R. Bijlsma

Environmental stress, adaptation and evolution: an overview

Extreme environments and adaptation.- The evolution of plants in metal-contaminated environments.- Responses of aquatic organisms to pollutant stress: Theoretical and practical implications.- Conifers from the cold.- Genetic variation and environmental stress.- Phenotypic plasticity and fluctuating asymmetry as responses to environmental stress in the butterflyBicyclus anynana.- Environmental stress and the expression of genetic variation.- Worldwide latitudinal clines for the alcohol dehydrogenase polymorphism in Drosophila melanogaster: What is the unit of selection?.- Stress and metabolic regulation inDrosophila.- Acclimation and response to thermal stress.- Phenotypic and evolutionary adaptation of a model bacterial system to stressful thermal environments.- Ecological and evolutionary physiology of heat shock proteins and the stress response inDrosophila: Complementary insights from genetic engineering and natural variation.- High-temperature stress and the evolution of thermal resistance inDrosophila.- Stress, selection and extinction.- Genetic and environmental stress, and the persistence of populations.- Adaptation and extinction in changing environments.- Environmental stress and evolution: A theoretical study.- Stress, developmental stability and sexual selection.- Evolution and stress.- Genetic variability and adaptation to stress.- Stress-resistance genotypes, metabolic efficiency and interpreting evolutionary change.- The Plus ca change model: Explaining stasis and evolution in response to abiotic stress over geological timescales.

Does inbreeding affect the extinction risk of small populations?: predictions from Drosophila

A fundamental assumption underlying the importance of genetic risks within conservation biology is that inbreeding increases the extinction probability of populations. Although inbreeding has been shown to have a detrimental impact on individual fitness, its contribution to extinction is still poorly understood. We have studied the consequences of different levels of prior inbreeding for the persistence of small populations using Drosophila melanogaster as a model organism. To this end, we determined the extinction rate of small vial populations differing in the level of inbreeding under both optimal and stress conditions, i.e. high temperature stress and ethanol stress. We show that inbred populations have a significantly higher short‐term probability of extinction than non‐inbred populations, even for low levels of inbreeding, and that the extinction probability increases with increasing inbreeding levels. In addition, we observed that the effects of inbreeding become greatly enhanced under stressful environmental conditions. More importantly, our results show that the impact of environmental stress becomes significantly greater for higher inbreeding levels, demonstrating explicitly that inbreeding and environmental stress are not independent but can act synergistically. These effects seem long lasting as the impact of prior inbreeding was still qualitatively the same after the inbred populations had been expanded to appreciable numbers and maintained as such for approximately 50 generations. Our observations have significant consequences for conservation biology.

DIRECT SELECTION ON LIFE-SPAN IN DROSOPHILA-MELANOGASTER

An important issue in the study of the evolution of aging in Drosophila melanogaster is whether decreased early fecundity is inextricably coupled with increased life span in selection experiments on age at reproduction. Here, this problem has been tackled using an experimental design in which selection is applied directly to longevity. Selection appeared successful for short and long life, in females as well as males. Progeny production of females selected for long life was lower than for short‐lived females throughout their whole life. No increase of late‐life reproduction in long‐lived females occurred, as has been found in selection experiments on age at reproduction. This discrepancy is explained in terms of the inadequacy of the latter design to separate selection on life span from selection on late‐life fecundity. Moreover, starvation resistance and fat content were lower for adults selected for short life. In general, the data support the negative‐pleiotropy–disposable‐soma theory of aging, and it is hypothesized that the pleiotropic allocation of resources to maintenance versus to reproduction as implicated in the theory might involve lipid metabolism. It is argued that further research on this suggestion is urgent and should certainly comprise observations on male reproduction because these are for the greater part still lacking. In conclusion, the longevity of D. melanogaster can be genetically altered in a direct‐selection design, and such an increase is accompanied by a decreased general reproduction and thus early reproduction.

The significance of genetic erosion in the process of extinction. I : Genetic differentiation in Salvia pratensis and Scabiosa columbaria in relation to population size

As part of a programme to determine the importance of the loss of genetic variation for the probability of population extinction, the amount of allozyme variation was determined in 14 populations of Salvia pratensis and in 12 populations of Scabiosa columbaria. Significant correlations were found between population size and the proportion of polymorphic loci (Salvia: r=0.619; Scabiosa: r=0.713) and between population size and mean observed number of alleles per locus (Salvia: r=0.540; Scabiosa: r=0.819). Genetic differentiation was substantially larger among small populations than among large populations: in Salvia GST was 0.181 and 0.115, respectively, and in Scabiosa 0.236 and 0.101, respectively. The results are discussed in relation to genetic drift, inbreeding and restricted gene flow.

Conservation genetics in transition to conservation genomics.

Over the past twenty years conservation genetics has progressed from being mainly a theory-based field of population biology to a full-grown empirical discipline. Technological developments in molecular genetics have led to extensive use of neutral molecular markers such as microsatellites in conservation biology. This has allowed assessment of the impact of genetic drift on genetic variation, of the level of inbreeding within populations, and of the amount of gene flow between or within populations. Recent developments in genomic techniques, including next generation sequencing, whole genome scans and gene-expression pattern analysis, have made it possible to step up from a limited number of neutral markers to genome-wide estimates of functional genetic variation. Here, we focus on how the transition of conservation genetics to conservation genomics leads to insights into the dynamics of selectively important variation and its interaction with environmental conditions, and into the mechanisms behind this interaction.

Environmental dependence of inbreeding depression and purging in Drosophila melanogaster

Elimination or reduction of inbreeding depression by natural selection at the contributing loci (purging) has been hypothesized to effectively mitigate the negative effects of inbreeding in small isolated populations. This may, however, only be valid when the environmental conditions are relatively constant. We tested this assumption using Drosophila melanogaster as a model organism. By means of chromosome balancers, chromosomes were sampled from a wild population and their viability was estimated in both homozygous and heterozygous conditions in a favourable environment. Around 50% of the chromosomes were found to carry a lethal or sublethal mutation, which upon inbreeding would cause a considerable amount of inbreeding depression. These detrimentals were artificially purged by selecting only chromosomes that in homozygous condition had a viability comparable to that of the heterozygotes (quasi‐normals), thereby removing most deleterious recessive alleles. Next, these quasi‐normals were tested both for egg‐to‐adult viability and for total fitness under different environmental stress conditions: high‐temperature stress, DDT stress, ethanol stress, and crowding. Under these altered stressful conditions, particularly for high temperature and DDT, novel recessive deleterious effects were expressed that were not apparent under control conditions. Some of these chromosomes were even found to carry lethal or near‐lethal mutations under stress. Compared with heterozygotes, homozygotes showed on average 25% additional reduction in total fitness. Our results show that, except for mutations that affect fitness under all environmental conditions, inbreeding depression may be due to different loci in different environments. Hence purging of deleterious recessive alleles can be effective only for the particular environment in which the purging occurred, because additional load will become expressed under changing environmental conditions. These results not only indicate that inbreeding depression is environment dependent, but also that inbreeding depression may become more severe under changing stressful conditions. These observations have significant consequences for conservation biology.

THE EFFECTS OF POPULATION-SIZE AND PLANT-DENSITY ON OUTCROSSING RATES IN LOCALLY ENDANGERED SALVIA-PRATENSIS

Multilocus outcrossing rates were estimated in natural and experimental populations of Salvia pratensis, an entomophilous, gynodioecious, protandrous perennial. Male steriles were used to check the estimation procedure of outcrossing rates in hermaphrodites. Estimates of outcrossing rates in hermaphroditic plants ranged from 38.2% to 81.8% in natural populations and from 71.5% to 95.5% in experimental populations. No correlations were found between outcrossing rates and population size. However, outcrossing in hermaphrodites was promoted by high plant densities and low frequencies of male steriles. It is argued that effective management to preserve genetic variation in populations of S. pratensis should provide for the maintenance of high plant densities.

Genetic erosion impedes adaptive responses to stressful environments

Biodiversity is increasingly subjected to human‐induced changes of the environment. To persist, populations continually have to adapt to these often stressful changes including pollution and climate change. Genetic erosion in small populations, owing to fragmentation of natural habitats, is expected to obstruct such adaptive responses: (i) genetic drift will cause a decrease in the level of adaptive genetic variation, thereby limiting evolutionary responses; (ii) inbreeding and the concomitant inbreeding depression will reduce individual fitness and, consequently, the tolerance of populations to environmental stress. Importantly, inbreeding generally increases the sensitivity of a population to stress, thereby increasing the amount of inbreeding depression. As adaptation to stress is most often accompanied by increased mortality (cost of selection), the increase in the ‘cost of inbreeding’ under stress is expected to severely hamper evolutionary adaptive processes. Inbreeding thus plays a pivotal role in this process and is expected to limit the probability of genetically eroded populations to successfully adapt to stressful environmental conditions. Consequently, the dynamics of small fragmented populations may differ considerably from large nonfragmented populations. The resilience of fragmented populations to changing and deteriorating environments is expected to be greatly decreased. Alleviating inbreeding depression, therefore, is crucial to ensure population persistence.

On the developmental theory of ageing. I, Starvation resistance and longevity in Drosophila melanogaster in relation to pre-adult breeding conditions

The developmental theory of ageing predicts a positive correlation between developmental time and adult longevity. Experiments that vary larval density and food level have been carried out to test this prediction. The results show differences in viability, developmental time, starvation resistance and adult longevity. It is concluded that pre-adult developmental time is not a causal factor for the determination of adult longevity in Drosophila melanogaster. The observed variation in adult longevity is discussed in relation to viability selection and changed adult physiology.

The significance of genetic erosion in the process of extinction. IV. Inbreeding depression and heterosis effects caused by selfing and outcrossing in Scabiosa columbaria

The effects of self‐fertilization, within‐population crosses (WPC) and between‐population crosses (BPC) on progeny fitness were investigated in the greenhouse for Scabiosa columbaria populations of varying size. Plants grown from field collected seeds were hand pollinated to produce selfed, WPC, and BPC progeny. The performance of these progenies was examined throughout the entire life cycle. The different pollination treatments did not significantly affect germination, seedling‐to‐adult survival, flowering percentage and the number of flower heads. But severe inbreeding depression was demonstrated for biomass production, root development, adult survival, and seed set. Additionally, multiplicative fitness functions were calculated to compare relative fitnesses for progeny. On average, WPC progeny showed a more than 4‐fold, and BPC progeny an almost 10‐fold, advantage over selfed progeny, indicating that S. columbaria is highly susceptible to inbreeding. No clear relationship was found between population size and level of inbreeding depression, suggesting that the genetic load has not yet been reduced substantially in the small populations. A significant positive correlation was found between plant dry weight and total fitness. In two out of six populations, the differences between the effects of the pollination treatments on dry weight increased significantly when seedlings were grown under competitive conditions. This result is interpreted as an enhancement of inbreeding depression under these conditions. It is argued that improvement of the genetic exchange between populations may lower the probability of population extinction.