Effects of heat stress on mammalian reproduction

Heat stress can have large effects on most aspects of reproductive function in mammals. These include disruptions in spermatogenesis and oocyte development, oocyte maturation, early embryonic development, foetal and placental growth and lactation. These deleterious effects of heat stress are the result of either the hyperthermia associated with heat stress or the physiological adjustments made by the heat-stressed animal to regulate body temperature. Many effects of elevated temperature on gametes and the early embryo involve increased production of reactive oxygen species. Genetic adaptation to heat stress is possible both with respect to regulation of body temperature and cellular resistance to elevated temperature.     

Selection for canalization at two extra dorsocentral bristles in Drosophila melanogaster.

From 10 isofemale lines of D. melanogaster, the D2 line was established with the aim of obtaining an invariant phenotype at two extra dorsocentral bristles. Line D2 was also subdivided into two other lines, SA and ASD, based on their different bristle patterns. The SA line was selected for two symmetrical anterior extra bristles, and the ASD line was selected for two asymmetrical extra bristles, one anterior and one posterior. Only the SA line showed any canalizing response (estimated by the width of the probit transformation) at the two-extra-bristle class. Nevertheless, the results from the different lines were more consistent with the independent ones of both the anterior and posterior regions of the extra dorsocentral bristles. This analysis showed some independent genetic systems for each region, developmental canalization being at two extra symmetrical bristles per region in all the selected lines (D2, ASD, and SA). Therefore, this canalization did not depend directly on the extra-bristle positional pattern used in the selection. The wild-type canalizing system is suggested to explain the fast canalizing response in a phenotype that had not been previously canalized by natural selection.     

Interactions between Pesticide Genes: Model and Experiment

In response to years of intense selection pressure by organophosphate insecticides, several different insecticide resistance mechanisms have evolved in natural populations of the mosquito Culex pipiens. We examined interactions between two of the most important mechanisms using a four-compartment model of insecticide pharmacokinetics. The joint effect of different mechanisms of resistance can be expressed in terms of epistasis at the physiological level in this model. The type of epistasis predicted by the model depends on the particular physiological mechanisms of resistance involved. Resistance due to a reduced penetration of the insecticide combines multiplicatively with other resistance factors, but resistance due to detoxicative processes and to insensitivity of the target site combines additively. How the pattern of epistasis at the physiological level is translated into fitness epistasis in natural populations of this mosquito depends on the intensity and pattern of insecticide selection in the field.     

细胞凋亡（Apoptosis）与癌基因

细胞凋亡是细胞衰老、死亡过程的主要形式.最近研究发现有多种癌基因与抑癌基因参与细胞凋亡过程.因此目前认为癌基因与抑癌基因不仅控制细胞增殖、分化,而且调节细胞凋亡.细胞凋亡受阻或缺陷可能是肿瘤发生的基础之一.     

Stress modulation of cellular metabolic sensors: interaction of stress from temperature and rainfall on the intertidal limpet Cellana toreuma

In the natural environment, organisms are exposed to large variations in physical conditions. Quantifying such physiological responses is, however, often performed in laboratory acclimation studies, in which usually only a single factor is varied. In contrast, field acclimatization may expose organisms to concurrent changes in several environmental variables. The interactions of these factors may have strong effects on organismal function. In particular, rare events that occur stochastically and have relatively short duration may have strong effects. The present experiments studied levels of expression of several genes associated with cellular stress and metabolic regulation in a field population of limpet Cellana toreuma that encountered a wide range of temperatures plus periodic rain events. Physiological responses to these variable conditions were quantified by measuring levels of mRNA of genes encoding heat‐shock proteins (Hsps) and metabolic sensors (AMPKs and Sirtuin 1). Our results reveal high ratios of individuals in upregulation group of stress‐related gene expression at high temperature and rainy days, indicating the occurrence of stress from both prevailing high summer temperatures and occasional rainfall during periods of emersion. At high temperature, stress due to exposure to rainfall may be more challenging than heat stress alone. The highly variable physiological performances of limpets in their natural habitats indicate the possible differences in capability for physiological regulation among individuals. Our results emphasize the importance of studies of field acclimatization in unravelling the effects of environmental change on organisms, notably in the context of multiple changes in abiotic factors that are accompanying global change.     

The evolutionary and ecological role of heat shock proteins

Most heat shock proteins (Hsp) function as molecular chaperones that help organisms to cope with stress of both an internal and external nature. Here, we review the recent evidence of the relationship between stress resistance and inducible Hsp expression, including a characterization of factors that induce the heat shock response and a discussion of the associated costs. We report on studies of stress resistance including mild stress, effects of high larval densities, inbreeding and age on Hsp expression, as well as on natural variation in the expression of Hsps. The relationship between Hsps and life history traits is discussed with special emphasis on the ecological and evolutionary relevance of Hsps. It is known that up‐regulation of the Hsps is a common cellular response to increased levels of non‐native proteins that facilitates correct protein folding/refolding or degradation of non‐functional proteins. However, we also suggest that the expression level of Hsp in each species and population is a balance between benefits and costs, i.e. a negative impact on growth, development rate and fertility as a result of overexpression of Hsps. To date, investigations have focused primarily on the Hsp70 family. There is evidence that representatives of this Hsp family and other molecular chaperones play significant roles in relation to stress resistance. Future studies including genomic and proteonomic analyses will increase our understanding of molecular chaperones in stress research.     

Diffuse selection on resistance to deer herbivory in the ivyleaf morning glory, Ipomoea hederacea

Recent work defines coevolution between plants and herbivores as pairwise when the pattern of selection on resistance traits and the response to selection are both independent of the presence or absence of other herbivores. In addition, for a pairwise response to selection, resistance to a focal herbivore must have the same genetic basis in the presence and absence of other herbivores. None of these conditions were satisfied for the ivyleaf morning glory, Ipomoea hederacea, and its insect, fungal, and mammalian natural enemies with a quantitative genetics field experiment. A significant negative genetic correlation exists between resistance to deer and generalist insect herbivory that would preclude an independent response to selection. In addition, resistance loci under selection differ depending on the composition of the natural enemy community as indicated by genetic correlations between deer resistances in the presence and absence of other natural enemies that differ substantially from 1. Finally, selection on deer resistance depends on the presence or absence of insects; in the presence of insects, greater deer resistance is favored, but in the absence of insects, deer resistance is effectively neutral. These results indicate that the composition of the natural enemy community can alter both the pattern of selection and the likely response to selection of resistance traits.     

The relation of changes in the individual levels of heat resistance of muscle tissue to their initial values during heat acclimation of Asellus aquaticus

1. 1.|In hog slater, Asellus aquaticus, five extremities were consecutively isolated in the course of heat acclimation to study the pattern of changes in the level of the heat resistance of muscle tissue of each single specimen.

2. 2.|The initial response of the population, during acclimation, is for the muscle resistance of different individuals to become less varied. Then a simultaneous increase in tissue resistance occurs in all ammals, which is complete by the 6th day of acclimation. Afterwards the heat resistance of muscles in the majority of animals shows little change and then, in spite of maintenance of acclimation, it starts to return to its initial level.

3. 3.|Thermal acclimation causes a temporary decrease in the variability of the heat resistance of the muscle tissue and also a temporary stabilization of this physiological characteristic to a new level. This phenomenon is a phenotypical masking of genotypic differences in a physiological characteristic in the population studied during changes in environmental temperature.

4. 4.|At all the stages of acclimation the relation of individual increases in cellular heat resistance to their initial levels follows a hyperbolic exponential equation. This implies that to a rise in environmental temperature a population responds as an integral functional system.

Physiological stress and the maintenance of adaptive genetic variation in a livebearing fish

The importance of genetic variation in evolution is well established. Yet, the mechanisms by which genetic variation—particularly variation in traits under selection—is maintained in natural populations has long been an evolutionary puzzle. Understanding individual variables driving selection and their functional mechanisms is increasingly important in the context of global change and its potential consequences for biodiversity. Here we examined intra-population performance among allelic variants of a pleiotropic locus in response to thermal stress in the variable platyfish, Xiphophorus variatus. The wild-type tailspot allele exhibited significantly lower heat tolerance than all three pattern alleles found in the population, conforming to predictions based on previously observed correlations between temperature and pattern frequencies in the wild. Furthermore, differences between tailspot pattern frequencies in adults and juveniles were broadly consistent with this trend. Thus, it appears that physiological stress and reduced performance of the wild-type allele at higher relative temperatures is a mechanism balancing its frequency in natural populations. Temperature variation and not dissolved oxygen alone, as previously reported, is likely a important abiotic variable contributing to the maintenance of adaptive polymorphism. Furthermore, our findings underscore the potential implications of rising temperatures and physiological stress for levels of genetic variation in natural populations.     

Epigenetic control of cell specification during female gametogenesis

In flowering plants, the formation of gametes depends on the differentiation of cellular precursors that divide meiotically before giving rise to a multicellular gametophyte. The establishment of this gametophytic phase presents an opportunity for natural selection to act on the haploid plant genome by means of epigenetic mechanisms that ensure a tight regulation of plant reproductive development. Despite this early acting selective pressure, there are numerous examples of naturally occurring developmental alternatives that suggest a flexible regulatory control of cell specification and subsequent gamete formation in flowering plants. In this review, we discuss recent findings indicating that epigenetic mechanisms related to the activity of small RNA pathways prevailing during ovule formation play an essential role in cell specification and genome integrity. We also compare these findings to small RNA pathways acting during gametogenesis in animals and discuss their implications for the understanding of the mechanisms that control the establishment of the female gametophytic lineage during both sexual reproduction and apomixis.     

Action of natural selection in lines of Drosophila selected for a new level of canalization

Summary Four lines of Drosophila melanogaster previously selected for a stabilized phenotype of two extra dorsocentral bristles were examined for 20 generations of canalizing selection and relaxation of selection. A substantial frequency of flies with either two anterior or two posterior extra bristles was maintained in the relaxed lines. These patterns were the only ones tolerated by natural selection, i.e., the only symmetric ones. It was concluded that anterior and posterior dorsocentral bristles are two independent development structures, and the results are discussed in relation to two proposed genetic systems for bristle determination.     

MATING PATTERN AND FITNESS-COMPONENT ANALYSIS ASSOCIATED WITH INVERSION POLYMORPHISM IN A NATURAL POPULATION OF DROSOPHILA BUZZATII

Direct studies of mating success or mating pattern associated with Mendelian factors rarely have been carried out in nature. From the samples taken for the standard analyses of selection components, it is not usually possible to obtain the mating table, and only directional selection for male mating success can be detected. Both processes, mating pattern and differential mating probability, together with other fitness components, have been investigated for the inversion polymorphism of a natural population of the cactophilic species Drosophila buzzatii. Two independent samples of adult flies were collected: nonmating or single individuals (base population) and mating pairs (mating population). All individuals were karyotyped for the second and fourth chromosomes. A sequence of models with increasing simplicity was fitted to the data to test null hypotheses of no selection and random union of gametes and karyotypes. The main results were (1) no deviations from random mating were found; (2) differential mating probability was nonsignificant in both sexes; (3) inversion and karyotypic frequencies did not differ between sexes; and (4) karyotypic frequencies did not depart from Hardy-Weinberg expectations. These results are discussed in light of complementary evidence showing the need for interpreting with caution no-effect hypotheses such as the ones tested here. The use of complementary selective tests in these studies is suggested.     

Response of Two Heat Shock Genes to Selection for Knockdown Heat Resistance in Drosophila Melanogaster

To identify genes involved in stress resistance and heat hardening, replicate lines of Drosophila melanogaster were selected for increased resistance to knockdown by a 39° heat stress. Two selective regimes were used, one with and one without prior hardening. Mean knockdown times were increased from ~5 min to >20 min after 18 generations. Initial realized heritabilities were as high as 10% for lines selected without hardening, and crosses between lines indicated simple additive gene effects for the selected phenotypes. To survey allelic variation and correlated selection responses in two candidate stress genes, hsr-omega and hsp68, we applied denaturing gradient gel electrophoresis to amplified DNA sequences from small regions of these genes. After eight generations of selection, allele frequencies at both loci showed correlated responses for selection following hardening, but not without hardening. The hardening process itself was associated with a hsp68 frequency change in the opposite direction to that associated with selection that followed hardening. These stress loci are closely linked on chromosome III, and the hardening selection established a disequilibrium, suggesting an epistatic effect on resistance. The data indicate that molecular variation in both hsr-omega and hsp68 contribute to natural heritable variation for hardened heat resistance.     

A POPULATION GENETIC THEORY OF CANALIZATION

Canalization is the suppression of phenotypic variation. Depending on the causes of phenotypic variation, one speaks either of genetic or environmental canalization. Genetic canalization describes insensitivity of a character to mutations, and the insensitivity to environmental factors is called environmental canalization. Genetic canalization is of interest because it influences the availability of heritable phenotypic variation to natural selection, and is thus potentially important in determining the pattern of phenotypic evolution. In this paper a number of population genetic models are considered of a quantitative character under stabilizing selection. The main purpose of this study is to define the population genetic conditions and constraints for the evolution of canalization. Environmental canalization is modeled as genotype specific environmental variance. It is shown that stabilizing selection favors genes that decrease environmental variance of quantitative characters. However, the theoretical limit of zero environmental variance has never been observed. Of the many ways to explain this fact, two are addressed by our model. It is shown that a “canalization limit” is reached if canalizing effects of mutations are correlated with direct effects on the same character. This canalization limit is predicted to be independent of the strength of stabilizing selection, which is inconsistent with recent experimental data (Sterns et al. 1995). The second model assumes that the canalizing genes have deleterious pleiotropic effects. If these deleterious effects are of the same magnitude as all the other mutations affecting fitness very strong stabilizing selection is required to allow the evolution of environmental canalization. Genetic canalization is modeled as an influence on the average effect of mutations at a locus of other genes. It is found that the selection for genetic canalization critically depends on the amount of genetic variation present in the population. The more genetic variation, the stronger the selection for canalizing effects. All factors that increase genetic variation favor the evolution of genetic canalization (large population size, high mutation rate, large number of genes). If genetic variation is maintained by mutation-selection balance, strong stabilizing selection can inhibit the evolution of genetic canalization. Strong stabilizing selection eliminates genetic variation to a level where selection for canalization does not work anymore. It is predicted that the most important characters (in terms of fitness) are not necessarily the most canalized ones, if they are under very strong stabilizing selection (k > 0.2V_e). The rate of decrease of mutational variance V_m is found to be less than 10% of the initial V_m. From this result it is concluded that characters with typical mutational variances of about 10^–3 V_e are in a metastable state where further evolution of genetic canalization is too slow to be of importance at a microevolutionary time scale. The implications for the explanation of macroevolutionary patterns are discussed.     

The correlation between the heat resistance of the zygotes of the common frog Rana temporaria and its embryos at different developmental stages

A study has been made of a correlation between the heat resistance of zygotes and embryos of the same clutches at different stages of development of Rana temporaria L. The embryos were incubated at 19 degrees C, the injurious temperature being 37 degrees C. As criterion of heat resistance served the time of the injurious temperature action which leads by the ++stra of cleavage to a 50 per cent elimination of embryos (LD50). The correlation in question has been evaluated by the rank correlation coefficient (rho). The correlation between the heat resistance of zygotes and embryos at the start of gastrulation (stage 11) was moderate (rho = 0.47). At one of neurulation stages (stage 22) this correlation was weak (rho = 0.207), to become rather high just before the cleavage (stage 28, rho = 0.63). It is assumed that the thermal selection of zygotes, i.e. of the organism at the cellular stage of development, may be oriented to result in the survival of embryos, which just before the cleavage will show both elevated heat resistance and high heritability of this character.     

Population variation in the cost and benefit of tolerance and resistance against herbivory in Datura stramonium

In this study we examine the hypothesis that divergent natural selection produces genetic differentiation among populations in plant defensive strategies (tolerance and resistance) generating adaptive variation in defensive traits against herbivory. Controlled genetic material (paternal half-sib families) from two populations of the annual Datura stramonium genetically differentiated in tolerance and resistance to herbivory were used. This set of paternal half-sib families was planted at both sites of origin and the pattern of genotypic selection acting on tolerance and resistance was determined, as well as the presence and variation in the magnitude of allocational costs of tolerance. Selection analyses support the adaptive differentiation hypothesis. Tolerance was favored at the site with higher average level of tolerance, and resistance was favored at the site with higher average level of resistance. The presence of significant environmentally dependent costs of tolerance was in agreement with site variation in the adaptive value of tolerance. Our results support the expectation that environmentally dependent costs of plant defensive strategies can generate differences among populations in the evolutionary trajectory of defensive traits and promote the existence of a selection mosaic. The pattern of contrasting selection on tolerance suggests that, in some populations of D. stramonium, tolerance may alter the strength of reciprocal coevolution between plant resistance and natural enemies.     

A physiological barrier for the maintenance of anisogamous sex

There are at least four conceivable factors which could be responsible for the maintenance of anisogamous sex: (1) long-term group selection, (2) short-term ecological, natural selection, (3) trade-offs between female sexual fertility and parthenogenetic fertility and (4) female dependence on a male gametic contribution. There is evidence for the action of each of the first three factors, while the fourth seems to be evolutionarily impossible, at first sight. But selection on male and female gamete production physiology could, in principle, produce female germ lines which depend on male gametes for their perpetuation. There is circumstantial evidence that this has in fact occurred in the Mammalia, and critical experiments to test this hypothesis are proposed.     

EXPERIMENTAL MANIPULATION OF PUTATIVE SELECTIVE AGENTS PROVIDES EVIDENCE FOR THE ROLE OF NATURAL ENEMIES IN THE EVOLUTION OF PLANT DEFENSE

Although biologists have long assumed that plant resistance characters evolved under selection exerted by such natural enemies as herbivores and pathogens, experimental evidence for this assumption is sparse. We present evidence that natural enemies exert selection on particular plant resistance characters. Specifically, we demonstrate that elimination of natural enemies from an experimental field population of Arabidopsis thaliana alters the pattern of selection on genetic variation in two characters that have been shown to reduce herbivore damage in the field: total glucosinolate concentration and trichome density. The change in pattern of selection reveals that natural enemies imposed selection favoring increased glucosinolate concentration and increased trichome density, and thus, supports one of the major assumptions of the coevolution hypothesis. We also demonstrate that a pattern of stabilizing selection on glucosinolate concentration results from a balance between the costs and benefits associated with increasing levels of this resistance character. This result provides direct confirmation of the appropriateness of cost-benefit models for characterizing the evolution of plant defenses.