Environmental variation and life-history evolution: experiments on Caenorhabditis remanei

QL Zoology;Q Science (General)

Diaz-Palacios, Sylvia Anaid ; Lindstrom, Jan

University:University of Glasgow

关键词: Phenotypic plasticity;    adaptation;    environmental variation;    life-history evolution;    Caenorhabditis remanei;    selection experiment;   

Others  :  http://theses.gla.ac.uk/744/1/2009DiazPalaciosphd.pdf

来源: University of Glasgow

【 摘 要 】

Organisms are constantly altering their phenotypes in response to changing environments.Many of these differences are known to be due to genetic changes. However, some of the differences between individuals will be due to phenotypic plasticity. Phenotypic plasticity is the property of a given genotype to produce different phenotypes in response to distinct environments (Pigliucci 2001). Phenotypic plasticity can be adaptive and may provide with the means to thrive across a wide range of environments. Thus it represents one solution to surviving in a variable environment.Maintaining high population genetic variance is also recognized as enabling a population to respond to a changing environment. Both constitute phenotypic responses to changing environments, but rely on quite different mechanisms. The purpose of my project is to examine by what means, population history can influence the responsiveness of populations to environmental change. In order to approach this question I used a model species (Caenorhabditis remanei) and selection experiments in the laboratory. Caenorhabditis species are widely used in research, for instance, to study mechanisms affecting gene expression and their effects on individual’s phenotype. Despite this, we have a limited understanding of the importance of environmental factors that controltheir demography in the laboratory or in nature. Particularly, the demography of other nematode species other than C. elegans has until very recently been ignored. Thus, I described the basic demography of C. remanei cultured under standard laboratory conditions.I compared the life history of two geographically distant populations of C. remanei under standard laboratory conditions. Differences between populations were expected to be present as a consequence of local adaptation to environmental conditions. My results show that C. remanei cultured in the laboratory has a short generation time, but it is surprisingly similar to the generation time of C. elegans. Moreover, I found that there was little difference in the life history across populations. Between individuals, I found high phenotypic variance, which would be partially the result of high genetic diversity within the population.C. elegans and C. remanei are morphologically indistinguishable. However, they differ in their reproductive biology; the former facutatively reproduces by selfing, whereas the latter can only produce progeny by crossing (hermaphroditism and gonochorism, respectively). Sexual conflict, different reproductive strategies between males and females, has previously been identified in the soil nematode of C. elegans. However, evidence of sexual conflict is lacking in gonochoristic species of nematode. Thus, I conducted an experiment to examine the effect of the number of males present on females’ fecundity and survival rate.My results show that increasing the number of males increases female fecundity. Thus, suggesting that C. remanei females are sperm limited. However, there is a threshold, a further increase in the number of males reduced survival rate. These results are in agreement with the theory of sexual conflict.Environmentally-dependent traits are universally common across species.For C. remanei, life-history traits such as fecundity and survival are expected to be genetic and environmentally dependent, but these dependencies remain very poorly understood. Thus, in order to improve our understanding of the response of C. remanei’s life history traits to changing environments; I exposed three populations of worms (two wild type isolates and a half-diall cross between them) to six temperatures and assessed their response. I used a half-sib breeding design as a means to estimate gene-environment interaction for all traits. Differences between populations were expected to be due to differences in genetic composition. I found that C. remanei fecundity is optimal at 17 °C, a higher growth temperature than that established for C. elegans. Although worms cultured at 5 and 30 °C significantly reduced their fecundity, it was still permissive for some individuals.Not all plastic traits are expected to be adaptive. It is recognised that heterogeneous environments select for plasticity. Thus, in order to manipulate the plasticity levels, I maintained populations for 50 generations in two different environments: constant temperature and predictably fluctuating temperature. Life-history components were quantified at three times during the course of the experiment (generation 1, 20 and 50). If plasticity is adaptive, it could be under strong selection in the fluctuating environment. After the selection experiment, comparisons between populations evolved in these different environments allowed me to quantify how two different evolutionary pressures shaped strains’ life history, and how this response depended on likely levels of genetic diversity (i.e. between the pure strains and the hybrid). In both environments, I found changes in the reproductive schedules. Although I did not detect significant changes in the lifetime fecundity after the selection experiment, females showed an increase in their early fecundity. This shift in reproductive parameters shows adaptation as a consequence of the environmental pressures. These results are in agreement with the theory of life-history evolution.In theory, a plastic genotype has a wider ecological breath compared with one with reduced or no plasticity. After 50 generations in each environment, populations were assayed at three temperatures to assess whether population history can influence the responsiveness of populations (e.g. tolerance to temperature). Higher levels of plasticity (i.e. tolerance) were expected in populations maintained in a fluctuating environment compared to the more stable environment. I found that worms from a fluctuating environment showed an increase in their tolerance to stressful conditions, while worms cultured in a constant environment showed no change. Thus, I successfully selected for populations with high and low levels of plasticity.Adaptive plasticity is expected to increase individual’s fitness across a range of environments because it expresses the “matching” phenotype according to environmental cues. However, a plastic genotype with the machinery to match the environment could be at disadvantage compared to a less plastic genotype when the environment is not changing. This disadvantage is expected to be linked to the reallocation of resources in the maintenance of genetic and cellular machinery that enables it to detect changes in the environment and in the production of the matching phenotype. Thus, to test this hypothesis, I translocated populations between the two environments. After the translocation, plastic worms moved back into the constant environment reproduced very poorly compared to worms before the selection took place and compared to the less plastic worms (reared in a constant environment). This strongly supports the idea that plastic strategies can turn an individual into “The Jack of all trades, but Master of none”.