ECOTYPIC DIVERGENCE IN ALPINE POLEMONIUM VISCOSUM: GENETIC STRUCTURE,QUANTITATIVE VARIATION,AND LOCAL ADAPTATION

Polemonium viscosum has a continuous distribution from 3,500 m in the krummholz to 4,025 m on the summit ridges of Pennsylvania Mountain, Colorado. Seeds produced by plants at opposite ends of this cline, 1.5 km apart, differed significantly at allozyme loci in two consecutive breeding seasons. Mean multilocus F_st values for both years (0.015 and 0.069) were significantly different from zero, indicating restricted gene exchange between subpopulations. Average allele frequencies at two individual loci also differed significantly between families comprising krummholz and summit subpopulations. Progeny of plants growing on the summit had higher leaf production rates, more densely packed leaflets, and lower resistance to aphids than progeny of plants growing in the krummholz site, when tested under greenhouse conditions. These differences probably reflect the restricted opportunities for growth and severe exposure at high elevations, and the increased risk from herbivores near timberline. The two subpopulations did not differ in leaf length (stature), leaf width, or pubescence. Reciprocal transplanting of seedlings between krummholz and summit sites confirmed that the differences were adaptive, since progeny from each subpopulation performed significantly better in their parent's habitat. Coordinated studies of genetic structure, quantitative variation, and local adaptation across the elevational range of P. viscosum provide a comprehensive view of ecotypic divergence in this widespread alpine plant.     

PHENOTYPIC PLASTICITY IN POLYGONUM PERSICARIA. II. NORMS OF REACTION TO SOIL MOISTURE AND THE MAINTENANCE OF GENETIC DIVERSITY

Adaptive phenotypic plasticity is the predicted evolutionary response to fine-grained fluctuation in major environmental factors, such as soil moisture in plant habitats. This study examines genotypes from two natural populations of Polygonum persicaria, one from a relatively homogeneous, moderately moist site, and one from a site in which severe drought and root flooding occur within single growth seasons. Norms of reaction (phenotypic response curves) were determined for a random sample of eight and ten cloned genotypes, respectively, from each of the populations over a controlled moisture gradient ranging from drought to flooding.     

PHENOTYPIC PLASTICITY IN CHRYSOPERLA: GENETIC VARIATION IN THE SENSORY MECHANISM AND IN CORRELATED REPRODUCTIVE TRAITS

A genetically variable sensory mechanism provides phenotypic plasticity in the seasonal cycle of the Chrysoperla carnea species-complex of green lacewings. The mechanism functions as a switch during the pupal and early imaginal stages to determine aestival reproduction versus aestival dormancy, and it has two major components: (1) response to photoperiod and (2) response to a stimulus(i) associated with the prey of the larvae. Ultimately, the switch is based on the response to photoperiod—an all-or-nothing trait whose variation (long-day reproduction versus a short-day/long-day requirement for reproduction) is determined by alleles at two unlinked autosomal loci. In eastern North America, variation in this component of the switch differentiates two reproductively isolated “species” that are sympatric throughout the region: Chrysoperla carnea, in which both loci are homozygous for the dominant alleles that determine long-day, spring and summer reproduction and thus multivoltinism, and C. downesi, which has a very high incidence of the recessive alleles for the short-day/long-day requirement, and thus univoltine spring breeding. In contrast, geographical populations in western North America harbor variable amounts of within-and among-family genetic variation for the photoperiodic responses and also for the switch's second component—adult responsiveness to the prey of the larvae. The geographic pattern of genetic variation in the two components of the switch indicates that it is a highly integrated adaptation to environmental heterogeneity. Expression of among-family variation in the prey component of the switch is highly dependent on photoperiodic conditions and genotype (it requires a constant long daylength and the recessive short-day/long-day genotype). Thus, we infer that responsiveness to prey evolved as a modifier of the photoperiodic trait. The switch has a significant negative effect on a major determinant of fitness; it lengthens the preoviposition period in nondiapausing reproductives. This negative effect may result in temporal variation in the direction of selection, which helps maintain genetic variability in the switch mechanisms of western populations. Also, the photoperiodic and prey components of the switch are positively correlated with fecundity in nondiapausing reproductives; however, the strong influence of environmental factors—presence or absence of prey—leaves open the question whether the correlated effects on fecundity are expressed in nature.     

QUANTITATIVE GENETICS OF BRYOZOAN PHENOTYPIC EVOLUTION. III. PHENOTYPIC PLASTICITY AND THE MAINTENANCE OF GENETIC VARIATION

Cheilostome bryozoan species show long-term morphologic stasis, implying stabilizing selection sustained for millions of years, but nevertheless retain significant heritable variation in traits of skeletal morphology. The possible role of within-genotype (within-colony) phenotypic variability in preserving genetic diversity was analyzed using breeding data for two species of Stylopoma from sites along 110 km of the Caribbean coast of Panama. Variation among zooids within colonies accounts for nearly two-thirds of the phenotypic variance on average, increases with environmental heterogeneity, and includes significant genotype-environment interaction. Thus, within-colony variability apparently represents phenotypic plasticity, at least some of which is heritable, rather than random “developmental noise.” Almost all of the among-colonies component of phenotypic variance is accounted for by additive genetic differences in trait means, suggesting that within-colony plasticity includes virtually all of the environmental component of phenotypic variance in these populations of Stylopoma. Thus, heritable within-colony plasticity could play a significant part in maintaining genetic diversity in cheilostomes, but it is also possible that rates of polygenic mutation alone are sufficient to balance the effects of selection.     

LONG-TERM COST OF REPRODUCTION WITH AND WITHOUT ACCELERATED SENESCENCE IN CALLOSOBRUCHUS MACULATUS: ANALYSIS OF AGE-SPECIFIC MORTALITY

Age-specific mortality is measured to characterize the costs of reproduction in the beetle Callosobruchus maculatus, providing explicit details of the timing, duration, magnitude, and acceleration of mortality. We experimentally manipulated reproductive effort in four cohorts of 200 individually housed females by controlling exposure to males and to an artificial oviposition substrate. We demonstrate that (1) early reproduction produces long-term increases in age-specific mortality; (2) egg-laying effort affects the onset of age-specific mortality but not its shape or rate of change; and (3) mating with subsequent reproduction increases the rate of change in age-specific mortality relative to virgins. Accelerated senescence is defined demographically as an increase in the rate of change of age-specific mortality. Our results challenge the hypothesis that reproductive effort accelerates senescence but provides evidence that mating itself may have this effect.     

QUANTITATIVE GENETICS OF BRYOZOAN PHENOTYPIC EVOLUTION. II. ANALYSIS OF SELECTION AND RANDOM CHANGE IN FOSSIL SPECIES USING RECONSTRUCTED GENETIC PARAMETERS

The roles of natural selection and random genetic change in the punctuated phenotypic evolution of eight Miocene-Pliocene tropical American species of the cheilostome bryozoan Metrarabdotos are analyzed by quantitative genetic methods. Trait heritabilities and genetic covariances reconstructed by partitioning within- and among-colony phenotypic variance are similar to those previously obtained for living species of the cheilostome Stylopoma using breeding data. The hypothesis that differences in skeletal morphology between species of Metrarabdotos are entirely due to mutation and genetic drift cannot be rejected for reasonable rates of mutation maintained for periods brief enough to account for the geologically abrupt appearances of these species in the fossil record. Except for one pair of species, separated by the largest morphologic distance, directional selection acting alone would require unrealistically high rates of selective mortality to be maintained for these periods. Thus, directional selection is not strongly implicated in the divergence of Metrarabdotos species. Within species, rates of net phenotypic change are slow enough to require stabilizing selection, but mask large, relatively rapid fluctuations, all of which, however, can be attributed to chance departures from the mean phenotype by mutation and genetic drift, rather than to tracking environmental fluctuation by directional selection. The results are consistent with genetic models involving shifts between multiple adaptive peaks on which phenotypes remain more or less static through long-term stabilizing selection. Regardless of the degree to which directional selection may be involved in peak shifts, phenotypic differentiation is thus related to processes different than the pervasive stabilizing selection acting within species.     

INTERACTIVE EFFECTS OF OFFSPRING SIZE AND TIMING OF REPRODUCTION ON OFFSPRING REPRODUCTION: EXPERIMENTAL,MATERNAL, AND QUANTITATIVE GENETIC ASPECTS

We demonstrate that egg size in side-blotched lizards is heritable (parent-offspring regressions) and thus will respond to natural selection. Because our estimate of heritability is derived from free-ranging lizards, it is useful for predicting evolutionary response to selection in wild populations. Moreover, our estimate for the heritability of egg size is not likely to be confounded by nongenetic maternal effects that might arise from egg size per se because we estimate a significant parent-offspring correlation for egg size in the face of dramatic experimental manipulation of yolk volume of the egg. Furthermore, we also demonstrate a significant correlation between egg size of the female parent and clutch size of her offspring. Because this correlation is not related to experimentally induced maternal effects, we suggest that it is indicative of a genetic correlation between egg size and clutch size. We synthesize our results from genetic analyses of the trade-off between egg size and clutch size with previously published experiments that document the mechanistic basis of this trade-off. Experimental manipulation of yolk volume has no effect on offspring reproductive traits such as egg size, clutch size, size at maturity, or oviposition date. However, egg size was related to offspring survival during adult phases of the life history. We partitioned survival of offspring during the adult phase of the life history into (1) survival of offspring from winter emergence to the production of the first clutch (i.e., the vitellogenic phase of the first clutch), and (2) survival of the offspring from the production of the first clutch to the end of the reproductive season. Offspring from the first clutch of the reproductive season in the previous year had higher survival during vitellogenesis of their first clutch if these offspring came from small eggs. We did not observe selection during these prelaying phases of adulthood for offspring from later clutches. However, we did find that later clutch offspring from large eggs had the highest survival over the first season of reproduction. The differences in selection on adult survival arising from maternal effects would reinforce previously documented selection that favors the production of small offspring early in the season and large offspring later in the season—a seasonal shift in maternal provisioning. We also report on a significant parent-offspring correlation in lay date and thus significant heritable variation in lay date. We can rule out the possibility of yolk volume as a confounding maternal effect—experimental manipulation of yolk volume has no effect on lay date of offspring. However, we cannot distinguish between genetic effects (i.e., heritable) and nongenetic maternal effects acting on lay date that arise from the maternal trait lay date per se (or other unidentified maternal traits). Nevertheless, we demonstrate how the timing of female reproduction (e.g., date of oviposition and date of hatching) affect reproductive attributes of offspring. Notably, we find that date of hatching has effects on body size at maturity and fecundity of offspring from later clutches. We did not detect comparable effects of lay date on offspring from the first clutch.     

ON GENETIC SEGREGATION AND THE EVOLUTION OF SEX

It has recently been argued that because the genetic load borne by an asexual species resulting from segregation, relative to a comparable sexual population, is greater than two, sex can overcome its twofold disadvantage and succeed. We evaluate some of the assumptions underlying this argument and discuss alternative assumptions. Further, we simulate the dynamics of competition between sexual and asexual types. We find that for populations of size 100 and 500 the advantages of segregation do not outweigh the cost of producing males. We conclude that, at least for small populations, drift and the cost of sex govern the evolution of sexuality, not selection or segregation. We believe, however, that if sexual and asexual populations were isolated for a sufficiently long period, segregation might impart a fitness advantage upon sexuals that could compensate for the cost of sex and allow sexuals to outcompete asexuals upon their reunion.     

COMPONENTS OF PHENOTYPIC SELECTION: POLLEN EXPORT AND FLOWER COROLLA WIDTH IN IPOMOPSIS AGGREGATA

In the hummingbird-pollinated herb Ipomopsis aggregata, selection through male function during pollination favors wide corolla tubes. We explored the mechanisms behind this selection, using phenotypic selection analysis to compare effects of corolla width on two components of male pollination success, pollinator visit rate and pollen exported per visit. During single visits by captive hummingbirds, flowers with wider corollas exported more pollen, and more dye used as a pollen analogue, to stigmas of recipient flowers. Corolla width was less strongly related to visit rate in the field, and had no direct effect on visit rate after nectar production and corolla length were controlled for. Moreover, the phenotypic selection differential was 80% higher for the effect on pollen exported per visit, suggesting that this is the more important mechanism of selection.     

QUANTITATIVE GENETIC MODELS FOR DEVELOPMENT,EPIGENETIC SELECTION,AND PHENOTYPIC EVOLUTION

Herein we describe a general multivariate quantitative genetic model that incorporates two potentially important developmental phenomena, maternal effects and epigenetic effects. Maternal and epigenetic effects are defined as partial regression coefficients and phenotypic variances are derived in terms of age-specific genetic and environmental variances. As a starting point, the traditional quantitative genetic model of additive gene effects and random environmental effects is cast in a developmental time framework. From this framework, we first extend a maternal effects model to include multiple developmental ages for the occurrence of maternal effects. An example of maternal effects occurring at multiple developmental ages is prenatal and postnatal maternal effects in mammals. Subsequently, a model of intrinsic and epigenetic effects in the absence of maternal effects is described. It is shown that genetic correlations can arise through epigenetic effects, and in the absence of other developmental effects, epigenetic effects are in general confounded with age-specific intrinsic genetic effects. Finally, the two effects are incorporated into the basic quantitative genetic model. For this more biologically realistic model combining maternal and epigenetic effects, it is shown that the phenotypic regressions of offspring on mother and offspring on father can be used in some cases to estimate simultaneously maternal effects and epigenetic effects.     

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.     

LIGHT AND SCANNING ELECTRON MICROSCOPIC INVESTIGATIONS OF SEXUAL REPRODUCTION IN HYDRA CARNEA

In the freshwater hydrozoan Hydra carnea, the egg, matured and grown between the mesoglea and the epitheliomuscular cells of the ectoderm, is extruded after the emission of the second polar body. Surrounded by a clear jelly layer the egg remains attached to the polyp. Sperm which are released from the testicular sacs of male polyps are attracted by the egg. The jelly layer is penetrated only at the site of emission of the polar bodies by sperm which lack a structurally distinct acrosome. One sperm fuses with the egg at that site where the female pronucleus is found to lie close to the egg membrane. After fertilization and cleavage an acellular embryotheca is secreted by the blastomeres. These events were investigated by light and scanning electron microscopy.     

COSTS OF REPRODUCTION IN THE WILD: PATH ANALYSIS OF NATURAL SELECTION AND EXPERIMENTAL TESTS OF CAUSATION

During 1991 through 1994, natural selection on reproductive effort in side-blotched lizards was indexed by measuring total clutch mass produced on the first clutch of the reproductive season and assessing how such effort in current reproduction affects subsequent survival and clutch production. In addition, selection was also experimentally assessed in free-ranging female side-blotched lizards by (1) surgically decreasing total clutch mass (direct ovarian manipulation) and enhancing clutch mass using (2) exogenous gonadotropin, and (3) exogenous corticosterone. Surgical reduction of clutch mass uniformly enhanced survival. However, increasing clutch mass had more complex effects depending on year. Experimentally enhanced clutch mass enhanced survival in 1991, had no effect on survival in 1992, and decreased survival in 1993. Despite the complexity of these experimental results, they are corroborated by our comparative data. It is important to note that local environmental effects can obscure detection of costs arising from natural variation in reproductive effort, and we removed such effects using path analysis. The striking shift in natural selection favoring females laying a large clutch mass (1991) to selection against females laying a large clutch mass (1993) is associated with an end of a severe multiyear drought. Our natural-history observations suggest that the correlated increase in predatory snake activity on our study site, coincident with the end of the drought, is the agent of natural selection. Although the actual agents of selection (e.g., snake predation versus drought-related effects) are not resolved, the patterns of natural selection measured in our comparative and experimental data are also consistent with year-to-year changes in clutch mass and egg size that would be indicative of rapid short-term evolution in these traits.     

光周期对飞蝗三个地理种群胚胎滞育和生殖的影响(英文)

本文研究了光周期对飞蝗三个地理种群胚胎滞育和生殖的影响。结果表明飞蝗不同地理种群胚胎滞有特性存在显著差异。新疆和硕飞蝗种群无论在长光照还是短光照下均产滞育卵,属于专性滞育;天津北大港和新疆哈密飞蝗种群的胚胎滞育随光周期的不同而变化,长光照和短光照下分别产非滞有卵和滞育卵,属于不同类型的兼性滞育。在纬度等条件差别不大的情况下,海拔高度对滞有特性的影响可能起主导作用。长光照延长哈密和北大港飞蝗种群的产卵前期、降低其产卵频率及产卵量;但对和硕飞蝗种群的这些特性无明显影响。和硕飞蝗种群胚胎打破滞育所需的低温处理时间较哈密和北大港飞蝗种群所需时间长。     

LIFE-CYCLE COMPONENTS OF SELECTION IN ERIGERON ANNUUS: II. GENETIC VARIATION

Genetic variation for seedling and adult fitness components was measured under natural conditions to determine the relative importance of the seedling stage for lifetime fitness in Erigeron annuus. Variation in lifetime reproductive success can result from both the persistent effects of genetic variation expressed among seedlings and from variation in adult fitness components. Analysis of covariance was used to separate the stage specific from the cumulative effects of genetic variance expressed earlier in the life cycle. E. annuus produces seeds through apomixis, which allowed measurement of the fitness of replicate genotypes from germination through the entire life cycle. There were significant differences among genotypes for date of emergence, seedling size, survivorship and fecundity, but heritabilities were low, indicating slow response to selection. For all characters, environmental components of variance were one to two orders of magnitude larger than genetic variance components, resulting in broad sense heritabilities less than 0.1. For seedling size and fecundity, all of the genetic variance was in the form of genotype-environment interactions, often with large negative genetic correlations across environments. In contrast, genotypes differed in mean survivorship through one year, but there were no genotype-environment interactions for viability. Genetic differences in viability were primarily expressed as differences in overwinter survivorship. Genotype × environment interactions among sites and blocks were generated early in the life cycle while the genotype × environment interactions in response to competitive environment (open, annual cover, perennial cover) first appeared in adult fecundity. Genetic variation in lifetime fitness was not significant, despite a fourfold difference in mean fitness among genotypes.