MATERNAL INHERITANCE AND ITS EFFECT ON ADAPTIVE EVOLUTION: A QUANTITATIVE GENETIC ANALYSIS OF MATERNAL EFFECTS IN A NATURAL PLANT POPULATION

Genotype × environment interaction obscures genetic sources of variation in seed size in Dithyrea californica but provides the opportunity for selection on phenotypic plasticity

PREMISE

In many species, seed size influences individual fitness, but its heritability is low, impeding its evolution. In heterogeneous environments, even if heritability of seed size is low, genetic variation in phenotypic plasticity for seed size may provide the opportunity for selection, but this possibility has rarely been investigated in wild species. The evolutionary trajectory of seed size depends on whether additive, maternal, or non-additive genetic variance dominates; moreover, the expression of any of these sources of variance may be environment-dependent, reflecting genetic variation in plasticity. In this study, we examined these sources of variation in seed size and their response to drought in Dithyrea californica.

METHODS

We used a diallel design to estimate variance components for seed size in three greenhouse-raised populations sampled from California and northern Mexico. We replicated diallels in two watering treatments to examine genetic parameters and genotype × environment interactions affecting seed size. We estimated general (GCA) and specific (SCA) combining ability, reciprocal effects (RGCA and RSCA), and their interactions with water availability, and we sought evidence that sexual conflict influences seed size.

RESULTS

Norms of reaction revealed genetic variation in plasticity for seed size in each population. Seed size in D. californica is determined by the combination of watering treatment, GCA and RGCA; parental identity and water availability do not consistently affect seed size, and we detected no evidence for sexual conflict.

CONCLUSIONS

Multiple sources of genetic variation in phenotypic plasticity for seed size have the potential to influence its evolutionary trajectory in heterogenous environments.

Transgenerational Genetic Effects Help Explain Latitudinal Variation in Seed Mass and Germination Timing in Plantago lanceolata

We know little about the underlying genetic control of phenotypic patterns of seed traits across large-scale geographic and environmental gradients. Such knowledge is important for understanding the evolution of populations within species and for improving species conservation. Therefore, to test for genetic variation in Plantago lanceolata, we made reciprocal crosses between northern and southern genotypes that span the species’ range in Europe. The results provide evidence of transgenerational genetic effects on seed mass and germination timing. Northern mothers produced larger seeds with delayed germination, in contrast to southern mothers, which produced smaller seeds with accelerated germination. A maternal latitude affected both the seed coat, solely maternal tissue, and embryo/endosperm tissues. Thus, latitudinal variation in seed size and germination timing can be explained, in part, by the direct influence of maternal genotype, independent of zygotic genes that parents pass directly to the embryo and endosperm. Data suggest that researchers exploring the existence and evolution of large-scale geographic variation within species test for transgenerational genetic effects. In addition, data suggest that transgenerational control of seed traits should be considered when developing procedures designed to facilitate species conservation and restoration.

How early does the selfing syndrome arise? Associations between selfing ability and flower size within populations of the mixed-mater Collinsia verna

PREMISE

Widespread associations between selfing rate and floral size within and among taxa suggest that these traits may evolve in concert. Does this association develop immediately because of shared genetic and/or developmental control, or stepwise with selection shaping the evolution of one trait following the other? If the former, then association ought to appear within and across selfing populations. We explore this fundamental question in three populations of the mixed-mater Collinsia verna where autonomous selfing (AS) ability has been shown to be under selection by the pollination environment.

METHODS

We grew clonal replicates of C. verna in a controlled environment to characterize broad-sense genetic correlations among traits within populations and to assess whether divergence in mating system and floral traits among these populations is consistent with their previously observed selection pressures.

RESULTS

As predicted by their respective pollination environments, we demonstrate significant genetic divergence among populations in AS ability. However, patterns of divergence in floral traits (petal, stamen, and style size, stigmatic receptivity, and stigma-anther distance) were not as expected. Within populations, genetic variation in AS appeared largely independent from floral traits, except for a single weak negative association in one population between flower size and AS rate.

CONCLUSIONS

Together, these results suggest that associations between selfing rate and floral traits across Collinsia species are not reflected at microevolutionary scales. If C. verna were to continue evolving toward the selfing syndrome, floral trait evolution would likely follow stepwise from mating system evolution.

The transgenerational effects of solar short-UV radiation differed in two accessions of Vicia faba L. from contrasting UV environments

BACKGROUND AND AIMS

UVB radiation can rapidly induce gene regulation leading to cumulative changes for plant physiology and morphology. We hypothesized that a transgenerational effect of chronic exposure to solar short UV modulates the offspring's responses to UVB and blue light, and that the transgenerational effect is genotype dependent.

METHODS

We established a factorial experiment combining two Vicia faba L. accessions, two parental UV treatments (full sunlight and exclusion of short UV, 290-350 nm), and four offspring light treatments from the factorial combination of UVB and blue light. The accessions were Aurora from southern Sweden, and ILB938 from Andean region of Colombia and Ecuador.

KEY RESULTS

The transgenerational effect influenced morphological responses to blue light differently in the two accessions. In Aurora, when UVB was absent, blue light increased shoot dry mass only in plants whose parents were protected from short UV. In ILB938, blue light increased leaf area and shoot dry mass more in plants whose parents were exposed to short UV than those that were not. Moreover, when the offspring was exposed to UVB, the transgenerational effect decreased in ILB938 and disappeared in Aurora. For flavonoids, the transgenerational effect was detected only in Aurora: parental exposure to short UV was associated with a greater induction of total quercetin in response to UVB. Transcript abundance was higher in Aurora than in ILB938 for both CHALCONE SYNTHASE (99-fold) and DON-GLUCOSYLTRANSFERASE 1 (19-fold).

CONCLUSIONS

The results supported both hypotheses. Solar short UV had transgenerational effects on progeny responses to blue and UVB radiation, and they differed between the accessions. These transgenerational effects could be adaptive by acclimation of slow and cumulative morphological change, and by early build-up of UV protection through flavonoid accumulation on UVB exposure. The differences between the two accessions aligned with their adaptation to contrasting UV environments.

Assessing How the Aluminum-Resistance Traits in Wheat and Rye Transfer to Hexaploid and Octoploid Triticale

The mechanisms of aluminum (Al) resistance in wheat and rye involve the release of citrate and malate anions from the root apices. Many of the genes controlling these processes have been identified and their responses to Al treatment described in detail. This study investigated how the major Al resistance traits of wheat and rye are transferred to triticale (x Tritosecale Wittmack) which is a hybrid between wheat and rye. We generated octoploid and hexaploid triticale lines and compared them with the parental lines for their relative resistance to Al, organic anion efflux and expression of some of the genes encoding the transporters involved. We report that the strong Al resistance of rye was incompletely transferred to octoploid and hexaploid triticale. The wheat and rye parents contributed to the Al-resistance of octoploid triticale but the phenotypes were not additive. The Al resistance genes of hexaploid wheat, TaALMT1, and TaMATE1B, were more successfully expressed in octoploid triticale than the Al resistance genes in rye tested, ScALMT1 and ScFRDL2. This study demonstrates that an important stress-tolerance trait derived from hexaploid wheat was expressed in octoploid triticale. Since most commercial triticale lines are largely hexaploid types it would be beneficial to develop techniques to generate genetically-stable octoploid triticale material. This would enable other useful traits that are present in hexaploid but not tetraploid wheat, to be transferred to triticale.

Local adaptation in transgenerational responses to predators

Environmental signals can induce phenotypic changes that span multiple generations. Along with phenotypic responses that occur during development (i.e. 'within-generation' plasticity), such 'transgenerational plasticity' (TGP) has been documented in a diverse array of taxa spanning many environmental perturbations. New theory predicts that temporal stability is a key driver of the evolution of TGP. We tested this prediction using natural populations of zooplankton from lakes in Connecticut that span a large gradient in the temporal dynamics of predator-induced mortality. We reared more than 120 clones of Daphnia ambigua from nine lakes for multiple generations in the presence/absence of predator cues. We found that temporal variation in mortality selects for within-generation plasticity while consistently strong (or weak) mortality selects for increased TGP. Such results provide us the first evidence for local adaptation in TGP and argue that divergent ecological conditions select for phenotypic responses within and across generations.

Geographic patterns of seed mass are associated with climate factors, but relationships vary between species

PREMISE OF THE STUDY

Seed size is a critical life history attribute with fitness effects that cascade throughout the lifespan of plants. Interspecific studies repeatedly report a negative correlation between seed mass and latitude. Yet, despite its importance, little is known about geographic variation in seed size within species' ranges.

METHODS

To improve our understanding of intraspecific geographic variation in seed size, we collected and weighed seeds by maternal line from 8 to 17 populations of seven herbaceous plant species spanning large geographic areas, and measured a dispersal trait, awn length, for two grass species. We examined the overall relationship between seed mass and latitude, then divided the data into species-specific subsets to compare the fit of three models to explain seed mass and awn length: (1) latitude and longitude, (2) long-term climate, and (3) collection-year weather.

KEY RESULTS

Like previous work, we found a negative relationship between interspecific seed mass and latitude. However, the best-fit models explaining seed size and awn length differed between individual species and often included significant interaction terms. For all species, the best model was either long-term or collection-year climate data instead of latitude and longitude.

CONCLUSIONS

Intraspecific geographic patterns for seed traits were remarkably inconsistent, covarying both negatively and positively with temperature and precipitation. The only apparent generalization is that annual species' seed mass corresponded more with collection-year weather while perennial species covaried more with long-term climate. Overall, this study suggests that the scale of climate variation that molds seed traits is highly species-specific.

Recurrent selection for wider seedling leaves increases early biomass and leaf area in wheat (Triticum aestivum L.)

The breeding of wheat with greater early vigour has potential to increase water- and nutrient-use efficiency, as well as to improve weed competitiveness to raise crop yields profitably. Given that wheat is inherently conservative in its early growth, a sustained breeding effort was initiated to increase genetically seedling leaf area in developing novel high vigour germplasm. A recurrent selection programme was initiated by intercrossing a genetically diverse set of 28 vigorous wheat lines identified globally. These were intercrossed at random and S1:2 progeny with the largest leaf 1 and 2 widths were intermated to develop new populations for assessment of early growth. This procedure was repeated for up to 60 segregating families per cycle across six cycles over 15 years. Thirty random S1:2 progeny were retained from each cycle and seed-increased together to produce seed for early vigour assessment in multiple sowings. The most vigorous wheat seedlings were identified in later cycles, with some lines producing more than double the leaf area and biomass of elite commercial wheat varieties. Phenotypic selection for greater leaf width was associated with a realized significant (P<0.01) linear increase per seedling of 0.41 mm per cycle (+7.1%) for mean leaf width, and correlated linear increases in total leaf area and biomass of 4.48 cm(2) per cycle (+10.3%) and 10.8 mg per cycle (+5.3%), respectively. Genetic gains in widths of leaves 2 (+8.4%) and 3 (+11.5%) were significantly (P<0.01) greater than for leaf 1 (+5.3%). Selection for greater leaf width was associated with linear increases in coleoptile tiller leaf area, small curvilinear increases in leaf 1 length, and reductions in numbers of leaves and mainstem tillers. Genetic variances were large and heritabilities high for leaf width and total leaf area in each cycle, but reduced linearly in size with selection across cycles. Coupling diverse germplasm with a simple, inexpensive, and repeatable selection process has confirmed the value of recurrent selection in developing uniquely vigorous wheat germplasm for use as parents in commercial breeding.

The evolution of multivariate maternal effects

There is a growing interest in predicting the social and ecological contexts that favor the evolution of maternal effects. Most predictions focus, however, on maternal effects that affect only a single character, whereas the evolution of maternal effects is poorly understood in the presence of suites of interacting traits. To overcome this, we simulate the evolution of multivariate maternal effects (captured by the matrix M) in a fluctuating environment. We find that the rate of environmental fluctuations has a substantial effect on the properties of M: in slowly changing environments, offspring are selected to have a multivariate phenotype roughly similar to the maternal phenotype, so that M is characterized by positive dominant eigenvalues; by contrast, rapidly changing environments favor Ms with dominant eigenvalues that are negative, as offspring favor a phenotype which substantially differs from the maternal phenotype. Moreover, when fluctuating selection on one maternal character is temporally delayed relative to selection on other traits, we find a striking pattern of cross-trait maternal effects in which maternal characters influence not only the same character in offspring, but also other offspring characters. Additionally, when selection on one character contains more stochastic noise relative to selection on other traits, large cross-trait maternal effects evolve from those maternal traits that experience the smallest amounts of noise. The presence of these cross-trait maternal effects shows that individual maternal effects cannot be studied in isolation, and that their study in a multivariate context may provide important insights about the nature of past selection. Our results call for more studies that measure multivariate maternal effects in wild populations.

In numerous organisms, mothers influence the phenotype of their offspring by transmitting hormones, antibodies and nutrients to the embryo. Evolutionary studies that make predictions about the evolution of these maternal effects typically focus, however, on single maternal characters only, in isolation of other traits. This contrasts with insights from quantitative genetics where reliable predictions about evolutionary change can only be made when measuring multiple traits simultaneously. The current study is therefore the first to make formal predictions about the evolutionary properties of multiple maternal effects. We show that maternal phenotypic characters generally give rise to developmental interactions in which one maternal character affects multiple offspring characters. In turn, such interactions can give rise to correlations between different traits in parent and offspring, which constrain evolutionary responses to sudden change. In addition, we find that the rate of environmental change directly affects some of the measurable properties of maternal effects: in rapidly changing environments, multivariate maternal effects are negative, so that offspring attain phenotypes that are different from their mothers, whereas positive maternal effects where offspring are more similar to their mothers occur in slowly changing environments. Hence, multivariate maternal effects provide a clear signature of the past selective environment experienced by organisms.

Parents lend a helping hand to their offspring in plant defence

Plants under attack by pathogens and pests can mount a range of inducible defences, encompassing both chemical and structural changes. Although few reports exist, it appears that plants responding to pathogen or herbivore attack, or chemical defence elicitors, may produce progeny that are better able to defend themselves against attack, compared with progeny from unthreatened or untreated plants. To date, all research on transgenerational effects of biotic stress has been conducted on dicotyledenous plants. We examined the possibility that resistance induced by application of chemical defence elicitors to the monocot plant barley, could be passed on to the progeny. Plants were treated with acibenzolar-S-methyl (ASM) or saccharin, and grain harvested at maturity. Germination was unaffected in seed collected from plants treated with saccharin, while germination was reduced significantly in seed collected from ASM-treated plants. The subsequent growth of the seedlings was not significantly different in any of the treatments. However, plants from parents treated with both ASM or saccharin exhibited significantly enhanced resistance to infection by Rhynchosporium commune, despite not being treated with elicitor themselves. These data hint at the possibility of producing disease-resistant plants by exposing parent plants to chemical elicitors.

The benefits of maternal effects in novel and in stable environments

Natural selection favours phenotypes that match prevailing ecological conditions. A rapid process of adaptation is therefore required in changing environments. Maternal effects can facilitate such responses, but it is currently poorly understood under which circumstances maternal effects may accelerate or slow down the rate of phenotypic evolution. Here, we use a quantitative genetic model, including phenotypic plasticity and maternal effects, to suggest that the relationship between fitness and phenotypic variance plays an important role. Intuitive expectations that positive maternal effects are beneficial are supported following an extreme environmental shift, but, if too strong, that shift can also generate oscillatory dynamics that overshoot the optimal phenotype. In a stable environment, negative maternal effects that slow phenotypic evolution actually minimize variance around the optimum phenotype and thus maximize population mean fitness.

Contribution of direct and maternal genetic effects to life-history evolution

Maternal effects are ubiquitous in nature. In plants, most work has focused on the effects of maternal environments on offspring trait expression. Less is known about the prevalence of genetic maternal effects and how they influence adaptive evolution. Here, we used multivariate genetic models to estimate the contributions of maternal and direct genetic (co)variance, the cross-generation direct-maternal covariance, and M, the matrix of maternal effect coefficients, for life-history traits in Campanulastrum americanum, a monocarpic herb. Following a three-generation breeding design, we grew paternal half-sib families with full-sib relatives of each parent and measured juvenile and adult traits. Seed size was influenced exclusively by maternal environmental effects, whereas maternal genetic effects influenced traits throughout the life cycle, including strong direct and maternal additive genetic correlations within and between generations for phenological and size traits. Examination of M suggested that both juvenile and adult traits in maternal plants influenced the expression of offspring traits. This study reveals substantial potential for genetic maternal effects to contribute to adaptive evolution including cross-generation direct-maternal correlations that may slow selection response, maternal effects on phenology that reinforce genetic correlations, and within- and between-generation genetic correlations that may influence life-history polymorphism.

What are maternal effects (and what are they not)?

Maternal effects can play an important role in a diversity of ecological and evolutionary processes such as population dynamics, phenotypic plasticity, niche construction, life-history evolution and the evolutionary response to selection. However, although maternal effects were defined by quantitative geneticists well over half a century ago, there remains some confusion over exactly what phenomena should be characterized as maternal effects and, more importantly, why it matters and how they are defined. We suggest a definition of maternal effects as the causal influence of the maternal genotype or phenotype on the offspring phenotype. This definition differs from some definitions in that it treats maternal effects as a phenomenon, not as a statistical construct. The causal link to maternal genotype or phenotype is the critical component of this definition providing the link between maternal effects and evolutionary and ecological processes. We show why phenomena such as maternal cytoplasmic inheritance and genomic imprinting are distinct genetically from and have different evolutionary consequences than true maternal effects. We also argue that one should consider cases where the maternal effect is conditional on offspring genotype as a class of maternal effects.

Heritability and correlation structure of nectar and floral morphology traits in Nicotiana alata

The heritability and genetic basis of nectar traits have been rarely studied in the field, where plants are exposed to environmental factors that could mask underlying genetic effects. Heritabilities and variance components were estimated for nectar and morphological traits of Nicotiana alata, using a partial diallel design. The main experiment was conducted in a Missouri experimental garden using a randomized block design with three plant density treatments, whereas a smaller experiment was conducted near native Brazil habitat to compare the environmental variance in traits between Missouri and Brazil. Significant heritability was detected for nectar volume and energy content, and for corolla tube length. Phenotypic correlations were significant between all traits investigated, whereas significant genetic correlations were only found between nectar volume and energy and between corolla limb width and mouth diameter. There were no significant family-by-density interactions detected in the Missouri field environment. All traits differed significantly between Missouri and Brazil environments, but significant genetic by environment (G x E) interactions between Missouri and Brazil were detected for only one trait. This study shows that nectar traits can be heritable despite considerable environmental variation.

Maternal inheritance and rapid evolution of sexual size dimorphism: passive effects or active strategies?

Adaptive evolution is often strongly influenced by maternal inheritance that transfers the parental strategies across generations. The consequences of maternal effects for the offspring generation depend on the between-generation similarity in environments and on the evolved sensitivity of the offspring's ontogeny to maternal effects. When these factors differ between sons and daughters, maternal effects can influence the evolution of sexual dimorphism. The establishment of house finch populations across western Montana during the last 30 years was accompanied by rapid evolutionary change in sexual size dimorphism. Here I show that traits that changed the most across generations were most influenced by maternal effects in males but not females. Maternal effects differentially affected sons' and daughters' survival; greater maternal effects were commonly associated with higher survival of sons, especially when maternal and offspring environments were similar. Stronger maternal effects extended preselection phenotypic variance in morphological traits of males, thereby producing some locally adaptive phenotypes and lessening juvenile mortality. Thus, the observed sex-specific maternal effects and their contribution to the evolution of sexual size dimorphism are likely a passive consequence of the distinct sensitivity of sons and daughters to maternal adaptations to breeding in ecologically distinct parts of the house finch's expanding range.

Maternal effects and heterosis influence the fitness of plant hybrids

Here we tested two possible nonexclusive explanations for the maintenance of a hybrid swarm between Senecio jacobaea and Senecio aquaticus; first, that genotype-by-environment interactions involving water and nutrient clines are involved in hybrid fitness, and second, heterosis in early hybrid generations may provide an initial hybrid advantage that contributes to hybrid persistence. In three climate chamber studies, fitness and root growth were measured for parental species and natural and artificial F1 hybrids, in order to determine whether hybrids occur in habitats where they are more fit than parental species. Natural hybrids, which are generally back-crossed to S. jacobaea, always equaled S. jacobaea in growth characteristics. Maternal effects played a role in the fitness of F1 hybrids, with offspring from S. jacobaea mothers exhibiting higher fitness than those from S. aquaticus mothers, and compared with parental species and natural hybrids. Natural hybrids are not distributed in zones where they are most fit with respect to nutrient and water regimes. Superior fitness of early generation hybrids may contribute to hybrid swarm stability.

Maternal effects and the response to selection in red squirrels

Mothers often provide much of the early environment for their offspring. These maternal effects are predicted to result in unusual evolutionary dynamics in offspring traits if they are themselves heritable, but these important predictions have not previously, to our knowledge, been tested in the wild. Here, we quantified the responses of red squirrels (Tamiasciurus hudsonicus) to documented episodes of natural selection and found support for both of the fundamental predictions of models that describe maternal effect evolution. First, changes in juvenile growth rates across one generation of selection were five times greater than predicted by heritability (h2) alone, but were consistent with the additional contribution of maternal genetic effects. Second, responses to selection were influenced not only by the strength of selection in the current generation, but also by selection in the previous generation, indicating the presence of evolutionary momentum. These results were in agreement with predictions of a simple model including litter size as the only maternal effect, and provide, to our knowledge, the first empirical evidence for the importance of maternal effects to evolutionary dynamics in a natural population.

The influence of light on paternal plants in Campanula americana (Campanulaceae): pollen characteristics and offspring traits

Offspring trait expression is determined by the combination of parental genes and parental environments. Although maternal environmental effects have been widely characterized, few studies have focused on paternal environmental effects. To determine whether light availability influences pollen and offspring traits in the woodland herb Campanula americana, we reared clones of 12 genotypes in two light levels. In the parental generation we measured pollen number and size. Plants grown under high light produced more pollen grains per flower than those grown under low light. However, the response was genotype specific; some individuals responded little to changes in light availability while others substantially reduced pollen production. As a consequence, paternity ratios may vary between light environments if more pollen is associated with greater siring success. We crossed a subset of these plants to produce the offspring generation. The paternal and maternal light environments influenced offspring seed mass, percentage germination, and days to germination, while only maternal light levels influenced later life traits, such as leaf number and size. Maternal and paternal environmental effects had opposite influences on seed mass, percentage germination and days to germination. Finally, there was no direct relationship between light effects on pollen production and offspring trait expression.

Heritable variation in quinone-induced haustorium development in the parasitic plant Triphysaria

We are using the facultative hemiparasite, Triphysaria, as a model for studying host-parasite signaling in the Scrophulariaceae. Parasitic members of this family form subterranean connections, or haustoria, on neighboring host roots to access host water and nutrients. These parasitic organs develop in response to haustorial-inducing factors contained in host root exudates. A well-characterized inducing factor, 2, 6-dimethoxy-p-benzoquinone (DMBQ), can be used to trigger in vitro haustorium formation in the roots of Triphysaria. We have assayed three species, Triphysaria eriantha (Benth.) Chuang and Heckard, Triphysaria pusilla (Benth.) Chuang and Heckard, and Triphysaria versicolor Fischer and C. Meyer, for haustorium development in response to DMBQ. There were significant differences between the species in their ability to recognize and respond to this quinone. Ninety percent of T. versicolor individuals responded, whereas only 40% of T. pusilla and less than 10% of T. eriantha formed haustoria. Within field collections of self-pollinating T. pusilla, differential responsiveness to DMBQ was seen in distinct maternal families. Assaying haustorium development in subsequent generations of self-pollinated T. pusilla showed that DMBQ responsiveness was heritable. Reciprocal crosses between T. eriantha and T. versicolor demonstrated that DMBQ responsiveness was influenced by maternal factors. These results demonstrate heritable, natural variation in the recognition of a haustorial-inducing factor by a parasitic member of the Scrophulariaceae.

Capitulum characters in a seed heteromorphic plant, Crepis sancta (Asteraceae): variance partitioning and inference for the evolution of dispersal rate

In Crepis sancta (Asteraceae), achenes produced in the periphery of the flower head have reduced dispersal ability and are larger than achenes produced in the centre of the head, which disperse farther. The proportion of central achenes produced by a single individual represents the potential dispersal rate of its progeny. Seed variation in dispersal ability may be important where there is spatio-temporal variability of habitats, but its evolutionary significance mainly depends on the heritability of the relative proportions of each achene morph. However, the number of peripheral achenes in a capitulum, and that of involucral bracts are suggested to depend on the number of parastichies, a canalized character. From a diallel cross design, phenotypic variance for several capitulum traits was partitioned among six variance components, including the additive variance. The phenotypic values of some head traits reflected the expected frequency due to ontogeny, in particular the number of involucral bracts. Yet, this character also had a significant heritability, suggesting that variation around the mode of the distribution was not only due to developmental noise. The additive variance for number of peripheral and central achenes was not significantly different from zero. In contrast, their respective proportion had a narrow sense heritability greater than 0.20. The present results suggest that the percentage of central achenes per individual, and thus the potential dispersal rate in Crepis sancta, is under quantitative genetic control, and could undergo microevolutionary changes in natural populations.

Uterine and postnatal maternal effects in mice selected for differential rate of early development

A series of mouse lines was produced by long-term restricted index selection for divergent rate of growth during early and late postnatal development. The selection program was based on the following treatments: E(+) and E(-) lines were selected to alter birth to 10-day weight gain while holding late gain for both lines constant and a control line was established via random selection. Using embryo transfer and crossfostering methodology, we partitioned postnatal growth for E(+), E(-), and C lines into progeny genetic, uterine maternal, and nurse maternal components. Selection for differential early growth resulted in correlated response in uterine and nurse maternal effects on body weights, with significant genetic-by-environment interactions. Significant uterine effects were also observed in tail length measurements. Direct uterine effects on body weight were relatively small and resulted in growth rate differences early in development. Nurse effects were large, resulting in modification of progeny growth trajectory especially during early postnatal development. Genetic-by-uterine interactions were large and demonstrate progeny-specific effects of the prenatal uterine environment.