behavioral means.     

DEVELOPMENT,FUNCTION, AND THE QUANTITATIVE GENETICS OF WING MELANIN PATTERN IN PIERIS BUTTERFLIES

Do genetic correlations among phenotypic characters reflect developmental organization or functional coadaptation of the characters? We test these hypotheses for the wing melanin pattern of Pieris occidentalis butterflies, by comparing estimated genetic correlations among wing melanin characters with a priori predictions of the developmental organization and the functional (thermoregulatory) organization of melanin pattern. There were significant broad-sense heritabilities and significant genetic correlations for most melanin characters. Matrix correlation tests revealed significant agreement between the observed genetic correlations and both developmental and functional predictions in most cases; this occurred even when the overlap between developmental and functional predictions was eliminated. These results suggest that both developmental organization and functional coadaptation among melanin characters influence the genetic correlation structure of melanin pattern in this species. These results have two important implications for the evolution of melanin pattern in P. occidentalis and other butterflies: 1) most phenotypic variation in pattern may reflect variation among, rather than within, sets of developmentally homologous wing melanin characters; and 2) in a changing selective environment, genetic correlations may retard the disruption of functionally coupled melanin characters, thus affecting the evolutionary response to selection.     

QUANTITATIVE GENETICS OF SURVIVAL AND GROWTH IN IMPATIENS CAPENSIS

When variation in life-history characters is caused by many genes of small effect, then quantitative-genetic parameters may quantify constraints on rate and direction of microevolutionary change. I estimated heritabilities and genetic correlations for 16 life-history and morphological characters in two populations of Impatiens capensis, a partially self-pollinating herbaceous annual. The Madison population had little or no additive genetic variance for any of these characters, while the Milwaukee population had significant narrowsense heritabilities and genetic correlations for several traits, including adult size, which is highly correlated with fitness. All genetic correlations among fitness components were positive, hence there is no evidence for antagonistic pleiotropy among these traits. Dissimilarity of heritabilities in the two populations supports theoretical predictions that long-term changes in genetic variance-covariance patterns may occur when population sizes are small and selection is strong, as may occur in many plant species.     

SEASONALLY VARYING DIET QUALITY AND THE QUANTITATIVE GENETICS OF DEVELOPMENT TIME AND BODY SIZE IN BIRCH FEEDING INSECTS

Abstract Genetic variance‐covariance structures (G), describing genetic constraints on microevolutionary changes of populations, have a central role in the current theories of life‐history evolution. However, the evolution of Gs in natural environments has been poorly documented. Resource quality and quantity for many animals and plants vary seasonally, which may shape genetic architectures of their life histories. In the mountain birch‐insect herbivore community, leaf quality of birch for insect herbivores declines profoundly during both leaf growth and senescence, but remains stable during midsummer. Using six sawfly species specialized on the mountain birch foliage, we tested the ways in which the seasonal variation in foliage quality of birch is related to the genetic architectures of larval development time and body size. In the species consuming mature birch leaves of stable quality, that is, without diet‐imposed time constraints for development time, long development led to high body mass. This was revealed by the strongly positive phenotypic and genetic correlations between the traits. In the species consuming growing or senescing leaves, on the other hand, the rapidly deteriorating leaf quality prevented the larvae from gaining high body mass after long development. In these species, the phenotypic and genetic correlations between development time and final mass were negative or zero. In the early‐summer species with strong selection for rapid development, genetic variation in development time was low. These results show that the intuitively obvious positive genetic relationship between development time and final body mass is a probable outcome only when the constraints for long development are relaxed. Our study provides the first example of a modification in guild‐wide patterns in the genetic architectures brought about by seasonal variation in resource quality.     

QUANTITATIVE GENETICS OF SIZE AND PHENOLOGY OF LIFE-HISTORY TRAITS IN CHAMAECRISTA FASCICULATA

Despite numerous adaptive scenarios concerning the evolution of plant life-history phenologies few studies have examined the heritable basis for and genetic correlations among these phenologies. Documentation of genetic variation for and covariation among reproductive phenologies is important because it is this variation/covariation that will determine the potential for response to evolutionary forces. To address this problem, I conducted a breeding experiment to determine narrow-sense heritabilities for and genetic correlations among the phenologies of life-history events and plant size in Chamaecristafasciculata, a temperate summer annual plant species. Paternal families showed no evidence of heritable variation for two estimates of plant size, six measures of reproductive phenology or two fitness components. Similarly, paternal estimates of genetic correlations among these traits were low or zero. In contrast, maternal estimates of heritability suggested the influence of maternal parent on one estimate of plant size and four phenological traits. Likewise, maternal effects influenced maternal estimates of genetic correlations. These maternal effects can arise from three sources: endosperm nuclear, cytoplasmic genetic and/or maternal phenotypic. The degree to which the phenology of one life-history trait acts as a constraint on the evolution of other phenological traits depends on the source of the maternal influence in this species.     

MOLECULAR GENETICS OF ADAPTATION IN AN EXPERIMENTAL MODEL OF COOPERATION

The evolution of cooperation was studied in an empirical system utilizing a parasitic bacteriophage (f1) and a bacterial host. Infected cells were propagated by serial passage so that a phage could increase its representation among infected hosts only by enhancing the rate of growth of its host. Loss of infectivity was therefore without selective penalty, and phage benevolence could potentially evolve through a variety of genetic changes. The infected hosts evolved to grow faster over the course of the study, but the genetic bases of this phenotypic change were more difficult to anticipate. Two fundamentally different types of genetic changes in the phage were revealed. One involved the loss of some phage genes, resulting in a noninfectious plasmid that continued to replicate via the parental phage replicon. The second change involved integration of the phage genome into host DNA by a process that, at low frequency, could be reversed to produce infectious phage particles. Integration is a previously unknown property of wild-type f1, and in the system studied, may have resulted from the use of a phage bearing an insert containing nonfunctional DNA. The evolution of this novel function apparently depended only on the presence of a small region in the phage genome that provided some homology to the host DNA, with the host providing all necessary functions. Although f1 is one of the simplest phages known, these observations suggest that host-parasite interactions of the filamentous phages are more complicated than previously thought. More generally, the f1 system offers a useful model for many problems concerning the genetic basis of adaptation.     

QUANTITATIVE GENETICS AND THE EVOLUTION OF REACTION NORMS

We extend methods of quantitative genetics to studies of the evolution of reaction norms defined over continuous environments. Our models consider both spatial variation (hard and soft selection) and temporal variation (within a generation and between generations). These different forms of environmental variation can produce different evolutionary trajectories even when they favor the same optimal reaction norm. When genetic constraints limit the types of evolutionary changes available to a reaction norm, different forms of environmental variation can also produce different evolutionary equilibria. The methods and models presented here provide a framework in which empiricists may determine whether a reaction norm is optimal and, if it is not, to evaluate hypotheses for why it is not.     

DEVELOPMENTAL QUANTITATIVE GENETICS AND THE EVOLUTION OF ONTOGENIES

A model is presented which permits integration of developmental information into genetic discussions about evolutionary change in morphology. Development of a trait is described in terms of an ontogenetic trajectory whose properties are defined by a small number of parameters. Some evolutionary aspects of development are examined from the perspective of this quantitative genetic model. Particular attention is given to the developmental origin of pleiotropic effects, developmental constraints, heterochrony, and the growth and morphogenesis of complex morphologies. The role of genetic maternal effects in mammalian development is briefly examined, particularly as it relates to selection on developmental traits.     

QUANTITATIVE GENETICS OF HETEROCHRONY

A model of quantitative-genetic variation in developmental processes is introduced and analyzed. The model is of a bifurcating sequence of events in which traits develop from the same tissue until a transition occurs, after which they develop partially independently. Genetic and environmental variation in both the rates of tissue growth and in the timing of transitions is considered. The model shows how genetic variation in developmental parameters governs variation and covariation in phenotypic traits and how selection on the phenotype alters the distributions of developmental parameters. Particular attention is paid to the conditions under which selection will lead to changes in the average times of developmental events.     

OPTIMIZATION MODELS,QUANTITATIVE GENETICS,AND MUTATION

The process of selection on a multivariate set of characters subject to functional constraints is considered from the points of view of both evolutionary optimization theory and quantitative genetics. Special attention is given to life-history characteristics. It is shown that, under suitable conditions (including weak selection), useful approximate formulas for the relations between the functional constraints and the additive genetic variance-covariance matrix can be derived. These can be used to show that the conditions for equilibrium under selection according to the two different approaches are approximately equivalent. Although large negative genetic correlations are to be expected between some pairs of life-history traits in populations at equilibrium under selection, in general some small negative genetic correlations and some positive genetic correlations will also be present. Thus, the observation of a positive genetic correlation between a pair of life-history traits does not necessarily refute the possibility of trade-offs among a multivariate set of traits that contains the pair in question. The relation between the pattern of functional constraints and the genetic correlations is often complex, and little insight into the former can be derived from the latter. The effects of mutations that lower the overall efficiency of resource utilization, thereby creating a positive component to the genetic covariances among life-history traits, are also considered for a specific model. Although such mutations can have a substantial effect on the form of the life history, extreme conditions seem to be needed for them to produce a large effect on the pattern of genetic correlations in a random-mating population. They can, however, cause the appearance of positive correlations following inbreeding, due to the exposure of deleterious recessive or partially recessive mutations. The analysis also suggests that the population means of individual components of a constrained multivariate system may often equilibrate at values that are far from the optima that would be attained if they were selected in isolation from the other members of the system.     

ADAPTATION, CONSTRAINT, AND COMPROMISE IN AVIAN POSTNATAL DEVELOPMENT

1. This paper discusses factors that influence the evolution of growth rate and determine its variation among species of birds. Growth rate is related to evolutionary fitness through the use of time, energy, and nutrients. In addition, balances between factors favouring rapid growth and those favouring slow growth may be investigated directly by experiment and by comparative observation. 2. David Lack (1968) proposed that the growth rate of the young is the optimum balance between selection for rapid growth to reduce the vulnerable period of development and selection for slow growth to reduce the energy requirements of the young. 3. To test Lack's hypothesis, the growth rates of birds, estimated by fitting sigmoid equations to curves relating weight to age, were surveyed widely from the literature. Among all species examined, growth rate was inversely related to adult weight. Among birds of similar size, most variation in growth rate was related to the degree of maturity of the neonate. Altricial chicks, which depend upon their parents for food and warmth, grow more rapidly than precocial chicks, which are self-sufficient shortly after hatching. Lack's hypothesis, which predicts a direct relationship between growth rate and mortality rate, was not supported. 4. I propose that the key to understanding variation in growth rate among birds lies in the balance between rate of cell proliferation or cell growth, on one hand, and acquisition of mature function, on the other. This idea is consistent with principles of cellular and developmental biology. It is supported by comparisons of (a) the neonates of different species, (b) the individual over the course of the developmental period, and (c) tissues whose use is acquired at different stages of development, wherein more mature individuals or tissues grow more slowly than those with less developed function. 5. Species of birds that are classified as semi-precocial develop precocially but grow rapidly. Although these seemingly violate the general rule relating growth rate to precocity, a closer inspection of their development reveals that they too support the rule. In the Common Tern, the legs, which are the key organ in precocial development, grow at the expected slow rate. The body as a whole grows rapidly because the growth increment of the legs is small and their growth is completed quickly. 6. Growth rates of precocial birds do not decrease abruptly at hatching. This points more to gradual tissue differentiation than to the pattern of procurement and allocation of energy as the primary control for growth rate. 7. Precocious development is favoured when the chicks are capable of self-feeding or when food supplies are distant from the next site and travelling time between one and the other is long. Precocity of the neonates frees both parents to feed at a distant food source. 8. Some species having diets with low levels of protein or other nutrients may grow slowly in order to match nutrient requirements to their availability in the diet. This pattern is indicated especially among the Procellariiformes, which feed an oily diet to their young, and also among tropical fruit-eating birds. 9. Some tropical, pelagically-feeding sea-birds that rear only one offspring at a time may not be able to procure food sufficient to support rapid chick growth. Alternative explanations for slow growth among these species include difficulty in obtaining essential nutrients and more precocious development of activity than in related species having more rapid growth.     

QUANTITATIVE GENETICS AND POPULATION DYNAMICS

This study examines the dynamics of a competition and a host-parasite model in which the interactions are determined by quantitative characters. Both models are extensions of one-dimensional difference equations that can exhibit complicated dynamics. Compared to these basic models, the phenotypic variability given by the quantitative characters reduces the size of the density fluctuations in asexual populations. With sexual reproduction, which is described by modeling the genetics of the quantitative character explicitly with many haploid loci that determine the character additively, this reduction in fitness variance is magnified. Moreover, quantitative genetics can induce simple dynamics. For example, the sexual population can have a two-cycle when the asexual system is chaotic. This paper discusses the consequences for the evolution of sex. The higher mean growth rate implied by the lower fitness variance in sexual populations is an advantage that can overcome a twofold intrinsic growth rate of asexuals. The advantage is bigger when the asexual population contains only a subset of the phenotypes present in the sexual population, which conforms with the tangled bank theory for the evolution of sex and shows that tangled bank effects also occur in host-parasite systems. The results suggest that explicitly describing the genetics of a quantitative character leads to more flexible models than the usual assumption of normal character distributions, and therefore to a better understanding of the character's impact on population dynamics.     

QUANTITATIVE GENETICS OF FEMALE MATE PREFERENCES IN AN ANCESTRAL AND A NOVEL ENVIRONMENT

A female's mate preference is a potentially complex function relating variation in multiple male phenotypes with her probability of accepting individual males as a mate. Estimating the quantitative genetic basis preference functions within a population is empirically challenging yet key to understanding preference evolution. We employed a recently described approach that uses random‐coefficient mixed models in the analysis of function‐valued traits. Using a half‐sibling breeding design in a laboratory‐adapted Drosophila serrata population, we estimated the genetic (co)variance function of female preference for male sexual displays composed of nine contact pheromones. The breeding design was performed across two environments: the food to which the population was well adapted and a novel food that reduced average female productivity by 35%. Significant genetic variance in female preference was detected and the majority (64.2%) was attributable to a single genetic dimension (eigenfunction), suggesting that preferences for different pheromones are not genetically independent. The second eigenfunction, accounting for 24% of the total genetic variance, approached significance in a conservative test, suggesting the existence of a second, independent genetic dimension. There was no evidence that the genetic basis of female preference differed between the two environments, suggesting the absence of genotype‐by‐environment interactions and hence a lack of condition‐dependent preference expression.     

GENETICS AND DEVELOPMENT OF AN UNSTABLE CHLOROPHYLL DEFICIENCY IN LYCOPERSICON ESCULENTUM

Rick , Charles M. (U. California, Davis.), Anson E. Thompson , and Oscar Brauer . Genetics and development of an unstable chlorophyll deficiency in Lycopersicon esculentum. Amer. Jour. Bot. 46(1) : 1-11. Illus. 1959.—A single gene gh determines a condition of variable chlorophyll deficiency in the tomato. Two independent mutations have been recorded at this locus, and both mutants exhibit the same phenotype. Linkage tests ascertained that the locus of gh lies between hl and j in group V. Graft and inoculation experiments prove that the gh phenotype is not caused by a virus. The mutant seedlings have cotyledons that can be identified by their partial chlorophyll deficiency. First true leaves may rarely have normal morphology and pigmentation; in most seedlings they show a mosaic of normal and chlorotic tissue. With subsequent growth, the plant rapidly reaches a condition of complete chlorophyll deficiency. Two constant levels of deficiency have been observed: a common white phase with very little or no chlorophyll and an infrequent yellow phase with about 5% of the normal chlorophyll content present in irregularly distributed chloroplasts. In both phases leaf lamina structure is drastically modified; petioles, stems, flowers, and fruits are much less affected. Sporadic islands of normal green coloration appear in the otherwise stable white or yellow phase. The degree of chlorophyll development is subject to great environmental modification but no evidence was obtained for genetic control in addition to the determination by gh. Depending upon environment, most gh seedlings perish before fruiting, but those with sufficient chlorophyll flower and set fruits with seeds after self-or cross-pollination. Most gh fruits remain white until mature, when the normal yellow epidermal pigment appears, the interior remaining white; occasional streaks or patches of chlorophyll that develop on immature fruits turn reddish at maturity. Seeds from such sectors, like all other seeds harvested from selfed gh plants, give rise only to gh progeny, thereby rendering improbable a mutational basis for the sporadic reversions to green. Applications of gh to further genetic and physiological studies are discussed.     

QUANTITATIVE GENETICS OF GEOMETRIC SHAPE IN THE MOUSE MANDIBLE

Abstract We combine the methods of geometric morphometrics and multivariate quantitative genetics to study the patterns of phenotypic and genetic variation of mandible shape in random‐bred mice. The data are the positions of 11 landmarks on the mandibles of 1241 mice from a parent‐offspring breeding design. We use Procrustes superimposition to extract shape variation and restricted maximum likelihood to estimate the additive genetic and environmental components of variance and covariance. Matrix permutation tests showed that the genetic and phenotypic as well as the genetic and environmental covariance matrices were similar, but not identical. Likewise, principal component analyses revealed correspondence in the patterns of phenotypic and genetic variation. Patterns revealed in these analyses also showed similarities to features previously found in the effects of quantitative trait loci and in the phenotypes generated in gene knockout experiments. We used the multivariate version of the breeder's equation to explore the potential for short‐term response to selection on shape. In general, the correlated response is substantial and regularly exceeds the direct response: Selection applied locally to one landmark usually produces a response in other parts of the mandible as well. Moreover, even selection for shifts of the same landmark in different directions can yield dramatically different responses. These results demonstrate the role of the geometry and anatomical structure of the mandible, which are key determinants of the patterns of the genetic and phenotypic covariance matrices, in molding the potential for adaptive evolution.