Evolutionary mechanisms responsible for genetic and phenotypic variation

2015-08-11Genetic variation is essential for natural selection to operate and facilitate evolutionary processes. Identifying the mechanisms that influence the degree of genetic variation is important for our understanding of population differences and species diversity. Mechanisms that produce genetic variation are those that generate mutations in nucleotide sequences; such mechanisms include replication error, DNA damage and insertions and deletions caused by transposable element transpositions. A portion of my dissertation work will focus on transposable elements, which are mobile genetic elements that replicate by inserting themselves into the host genome. Like mutations caused by replication error or DNA damage, these insertions are often deleterious; however, in some cases can lead to new genetic variants contributing to phenotypic variation. Although the strength and effectiveness of natural selection is proportional to the amount of genetic variation, high rates of mutation is detrimental and organisms have evolved mechanisms to prevent mutations from occurring. Chapter 2 examines the coevolution of transposable elements (TE) and their host to identify mechanisms that prevent transpositions and control TE copy number. This study focuses on genetic variation and the population dynamics of TE host silencing pathways, specifically the copia long-terminal-repeat retrotransposon in Drosophila melanogaster. ❧ Reduced genetic variation is an inevitable consequence of natural selection, but studies in both morphological and physiological traits have detected considerable amounts of genetic and phenotypic variation within and among populations and species. This indicates that there are evolutionary mechanisms that work to maintain genetic and phenotypic variation. Differential selection due to spatial and temporal environmental variation can lead to the maintenance of genetic and phenotypic variation. Genetic differentiation is often seen in populations isolated by large-scale geographical distances; however, population differentiation can also occur within smaller geographical distances, where microenvironmental variability within a single habitat can facilitate differential selection. Chapter 3 focuses on the maintenance of genetic and phenotypic variation in a natural plant population adapted to a heterogeneous environment. This study examines local adaptation of a single self-fertilizing population of an emerging model legume, Medicago truncatula, and identifies patterns of microgeographcial adaptation to a saline environment exhibiting high variability in soil salinity levels. ❧ Adaptation to spatial and temporal environmental variability has been hypothesized to favor the evolution of phenotypic plasticity. Phenotypic plasticity is defined as a single genotype expressing alternative phenotypes in response to different environments or environmental cues. This type of phenotypic flexibility allows organisms to adapt to climate change and inhabit a broader range of environments. Phenotypic plasticity is especially important for sessile organisms that have little choice in their growth conditions. Plants, for instance, rely on environmental cues to make developmental transitions as well as adaptively adjust physiological and developmental traits in response to abiotic and biotic stress. In order for plasticity to evolve, there must be genotypic differences in phenotypic plasticity—this kind of genetic variation is known as genotype-environment interactions. In addition to the immediate environment, the parental environment also plays a pivotal role in influencing plastic plant growth and development. This form of plasticity is known as transgenerational plasticity, where cues from the parental environment are transmitted to influence offspring phenotype. Transgenerational plasticity contributes to phenotypic variation and can influence the strength of selection and rate of adaptation. In order for transgenerational plasticity to itself evolve, it must be heritable and exhibit variation among genotypes. Chapter 4 test for genetic variation in transgenerational plasticity of natural Medicago truncatula genotypes and identifies novel molecular mechanisms that are influenced by the parental environment to facilitate changes in offspring development and response. This study examines parental exposure to salinity stress and test whether the parental environment influences offspring germination behavior and the expression of stored seed transcripts (necessary for seed germination) to infer biological pathways mediating transgenerational plastic offspring germination response