1,721,010 research outputs found
Exploring the Power of Experimental Evolution to Identify Dobzhansky-Muller Incompatibility Loci using Forward-time Simulations
Full text linkFitness reducing epistatic interactions referred to as Dobzhansky-Muller incompatibilities (DMIs) are responsible for the inviability and infertility prevalent in hybrid populations. As “evolutionary dead-ends,” these hybrid failures encourage reproductive isolation between two incipient species. Studies on hybrid genomes are therefore vital to understand the processes that underlie speciation events. My project aimed to determine the power of simulations to correctly identify genetic loci involved in reproductive isolation. Using SLiM, I created and evolved simulated hybrid populations with genomes containing user-defined DMIs. Biologically relevant parameters, such as recombination rates, selection coefficients, dominance effects, generation timepoints, and incompatibility types, differed among my simulations, allowing me to characterize the parameter spaces that resulted in high-powered estimates of DMI localization. My conclusions may act as a guide for evolve and re-sequence wet-lab experiments that aim to map real-world DMIs in Caenorhabditis hybrid populations.M.Sc
Experimental Optimization of High Throughput Fitness Assay Protocols using Caenorhabditis Nematodes
Full text linkHigh throughput experimentation (HTE) is an empirical technique that draws from technological advancement and automation to achieve precision and high replication, while decreasing labour, time, and resources. Initial applications in molecular and microbiology have inspired similar protocols in fields like evolutionary ecology and in eukaryotic organisms. HTE rapidly generates biological knowledge, which is beneficial when studying taxa that are not well characterized, like Caenorhabditis. I aim to optimize, determine the scope, and limitations of empirical data collection using 96-well plate spectrophotometric HTE for bacterial clearing as a proxy for nematode abundance and relative fitness. In nutrient-rich chronic exposure assays, nematode logistic growth rates were similar; saline media did not permit growth. Methods refinement may promote depth and breadth of understanding about Caenorhabditis nematodes across diverse genotypes, phenotypes, species, and selection gradients. Future HTE may employ other taxa and environmental monitoring to investigate biodiversity trends, including community assembly and niche dimensionality.M.Sc
The Evolution of Sperm Gigantism – Structure, Behaviour, and Consequences of Giant Sperm in Caenorhabditis Nematodes
Full text linkGametes of sexually reproducing species experience strong selection, due to the tight link between gamete performance and an individual’s reproductive success. Across animal taxa, spermatozoa display enormous diversity in specific cellular adaptations, modes of motility, shapes, and sizes. Increased sperm cell size is a particularly interesting shift in gamete characteristics due to its contradiction to the mathematically predicted, and commonly observed, strong advantage to males producing numerous quantities of small spermatozoa. Nonetheless, in the nematode genus Caenorhabditis, sperm cell gigantism evolved multiple times independently. To date, it is still unclear what unique advantage giant sperm might provide and whether species with sperm gigantism show consistent features of their reproductive traits generally, and sperm specifically. In this dissertation, I addressed questions of sperm size evolution across biological scales using species that have evolved giant versus standard-sized sperm. First, I quantified a suite of reproductive traits for 12 species that vary in average sperm cell size. Reproductive traits consistently associated with sperm gigantism point to convergent evolution of multiple reproductive trait phenotypes to define a sperm gigantism syndrome, potentially resulting from trade-offs and correlated selective pressures among reproductive traits. Next, using a case study of two species, C. nouraguensis and C. macrosperma, I quantified differences between standard-sized and giant sperm at the subcellular level via electron microscopy. By comparing relative investment in performance-affecting organelles, I advanced our understanding of how selection optimizes the subcellular composition of sperm cells for competition with other sperm and fertilization of eggs. In my final chapter, I shifted the investigation of sperm gigantism to address its consequences for ecological scales. I explored the hypothesis that sperm, though evolved in response to intraspecific sexual selection, can affect species coexistence if habitats overlap and species interbreed. I found that gamete-mediated reproductive interference is strong enough to affect species co-existence and consequently capable of shifting species community compositions. Collectively, my dissertation expands our understanding of sperm size evolution, and demonstrates the power of the Caenorhabditis nematode system for answering questions about complex trait evolution across multiple scales of biological organization.Ph.D
Mating costs and gametic barriers to reproduction
No full textMating is essential to many organisms for their continued existence, and yet mating is costly. However, the fitness costs–which may immediately affect the individuals involved, or may affect their future reproductive output, are not often quantified. I assess costs that are acquired at different periods of the mating process–in search of a mate and gametic interactions after copulation. I first quantify the cost of mate searching of a monogynous spider measured by male longevity and I additionally examine how mate searching affects subsequent mating behaviours and performance. I find evidence for behavioural plasticity in males influenced by mate search duration to maximize female egg laying which may be mediated by male seminal fluids and/or female cryptic choice. I then use transparent organisms, Caenorhabditis nematodes, to identify a novel form of gametic isolation using 10 different species. This sperm mediated form of gametic isolation likely evolved from intra-species sperm competition, resulting in incompatibilities as a byproduct of competitive sperm in inter-species matings that caused early death, sterility, and invasive sperm. I further explored sperm-mediated gametic isolation in Caenorhabditis from the perspective of inter-species interactions by experimentally evolving populations with mixed species for six generations to show reproductive interference at the population level. Finally, I screened 25 genetic mutants and phenotyped 99 nearly isogenic lines of Caenorhabditis to identify genetic contributions to gametic isolation (i.e. speciation genes). I identified candidate genes that fall into two categories of phenotypic defects: ovulation defects increase the incidence of sperm invasion, whereas sperm signaling defects reduce the incidence of sperm invasion. These findings and novel methods add to our understanding of how costly mating can be, and how gametic interactions, in particular, contribute to reproductive isolation in Caenorhabditis.Ph.D
Temperature-dependent Patterns of Gene Expression in Caenorhabditis briggsae
Full text linkDiscerning the genetic basis of adaptive phenotypes is a fundamental problem in biology that remains an open question. Studies using high-throughput sequencing methods of gene expression have contributed greatly to our understanding of how genotype becomes phenotype by treating gene expression as an intermediary phenotype, especially under variable environmental conditions. Using whole genome high-throughput RNAseq data, I characterized the responses of Temperate and Tropical genotypes of Caenorhabditis briggsae to chronic temperature stress. These genotypes show evidence of local adaptation, suggesting that differences in their responses to temperature may underlie adaptive phenotypes. I discovered that a large proportion of genes show genotype-specific changes in gene expression in response to temperature (genotype-by-temperature interactions), and that most of these genotype-specific responses occur under heat stress. These results suggest that responses to cold stress and heat stress are qualitatively different, and identify sets of genes that suggest further study into temperature-adapted phenotypes.M.Sc
Temperature-dependent Patterns of Gene Expression in Caenorhabditis briggsae
No full textDiscerning the genetic basis of adaptive phenotypes is a fundamental problem in biology that remains an open question. Studies using high-throughput sequencing methods of gene expression have contributed greatly to our understanding of how genotype becomes phenotype by treating gene expression as an intermediary phenotype, especially under variable environmental conditions. Using whole genome high-throughput RNAseq data, I characterized the responses of Temperate and Tropical genotypes of Caenorhabditis briggsae to chronic temperature stress. These genotypes show evidence of local adaptation, suggesting that differences in their responses to temperature may underlie adaptive phenotypes. I discovered that a large proportion of genes show genotype-specific changes in gene expression in response to temperature (genotype-by-temperature interactions), and that most of these genotype-specific responses occur under heat stress. These results suggest that responses to cold stress and heat stress are qualitatively different, and identify sets of genes that suggest further study into temperature-adapted phenotypes.M.Sc
Genetic and Developmental Contributions to Reproductive Isolation in Caenorhabditis Nematodes
Full text linkThe process of speciation is central to the origins of biodiversity. Therefore,
understanding how and why species form and persist represents one of the major
goals of evolutionary biology. Speciation involves the evolution of reproductive
isolation, which prevents gene flow between diverging lineages. Complete
reproductive isolation typically results from the accumulation of multiple
reproductive barriers. A particularly important reproductive barrier, when hybrid
offspring are intrinsically sterile or inviable, follows repeated genetic patterns in
many taxa. This consistency suggests fundamental rules to how speciation proceeds.
I analyzed these patterns using two pairs of species in the Caenorhabditis genus,
from the nematode phylum that has largely been neglected in the study of
speciation. First, I examined how inviability manifests over hybrid development and
found that, in contrast to predictions, nearly all inviability that occurs takes places
during embryogenesis, even in closely related species that produce fertile adult
offspring. This result indicates that the embryonic stage of development is
particularly sensitive to disruption in this genus. I explored whether the inviability
can be traced to the first cell division in hybrids, and found that the first cell division
is often disrupted in hybrids between distantly-related species, but not in those
between closely related species. I also compared the genetic complexity of hybrid
inviability and hybrid sterility, and found that hybrid male sterility is best explained
by a simple two-locus incompatibility while hybrid inviability is genetically complex,
in contrast to other taxa. I demonstrated that reproductive barriers that act after
mating but before zygote formation contribute to reproductive isolation by
depressing progeny production in hybrid crosses. Finally, I tested the role that
temperature, an environmental factor, might exert on intrinsic reproductive
isolation, to show that hybrid males are particularly sensitive to environmental
perturbations. Overall, my research reinforces the generality of common â rulesâ of
speciation across diverse taxa, and yet I also identified distinctive phenomena to
Caenorhabditis, highlighting the importance of extending speciation research into
new study organisms.Ph.D
Genetic and Developmental Contributions to Reproductive Isolation in Caenorhabditis Nematodes
No full textThe process of speciation is central to the origins of biodiversity. Therefore,
understanding how and why species form and persist represents one of the major
goals of evolutionary biology. Speciation involves the evolution of reproductive
isolation, which prevents gene flow between diverging lineages. Complete
reproductive isolation typically results from the accumulation of multiple
reproductive barriers. A particularly important reproductive barrier, when hybrid
offspring are intrinsically sterile or inviable, follows repeated genetic patterns in
many taxa. This consistency suggests fundamental rules to how speciation proceeds.
I analyzed these patterns using two pairs of species in the Caenorhabditis genus,
from the nematode phylum that has largely been neglected in the study of
speciation. First, I examined how inviability manifests over hybrid development and
found that, in contrast to predictions, nearly all inviability that occurs takes places
during embryogenesis, even in closely related species that produce fertile adult
offspring. This result indicates that the embryonic stage of development is
particularly sensitive to disruption in this genus. I explored whether the inviability
can be traced to the first cell division in hybrids, and found that the first cell division
is often disrupted in hybrids between distantly-related species, but not in those
between closely related species. I also compared the genetic complexity of hybrid
inviability and hybrid sterility, and found that hybrid male sterility is best explained
by a simple two-locus incompatibility while hybrid inviability is genetically complex,
in contrast to other taxa. I demonstrated that reproductive barriers that act after
mating but before zygote formation contribute to reproductive isolation by
depressing progeny production in hybrid crosses. Finally, I tested the role that
temperature, an environmental factor, might exert on intrinsic reproductive
isolation, to show that hybrid males are particularly sensitive to environmental
perturbations. Overall, my research reinforces the generality of common â rulesâ of
speciation across diverse taxa, and yet I also identified distinctive phenomena to
Caenorhabditis, highlighting the importance of extending speciation research into
new study organisms.Ph.D
Mating costs and gametic barriers to reproduction
Full text linkMating is essential to many organisms for their continued existence, and yet mating is costly. However, the fitness costs–which may immediately affect the individuals involved, or may affect their future reproductive output, are not often quantified. I assess costs that are acquired at different periods of the mating process–in search of a mate and gametic interactions after copulation. I first quantify the cost of mate searching of a monogynous spider measured by male longevity and I additionally examine how mate searching affects subsequent mating behaviours and performance. I find evidence for behavioural plasticity in males influenced by mate search duration to maximize female egg laying which may be mediated by male seminal fluids and/or female cryptic choice. I then use transparent organisms, Caenorhabditis nematodes, to identify a novel form of gametic isolation using 10 different species. This sperm mediated form of gametic isolation likely evolved from intra-species sperm competition, resulting in incompatibilities as a byproduct of competitive sperm in inter-species matings that caused early death, sterility, and invasive sperm. I further explored sperm-mediated gametic isolation in Caenorhabditis from the perspective of inter-species interactions by experimentally evolving populations with mixed species for six generations to show reproductive interference at the population level. Finally, I screened 25 genetic mutants and phenotyped 99 nearly isogenic lines of Caenorhabditis to identify genetic contributions to gametic isolation (i.e. speciation genes). I identified candidate genes that fall into two categories of phenotypic defects: ovulation defects increase the incidence of sperm invasion, whereas sperm signaling defects reduce the incidence of sperm invasion. These findings and novel methods add to our understanding of how costly mating can be, and how gametic interactions, in particular, contribute to reproductive isolation in Caenorhabditis.Ph.D
Genes, behaviour, and thermal reaction norms in Caenorhabditis nematodes
Full text linkUnderstanding how complex phenotypes like behaviour evolve requires that we determine the genetic contributions that get used by natural selection. Deciphering the genetic causes of differences in animal behaviour is challenging because behaviour phenotypes are strongly influenced by the environment, with phenotypic responses transduced through pleiotropic gene networks and the nervous system. My thesis simplifies these issues in behavioural genetics by working with a powerful genetic model animal system under tightly controlled laboratory environments that mimic natural environmental stimuli. I was able to quantify heritable natural variation in behaviour phenotypes among natural isolate strains ofPh.D
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