Levels Of Additive Genetic Variance Research Articles

Genetic correlation between biomass allocation to male and female functions in a natural population of Ipomopsis aggregata

Models for sex allocation assume that increased expenditure of resources on male function decreases the resources available for female function. Under some circumstances, a negative genetic correlation between investment in stamens and investment in pistil and seeds is expected. This study used a nested half-sibship design to test for genetic correlations between biomass of stamens, corolla, calyx, pistil and seeds in the hermaphroditic herb Ipomopsis aggregata. Over 5000 seeds from 32 paternal half-sib families and 229 maternal full-sib families were planted under natural field conditions and followed for up to 9 years until blooming. Stamen mass and corolla mass exhibited moderately high levels of additive genetic variance, and heritabilities of 0.32 and 0.40, respectively. Genetic correlations, as estimated with family mean correlations, were all either positive or not significantly different from zero. Even after controlling for plant size and flower number as indices of resource acquisition, paternal half-sib families that invested more biomass in stamens also invested more in the pistil and in the corolla.

Genetic correlation between biomass allocation to male and female functions in a natural population of Ipomopsis aggregata

Models for sex allocation assume that increased expenditure of resources on male function decreases the resources available for female function. Under some circumstances, a negative genetic correlation between investment in stamens and investment in pistil and seeds is expected. This study used a nested half-sibship design to test for genetic correlations between biomass of stamens, corolla, calyx, pistil and seeds in the hermaphroditic herb Ipomopsis aggregata. Over 5000 seeds from 32 paternal half-sib families and 229 maternal full-sib families were planted under natural field conditions and followed for up to 9 years until blooming. Stamen mass and corolla mass exhibited moderately high levels of additive genetic variance, and heritabilities of 0.32 and 0.40, respectively. Genetic correlations, as estimated with family mean correlations, were all either positive or not significantly different from zero. Even after controlling for plant size and flower number as indices of resource acquisition, paternal half-sib families that invested more biomass in stamens also invested more in the pistil and in the corolla.

GENETIC AND ENVIRONMENTAL VARIATION IN LIFE-HISTORY TRAITS OF A MONOCARPIC PERENNIAL: A DECADE-LONG FIELD EXPERIMENT.

Directional and stabilizing selection tend to deplete additive genetic variance. On the other hand, genetic variance in traits related to fitness could be retained through polygenic mutation, spatially varying selection, genotype-environment interaction, or antagonistic pleiotropy. Most estimates of genetic variance in fitness-related traits have come from laboratory studies, with few estimates of heritability made under natural conditions, particularly for longer lived organisms. Here I estimated additive genetic variance in life-history characters of a monocarpic herb, Ipomopsis aggregata, that lives for up to a decade. Experimental crosses yielded 229 full-sibships nested within 32 paternal half-sibships. More than 5000 offspring were planted as seeds into natural field sites and were followed in most cases through their entire life cycle. Survival showed substantial additive genetic variance (genetic coefficient of variation ≈ 54%). Small differences at seedling emergence were magnified over time, such that the genetic variability in survival was only detectable by tracking the success of offspring for several years starting from seed. In contrast to survival, reproductive traits such as flower number, seeds per flower, and age at flowering showed little or no genetic variability. Despite relatively high levels of additive genetic variation for some life-history characters, high environmental variance in survival resulted in very low heritabilities (0-9%) for all of these characters. Maternal effects were evident in seed mass and remained strong throughout the lengthy vegetative period. No negative genetic correlations between major components of female fitness were detected. Mean corolla width for a paternal family was, however, negatively correlated with the finite rate of increase based on female fitness. That negative correlation could help to maintain additive genetic variance in the face of strong selection through male function for wide corollas.

The biology of post-invasion events

In this contribution we consider the biology of invading organisms after they have become established. Adaptive radiation over the long term has been a favorite subject in evolutionary biology. Examples have been Darwin's finches in the Galapagos and the honeycreepers and Drosophila of Hawaii. Hawaiian honeycreepers have evolved from a finch-like ancestor into guilds of seed eaters, nectar feeders, and combined, nectar and insect feeders plus some species with unique beak structures. In the Hawaiian Drosophila sexual selection may have driven the extensive adaptive radiation and speciation in the group. The North American soapberry bug Jadera haemotoloma is an interesting model for post-invasion evolution in the short term. Some populations have moved onto introduced goldenrain trees Koelreuteria spp. and have evolved different stylet (mouthpart) lengths, as a function of fruit size, and new host preferences, all within the last 50 years. These rapid responses are possible because of high additive genetic variances for these traits. Similarly, there has been rapid evolution of life history variation in American shad introduced from east coast to west coast rivers. We postulate that invaders most likely to integrate successfully are those in which high levels of additive genetic variation are expressed in traits most likely to be adaptive in the new environment.

Sexual selection in seaweed flies: genetic variation in male size and its reliability as an indicator in natural populations

Characters that are evolving in response to indirect sexual selection operating through the production either of attractive sons (the Fisher process) or of progeny with high viability (good genes), are expected to exhibit high levels of additive genetic variation. In seaweed flies ( Coelopa frigida ) male size is subject to both direct and indirect sexual selection, whereas female size has probably evolved in response to direct selection alone. Coefficients of additive genetic variance ( CV A 9s) in adult size were shown to be larger in males than females. Virtually all the genetic variation is attributable to the αβ chromosomal inversion which is known to be associated with larval survival. Sexual selection appears to have led to differential expression of relevant loci in the two sexes, and in the focussing of this variation into a single region of the genome. Samples from 25 natural populations provided estimates of CV A 9s in male size. Although the CV A values suggest male size is, on average, a reliable indicator of offspring fitness, both spatial and temporal differences in CV A ’s were observed. We suggest that indirect sexual selection is a very inconstant force of evolution.

EPISTASIS AS A SOURCE OF INCREASED ADDITIVE GENETIC VARIANCE AT POPULATION BOTTLENECKS.

The role of epistasis in evolution and speciation has remained controversial. We use a new parameterization of physiological epistasis to examine the effects of epistasis on levels of additive genetic variance during a population bottleneck. We found that all forms of epistasis increase average additive genetic variance in finite populations derived from initial populations with intermediate allele frequencies. Average additive variance continues to increase over many generations, especially at larger population sizes (N = 32 to 64). Additive-by-additive epistasis is the most potent source of additive genetic variance in this situation, whereas dominance-by-dominance epistasis contributes smaller amounts of additive genetic variance. With additive-by-dominance epistasis, additive genetic variance decreases at a relatively high rate immediately after a population bottleneck, rebounding to higher levels after several generations. Empirical examples of epistasis for murine adult body weight based on measured genotypes are provided illustrating the varying effects of epistasis on additive genetic variance during population bottlenecks.

EXPERIMENTAL EVIDENCE FOR THE GENETIC-TRANSILIENCE MODEL OF SPECIATION.

Rice and Hostert (1993) recently presented a literature review of laboratory experiments on and concluded that there was little or no support for the model of speciation. This conclusion is flawed because the most relevant papers in the experimental literature on this topic are not cited by Rice and Hostert (1993), internally inconsistent criteria for evaluation are used in the contrast of bottleneck models versus the other models, and the predictions of the genetic-transilience model of are inaccurately portrayed. Before discussing the above issues, it is first necessary to clarify the modes of being discussed. Although Rice and Hostert (1993) acknowledge that three different models of founder-induced have been presented (Carson and Templeton 1984), they still proceed to equate the model of speciation to genetic revolution (p. 1638). A discussion of the distinction of the geneticrevolution model from the alternatives of genetic transilience and founder flush can be found in Carson and Templeton (1984), so only a brief distinction will be presented here. Genetic revolution occurs after an extreme founder event and subsequent small population size has eroded virtually all genetic variability from the founding population's gene pool. This is likely to occur only in a peripheral population that remains isolated from the ancestral population for many generations with persistent small variance effective size; hence, this mechanism is also called peripatric (Mayr 1982). None of the experimental protocols reviewed by Rice and Hostert (1993) tried to approximate this genetic situation, so the genetic-revolution model is irrelevant to their review. In contrast, both the genetic-transilience and founder-flush models stipulate that the founder event is followed by a large increase in population size; for example, colonization of a new territory with an open ecological niche for the founding population. Both of these models are also predicated upon the assumption that much nuclear genetic variability is retained in the founding population; and indeed both models actually predict an increase of certain classes of genetic variability in the founding population. However, the genetic-transilience and founder-flush models do differ in the type of genetic variability that is used in the process and in its selective impact. The genetic-transilience model focuses on polymorphic systems in the ancestral population that have a genetic architecture characterized by at most a few major segregating units (thereby including both genic and chromosomal variants as discussed in Templeton 1981, although for ease of presentation, the words and will be used hereafter) that have large phenotypic effects on syndromes with many pleiotropic effects on fitness-related traits. These pleiotropic effects in turn can be altered by modifying loci. Hence, there is at most a few major with many epistatically modifying loci. An example of this type of polymorphic system is provided by abnormal abdomen, as found in natural populations of Drosophila mercatorum. The major in this case are the ribosomal DNA (rDNA) multigene family complex on the X chromosome and a second X-linked locus that controls the pattern of somatic underreplication of rDNA in polytene tissues. These two loci are closely linked but separable through recombination, but due to linkage disequilibrium they form a supergene complex in natural populations (Hollocher et al. 1992). The abnormal-abdomen syndrome affects egg-to-adult developmental time, female agespecific fecundity, adult longevity, male reproductive maturity rates, male mating success, system of mating, and several morphological features. These pleiotropic effects are under intense natural selection (Templeton et al. 1989, 1990) and are subject to genetic modification in a trait specific fashion via epistatic loci scattered throughout the D. mercatorum genome (Templeton et al. 1985, 1993; Hollocher and Templeton 1994). Under genetic transilience, there is no radical overall reduction in levels of genetic variation in the founding population as in the genetic revolution model, but the initial founder effect can occasionally induce severe allele frequency alterations at one or more of the major loci, including fixation. Such a frequency alteration at the major locus then induces a cascading selective response at the modifier loci that are extremely responsive to selection in the founding population (Templeton 1980). The theoretical prediction of increased responsiveness to selection at the modifier loci has been elaborated and confirmed by subsequent theoretical work showing that founder events can indeed convert epistatic variance into additive genetic variance, thereby increasing-not diminishing-the overall levels of additive genetic variance and hence selective responsiveness immediately after the founder event (Goodnight 1988; Wagner et al. 1994). This responsiveness is also predicted to be enhanced by recombination through the creation of novel, selectively important allelic combinations (Templeton 1980); such as recombination between the major X-linked elements in the abnormal-abdomen syndrome. Thus, genetic transilience is triggered by drift effects

Genetic variation in height growth and leaf colour of Eucalyptus deglupta Blume at ages up to 16 months in Costa Rica

Results of three progeny tests of Eucalyptus deglupta in Costa Rica are described and discussed. High levels of additive genetic variation were found in height growth up to 16 months. Heritabilities were within the range typically found in forest trees. High levels of additive genetic variation and family heritabilities in leaf colour were also found. Purple leaves were associated with slow growth, and green leaves with fast growth. The best families showed a high degree of stability across the three sites. Predicted genetic gain from directed seed collections of the best mother-trees ranged from 12.4 to 15.8%.

Expression of a dispersal trait in a guild of mites colonizing transient habitats

Dispersal as a means of escape from deteriorating habitats is of particular ecological relevance for organisms such as certain astigmatic mites that colonize habitats which vary unpredictably in space and time. The mites meet these ecological challenges by a facultative dispersal morph, the heteromorphic deutonymph, also called hypopus. The appearance or absence of hypopodes in natural populations is attributable to two fundamentally different, albeit interacting, causes. Genetic polymorphism for the propensity to induce a hypopus provides for heritable variation within the population and allows selection to favor or eliminate certain genotypes. The genotypic composition of a population reflects selection forces previously acting on the population. But it holds no predictive power. Rather, it adapts the population to cope with unpredictably varying living conditions because it ensures instantaneous fit of certain genotypes of the population (those displaying hypopus-free development) to favorable (moist) environmental conditions, and others (those expressing a hypopus) to detrimental (dry) conditions. In contrast, environmentally cued inducibility allows mites to anticipate food quality inasmuch as it allows each genotype of the population to adjust its development rapidly to impending adversity or benefit. Inducibility occurs by means of a developmental switching mechanism and leads either to a developmental pathway with a hypopus or else one without. The expression of a hypopus depends on interacting genetic and environmental (trophic) factors. High levels of additive genetic variation combine with considerable genetic-trophical interaction (comprising a threshold for phenotypic expression of the trait) to control hypopus induction. The results are consistent with a variable threshold whose level depends on diet quality. Different trophic conditions set the threshold at different points along the genetic scale resulting in different proportions of hypopus-forming and directly developing individuals within the population. The threshold, therefore, converts the concealed continuous genetic variation underlying the trait into a discontinuous response of the mite.

A quantitative genetic analysis of an aposematic colour pattern and its ecological implications

The phenotypic and genetic variability of the aposematic colour pattern of the two-spot ladybird was investigated. There was significant variation in spot size (relative to elytron length) among wild populations of ladybirds. A quantitative genetic analysis revealed high heritability estimates for a number of individual colour pattern elements on the pronotum and elytra. The genetic covariances between the elytral spot and the pronotal spots were generally negative and all of the covariances among the pronotal spots were positive. This pattern of covariances could be explained if natural selection acts so as to maximize the combined anti-predator effect of the pronotal and elytral colour patterns by optimizing the rate of melanin production. An optimization process could also account for the significant levels of additive genetic variation found for each of the pattern elements considered. There was no evidence of any sex-linked gene expression for colour pattern, although female colour pattern may be influenced by maternally inherited factors.

THE EFFECTS OF HOST GENOTYPE AND SPATIAL DISTRIBUTION ON TREMATODE PARASITISM IN A BIVALVE POPULATION.

A basic assumption underlying models of host-parasite coevolution is the existence of additive genetic variation among hosts for resistance to parasites. However, estimates of additive genetic variation are lacking for natural populations of invertebrates. Testing this assumption is especially important in view of current models that suggest parasites may be responsible for the evolution of sex, such as the Red Queen hypothesis. This hypothesis suggests that the twofold reproductive disadvantage of sex relative to parthenogenesis can be overcome by the more rapid production of rare genotypes resistant to parasites. Here I present evidence of significant levels of additive genetic variance in parasite resistance for an invertebrate host-parasite system in nature. Using families of the bivalve mollusc, Transennella tantilla, cultured in the laboratory, then exposed to parasites in the field, I quantified heritable variation in parasite resistance under natural conditions. The spatial distribution of outplanted hosts was also varied to determine environmental contributions to levels of parasite infection and to estimate potential interactions of host genotype with environment. The results show moderate but significant levels of heritability for resistance to parasites (h2 = 0.36). The spatial distribution of hosts also significantly influenced parasite prevalence such that increased host aggregation resulted in decreased levels of parasite infection. Family mean correlations across environments were positive, indicating no genotype-environment interaction. Therefore, these results provide support for important assumptions underlying coevolutionary models of host-parasite systems.

The ecology and genetics of fitness in forest plants. IV. Quantitative genetics of fitness components in Impatiens pallida (Balsaminaceae)

Genetic and environmental variances were estimated for a number of characters in the annual plant Impatiens pallida by planting seed obtained through controlled crosses into their native field site or pots maintained in the greenhouse. Significant additive genetic variance was detected for three of 11 characters studied—germination date, cotyledon area, and date of first flower production. Significant dominance and/or maternal variance was found for seed weight, proportion of seeds germinating, cotyledon area, plant height, and number of leaves produced. Environmental variance was greater in the field compared with the greenhouse. Characters found to be under strong directional selection in a previous study showed no detectable additive genetic variance. While these populations exhibit conditions that in theory could contribute to the maintenance of genetic variation (limited pollen and seed dispersal distances and small‐scale variation for edaphic characteristics influencing plant growth), levels of additive genetic variance for most characters were not significantly different from zero.

Heritabilities and additive genetic coefficients of variation in forest trees

Estimates of individual-tree narrow-sense heritability and additive genetic coefficient of variation of seven traits of forest trees were compiled from 67 published papers. Distributions of the values for each trait were characterized and compared by calculating medians and running Kolmogorov–Smirnov and Wilcoxon signed-rank tests. Generalizations are possible about at least some of the traits examined. Heritability of wood specific gravity was almost always above 0.3 (median 0.48). Heritabilities for other traits tended to be low: medians ranged from 0.185 to 0.26, and individual values generally ranged from 0.1 to 0.4. Evidence that heritabilities of form traits tend to be higher than those of growth traits was weak. The analysis of additive genetic coefficients of variation suggested that specific gravity tends to have lower values than other traits (median 5.1%), while height and diameter (medians 8.5 and 8.6%, respectively) had lower values than straightness (median 11.65%). Individual-tree volume showed the highest levels of additive genetic coefficient of variation (median 20.3%). The levels of additive genetic variation and heritabilities suggest that reasonable levels of genetic gain can be achieved by screening relatively low numbers of trees.

ADAPTIVE GENETIC VARIATION IN GROWTH AND DEVELOPMENT OF TADPOLES OF THE HYBRIDOGENETIC RANA ESCULENTA COMPLEX.

The distribution and proportion of the sexual species Rana lessonae to the hemiclonal hybrid R. esculenta among natural habitats suggests that these anurans may differ in adaptive abilities. I used a half-sib design to partition phenotypic and quantitative genetic variation in tadpole responses at two food levels into causal variance components. Rana lessonae displays strong phenotypic variation across food levels. Growth rate is strictly determined by environmental factors and includes weak maternal effects. Larval period and body size at metamorphosis both contain moderate levels of additive genetic variance. The sire x food interactions and the lack of environmental correlations indicate that adaptive phenotypic plasticity is present in both of these traits. In contrast, R. esculenta displays less phenotypic variation across food levels, especially for larval period. Variation in body size at metamorphosis is underlain by genetic variation as shown by high levels of additive genetic variance, yet growth rate and larval period are not. Significant environmental correlations between larval period at high food level and growth, larval period, and body size at low food, indicate phenotypic plasticity is absent. A positive phenotypic correlation between body size at metamorphosis and larval period for R. lessonae at both food levels suggests a trade-off between growing large and metamorphosing quickly to escape predation or pond drying. The lack of a similar correlation for R. esculenta at the high food level suggests it may be less constrained. Different levels of adaptive genetic variation among larval traits suggest that the sexual species and the hybridogenetic hemiclone differ in their abilities to cope with temporally and spatially heterogeneous environments.

ON THE LOW HERITABILITY OF LIFE-HISTORY TRAITS.

Life-history traits such as longevity and fecundity often show low heritability. This is usually interpreted in terms of Fisher's fundamental theorem to mean that populations are near evolutionary equilibrium and genetic variance in total fitness is low. We develop the causal relationship between metric traits and life-history traits to show that a life-history trait is expected to have a low heritability whether or not the population is at equilibrium. This is because it is subject to all the environmental variation in the metric traits that affect it plus additional environmental variation. There is no simple prediction regarding levels of additive genetic variance in life-history traits, which may be high at equilibrium. Several other patterns in the inheritance of life-history traits are readily predicted from the causal model. These include the strength of genetic correlations between life-history traits, levels of nonadditive genetic variance, and the inevitability of genotype-environment interaction.

QUANTITATIVE GENETICS OF FITNESS TRAITS IN A WILD POPULATION OF PHLOX.

To determine if the evolution of fitness traits in the annual plant, Phlox drummondii, is constrained by lack of genetic variation, we calculated the heritability and genetic correlation of 16 traits in a field population. Full- and half-sib families of seeds were generated in the greenhouse and planted back into the study population. Of 855 seeds that germinated, 609 survived to produce fruit. For each plant we measured several aspects of plant size and three components of female fecundity: total number of fruits produced, number of seeds per fruit, and mass of individual seeds. Heritability of traits ranged from 0.00 to 0.15. Several traits showed significant levels of additive genetic variance, but we found no evidence of additive genetic variance in components of female fecundity and no evidence of negative genetic correlation between fitness traits. These results suggest that evolution in this population would be constrained by lack of heritable variation in fitness traits.

The effect of new environment on adapted genetic architecture

Central to the study of life cycle evolution is the concept of genetic trade-offs. Genetic trade-offs between life cycle characters develop as a result of the accumulation of genes with antagonistic pleiotropic effects. In the present study a comparison was made between the genetic architecture that had evolved in the ancestral environment and the way that this genetic architecture was disrupted following transfer to a new environment. It was predicted that, in the ancestral environment, genetic trade-offs should have evolved between each life cycle character and, as a result of these genetic trade-offs, significant levels of additive genetic variation should remain despite many generations of selection.Following transfer to a new environment different genes might be expressed. Therefore, it was predicted that in the new environment the levels of additive genetic variation should increase and that the genetic trade-offs should break down. The predictions were well supported by the data.

Genetic Improvement of Timber Yield and Wood Quality inEucalyptus Grandis(Hill) Maiden

SYNOPSIS Results of a partial diallel progeny test of Eucalyptus grandis assessed at ages 5, 36, 60 and 105 months are presented. Large genetic variations among full-sib families were found for all traits studied, e.g. the mean tree volume per family at 105 months varied from 0,226 m3 to 1,325 m3. Estimates of additive genetic variance (σ2 a) for volume production was slightly less than dominance variance (σ2 d) but for stem and crown form, and for taper σ2 a clearly exceeded σ2 d. Estimates of individual tree heritabilities were moderately high, varying from 0,28 to 0,54. Reciprocal effects were not significant. Genetic values, breeding values and specific combining ability values indicate that the relatively high level of additive genetic variation can be utilised for predicting which parent will produce the best offspring. Such offspring could be particularly useful for vegetative propagation. Genetic correlations are favourable among most growth traits, indicating that simultaneous improvements are possible even if selection is practised on one trait only. Strong age-age and trait-trait correlations imply that assessment procedures on E. grandis could be simplified substantially and that a strategy with rapid generation turnover could be adopted.

EXPERIMENTAL STUDIES OF PONDEROSA PINE. II. QUANTITATIVE GENETICS OF MORPHOLOGICAL TRAITS

We studied fourteen morphological attributes of 511 seedling stage progeny involving crosses between Pinus ponderosa ponderosa parents as well as intervarietal crosses of P. p. ponderosa × P. p. scopulorum. Intravarietal progeny were distinctly differentiated genetically from intervarietal sibships in seven of the fourteen characters examined. The patterns of differentiation observed strongly suggest a syndrome of selective responses to increased water stress in the progeny with a scopulorum parent and for comparatively more rapid, columnar growth in the progeny of the intravariety crosses which involved only P. p. ponderosa parents. These differences are consistent with known ecological distinctions between the habitats of the two varieties. Narrow‐sense heritability estimates, obtained from half‐sib progeny analysis, indicated that considerable levels of additive genetic variance remain present in several traits. We could detect no relationship between the amount of additive genetic variance present for a particular character and its presumed relevance to fitness.

Experimental Studies of Ponderosa Pine. II. Quantitative Genetics of Morphological Traits

We studied fourteen morphological attributes of 511 seedling stage progeny involving crosses between Pinus ponderosa ponderosa parents as well as intervarietal crosses of P. p. ponderosa × P. p. scopulorum. Intravarietal progeny were distinctly differentiated genetically from intervarietal sibships in seven of the fourteen characters examined. The patterns of differentiation observed strongly suggest a syndrome of selective responses to increased water stress in the progeny with a scopulorum parent and for comparatively more rapid, columnar growth in the progeny of the intravariety crosses which involved only P. p. ponderosa parents. These differences are consistent with known ecological distinctions between the habitats of the two varieties. Narrow-sense heritability estimates, obtained from half-sib progeny analysis, indicated that considerable levels of additive genetic variance remain present in several traits. We could detect no relationship between the amount of additive genetic variance present for a particular character and its presumed relevance to fitness.