Browsing by Subject "Mimulus"
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Item Open Access Evolution of Floral Color Patterning in Chilean Mimulus(2008-12-05) Cooley, Arielle MarieEvolution can be studied at many levels, from phenotypic to molecular, and from a variety of disciplines. An integrative approach can help provide a more complete understanding of the complexities of evolutionary change. This dissertation examines the ecology, genetics, and molecular mechanisms of the evolution of floral anthocyanin pigmentation in four species of Mimulus native to central Chile. Anthocyanins, which create red and purple colors in many plants, are a valuable model for studying evolutionary processes. They are ecologically important and highly variable both within and between species, and the underlying biosynthetic pathway is well characterized. The focus of this dissertation is dramatic diversification in anthocyanin coloration, in four taxa that are closely related to the genomic model system M. guttatus. I posed three primary questions: (1) Is floral diversification associated with pollinator divergence? (2) What is the genetic basis of the floral diversification? (3) What is the molecular mechanism of the increased production of anthocyanin pigment? The first question was addressed by evaluating patterns of pollinator visitation in natural populations of all four study taxa. The second question was explored using segregation analysis for a series of inter- and intraspecific crosses. One trait, increased petal anthocyanins in M. cupreus, was further dissected at the molecular level, using candidate gene testing and quantitative gene expression analysis. Pollinator studies showed little effect of flower color on pollinator behavior, implying that pollinator preference probably did not drive pigment evolution in this group. However, segregation analyses revealed that petal anthocyanin pigmentation has evolved three times independently in the study taxa, suggesting an adaptive origin. In addition to pollinator attraction, anthocyanins and their biochemical precursors protect against a variety of environmental stressors, and selection may have acted on these additional functions. Molecular analysis of petal anthocyanins in M. cupreus revealed that this single-locus trait maps to a transcription factor, McAn1, which is differentially expressed in high- versus low-pigmented flowers. Expression of the anthocyanin structural genes is tightly correlated with McAn1 expression. The results suggest that M. cupreus pigmentation evolved by a mutation cis to McAn1 that alters the intensity of anthocyanin biosynthesis.
Item Open Access One Gene or Many? Different Genetic Mechanisms Drive Convergent Evolution in Monkey Flowers(2016-04-24) Farnitano, MatthewEvolutionary history is riddled with examples of convergent evolution, in which the same adaptation appears independently across multiple populations as a result of similar selective pressures. Convergent evolution can proceed by identical, similar, or unrelated genetic mechanisms. The relative frequencies of these scenarios and the evolutionary constraints that produce them are poorly understood, partly due to a lack of diverse comparative models. One such promising model is repeated adaptation to serpentine soil, a soil environment characterized by abundant heavy metals, low nutrient content, and poor water retention. Many species of Mimulus monkey flowers have populations that can tolerate these soils, but most Mimulus populations cannot. I compared the genetic signatures of serpentine tolerance across the genomes of four tolerant Mimulus populations, in order to determine whether these different species employed similar genetic mechanisms. Previous work has identified a single locus strongly connected with serpentine tolerance in M. guttatus tolerant populations. I found that M. glaucescens also likely has a single or small number of loci responsible for this adaptation. However, serpentine tolerance in M. nudatus appears to be controlled by many genes of smaller effect, rather than a single locus. This vastly different genetic mechanism is surprising given the close evolutionary and ecological relationships of these species. The results of this study show that, even under similar circumstances, evolution can employ very different genetic strategies.Item Open Access The Genetic Basis of Local Adaptation to Serpentine Soils in Mimulus guttatus(2014) Selby, JessicaWhile local adaptation has been frequently demonstrated via reciprocal transplant experiments, our understanding of the genetic basis of it remains minimal. There is a notable lack of studies that identify naturally segregating variants, determine the traits controlled by these variants and characterize their fitness effects in the field. Such studies are critical for understanding how spatially varying selective pressures can drive population divergence and maintain genetic variation. The experiments presented here aim to characterize the genetic basis of local adaptation to serpentine soils in Mimulus guttatus. First, I show that serpentine and non-serpentine populations of M. guttatus are locally adapted to soil habitat wherein non-serpentine plants are unable to survive on serpentine soils. Serpentine tolerance appears to come at a cost as serpentine plants are smaller in the juvenile stage than non-serpentine plants when grown at non-serpentine field sites. These size differences may limit the competitive ability of serpentine tolerant plants in non-serpentine habitats which tend to be more heavily vegetated than serpentine habitats. Next I identify environmental variables that are important selective agents in the serpentine habitat. Using hydroponic assays to isolate an individual chemical variable of serpentine soils - low calcium levels to high magnesium levels (low Ca:Mg ratio) - I show that serpentine and non-serpentine populations of M. guttatus have significant differences in tolerance to low Ca:Mg. I then characterize the genetic basis of these ecotypic differences in survival and tolerance using quantitative trait locus (QTL) mapping. I identify a single, major QTL that controls both the ability to survive on serpentine soils and tolerance to low Ca:Mg ratio which suggests that M. guttatus populations have adapted to serpentine soils through an ability to tolerate the low levels of Ca while simultaneously not suffering from Mg toxicity. Furthermore, I show that this same QTL controls ability to survive on serpentine soils in a second, geographically distant population. However, preliminary work suggests that the two populations are not equally tolerant to each other's soils indicating that either other loci also contribute to serpentine tolerance and these are not shared between the two serpentine populations or that there are different serpentine tolerance alleles at the major QTL are not functionally equivalent. This work addresses long-standing questions in evolutionary biology regarding the number and effect size of loci that underlie adaptive traits by identifying a large effect locus that contributes to adaptive differences between M. guttatus populations.
Item Open Access The Role of Polyploidy in Phenotypic and Genomic Evolution in the Shy Monkeyflower, Mimulus sookensis(2012) Modliszewski, Jennifer LouiseIn an ever-changing world, evolution is an essential process that may allow organisms to adapt to their environment through natural selection. All evolutionary processes act through a single fundamental medium: genetic variation. Polyploidy, or whole genome duplication, is a major mechanism for evolutionary change because it is both widespread across taxa and results in a proliferation of genetic material that evolution can act upon. The key questions addressed here are: (1) How does chromosome pairing during meiosis in allopolyploids affect the magnitude of genetic variation?, (2) How does the genome of polyploids evolve following formation, and what genetic mechanisms govern this evolution?, and (3) How does genetic and genomic evolution in polyploids affect phenotypic evolution? I use the shy monkeyflower, Mimulus sookensis, a tetraploid of hybrid origin between Mimulus guttatus and Mimulus nasutus, to address these focal questions. In order to develop a foundation to aid in interpretation of my findings, I first investigate the evolutionary history of M. sookensis. Chromosome counts establish that M. sookensis is indeed an allotetraploid, and a review of taxonomic literature reveals that this species is heretofore undescribed. By analysing the patterns of genetic variation at chloroplast and nuclear loci in M. guttatus, M. nasutus, and M. sookensis, I show that M. sookensis has recurrently formed from M. guttatus and M. nasutus. Crossing experiments within M. sookensis indicate that recurrent origins can contribute to genetic diversity without contributing to reproductive isolation among independently arisen polyploid lineages.
To address my focal questions, I take advantage of an intriguing and striking difference in flower size among M. sookensis, M. guttatus, and M. nasutus. The flowers of M. sookensis and M. nasutus are small and remarkably similar to one another, while the flowers of M. guttatus and diploid and tetraploid F1 hybrids between M. guttatus and M. nasutus are large and showy. This phenotypic divergence in flower size between M. sookensis and M. guttatus-like hybrids indicates that small flower size has evolved in M. sookensis. Using genetic marker data and phenotypic measurements from synthetic neoallotetraploid Mimulus, I demonstrate that there are low levels of fragment loss and phenotypic variation in neoallotetraploids; this suggests that homeologous pairing and recombination following polyploidization is not a major source of genetic variation or phenotypic evolution in M. sookensis. Analysis of the whole genome sequence of two M. sookensis lines reveals that M. sookensis is a fixed heterozygote throughout its entire genome, in that it has retained both a M. guttatus-like and M. nasutus-like subgenome, neither of which have been removed through homeologous recombination. These subgenomes have been homogenized by widespread gene conversion, and do not appear to have been differentially affected by deletions or deleterious mutations. Finally, to directly characterize the genetic architecture of flower size in M. sookensis, I cross a large-flowered synthetic neoallotetraploid Mimulus to small-flowered M. sookensis. I then employ a novel genotyping-by-sequencing approach to identify quantitative trait loci (QTL) associated with flower size. I find that there is one locus that accounts for a large proportion of phenotypic variation, and four other loci also contribute to flower size variation between the parental lines. Some of these loci co-localize with previously identified loci for flower size in diploid Mimulus, while others do not. Altogether, genetic marker data, phenotypic analysis of neoallotetraploids, whole genome sequence data, and QTL mapping data suggest that the genetic variation necessary for flower size evolution was likely caused by both gene conversion and new mutations, but not homeologous recombination. These results suggest that trait evolution in polyploids may be affected by the unique attributes of polyploids, but that new mutations are always an important source of genetic variation, regardless of ploidy level.