One important source of phenotypic variation on which natural selection can act is developmental plasticity (the capacity of a single genotype to produce different environment-dependent forms). Therefore, studies of how the environment influences development can facilitate our understanding of how natural selection acts to yield phenotypic evolution. Using the Pacific barnacle (Balanus glandula Darwin), I explored how functionally independent appendages (the legs and unusually long penises of barnacles) respond to widespread spatial and temporal variation in water velocity and conspecific density. Through field surveys, reciprocal transplant experiments, and histological sectioning, I show that barnacle legs and penises appear remarkably well adapted to spatial and temporal variation in water velocity. Building on past work on leg form variation, I show that penises from exposed shores were shorter than, stouter than, and more than twice as massive for their length, as those from nearby protected bays (this effect holds true for artificially inflated penises as well). A transplant experiment confirmed that most of this variation in penis and leg form variation was due to developmental plasticity. Penises and legs of barnacles from an exposed shore also had thicker cuticle, and muscles with greater cross-sectional area (and shorter sarcomeres) compared to those from a protected shore. Form variation was consistent with numerous predictions from engineering theory suggesting that barnacles show dramatic, complex and likely adaptive variation in leg and penis form among sites that differ dramatically in water velocity. Additional experiments showed evidence for and against developmental limits to plasticity in barnacles. A transplant experiment identified an important (and asymmetrical) developmental limit to leg-length response time – likely mediated by food limitation – while a field survey showed that developmental coupling does not restrict adaptive plastic responses of legs and penises to multiple conflicting cues (conspecific density and water velocity). Finally, a two-year survey of natural populations revealed the first evidence that barnacles also change leg form seasonally. Together these results contribute valuable information on the mechanisms of phenotypic change. This research also sheds light on the circumstances that allow decoupling of developmental processes to produce novel combinations of characters on which natural selection can act. / Ecology

Thesis (MSc)--University of Stellenbosch, 2006. / Bibliography. Also available via the Internet.

Phenotypic plasticity, the ability of a single genotype to be expressed as a range of phenotypes in response to environmental variation, is a widespread phenomenon. Documented increasingly among the larval stages of marine organisms, phenotypic plasticity in the veliger larvae of the marine snail Littorina scutulata was investigated in response to predatory, nutritional, and gravitational stimuli. Veligers developed rounder shells, smaller apertures, and reinforced aperture margins in response to water-borne cues from predatory crab larvae. The nature and degree of the induced-morphologies depended on cue composition and conferred decreased vulnerability to predation. Food-limited veligers developed larger feeding and swimming structures (vela) with longer cilia relative to shell size compared to larvae raised with high food. This inducible offense corresponded with a decrease in vertical swimming speed, an unexpected result possibly reflecting behavioral manipulation of individual velar components. A cell proliferation assay indicated that growth of the larger structure was achieved partially by a steady rate of cell division over a longer period of time; an initially higher level of cell proliferation in veligers raised on high food dropped off sharply. Velar lobe asymmetry, where one lobe is larger than the other, may exist to offset an asymmetry in weight distribution due to how the larval shell is carried. The larger velar lobe overlies the protruding spire of the larval shell. Bi- and multi-lobed vela get bigger with shell size but follow different rules with regards to the relationship between velar asymmetry and shell asymmetry. Experimental alternations of mass distribution of the larval shell caused changes in the ratio of area between each side of the velum and total velar growth for larvae of L. scutulata. Following settlement and metamorphosis, juveniles of intertidal marine invertebrates are subject to additional stressors that can manifest as phenotypic variation. Color differences between juvenile and adult Strongylocentrotus purpuratus were shown to be caused by variation in light exposure. Green juveniles raised in sunlight turned purple (due to more pigment) and showed decreased susceptibility to artificial UVR than urchins kept in the dark, which remained green (due to less pigment). This dissertation includes previously unpublished co-authored material.

Virtually all multicellular organisms are capable of developing differently in response to environmental variation. At the molecular level, such developmental plasticity requires interpretation and perpetuation of environmental signals without changing the underlying genotype. Such non-genetic, heritable information is known as epigenetic information. This dissertation examines epigenetic information among social insects, and how differences in such information relate to phenotypic caste differences. The studies included herein primarily focus on one form of epigenetic information: DNA methylation. In particular, these studies explore DNA methylation as it relates to and impacts (i) alternative phenotype and particular gene expression differences in two social insect species, (ii) histone modifications, another important form of epigenetic information, in insect genomes, and (iii) molecular evolutionary rate of underlying actively transcribed gene sequences. We find that DNA methylation exhibits marked epigenetic and evolutionary associations, and is associated with alternative phenotype in multiple insect species. Thus, DNA methylation is emerging as one important epigenetic mediator of phenotypic plasticity in social insects.

Forensic entomology is a well-established science linking arthropod biology and ecology to legal investigations. Specifically, immature development on a decomposing corpse may give insight into the minimum time elapsed since death. Until recently, biological variation within a single species has been overlooked when estimating colonization events. Variation in the form of phenotypic plasticity, or the ability of a single genotype to produce multiple phenotypes under alternative stresses, has been documented in genetic and ecological literature and spans across all phyla. Taking this into account, different subpopulations of forensically pertinent insect species should also possess the ability to adapt to changing environments as geographic distribution increases. Thus, plastic responses of a species to alternative stresses may be measured in biological parameters, such as development time. In this research, three geographically distinct strains of the blow fly Cochliomyia macellaria Fabricius (Diptera Calliphoridae) were reared in two distinct environments in order to measure development time, as well as pupal and adult masses. Strains exhibited genetic variance when compared to each other, and each strain exhibited variable responses across environments (phenotypic plasticity). Plasticity in the form of genotype by environment (GxE) interactions was also exhibited by C. macellaria, although consistent adherence to any single rule explaining ontogenetic trends was not apparent. This research supports the existence of intraspecific variation in a common blow fly of forensic importance. Results of this study will impact the forensic entomology community by encouraging the generation of either strain;specific developmental datasets or statistical models to minimize variation caused by genetic, environment, or GxE effects in order to compare developmental data across strains.

No description available.

Evolutionary processes in any population depend upon patterns of phenotypic variation available to selection and their underlying heritability. In this thesis, I used the filamentous red seaweed Asparagopsis armata, with particular focus on its modularity, to test several key questions underlying its growth-form evolution in heterogeneous environments. I established that experimental manipulations of light quantity and quality mimicking variation in underwater light due to shading or depth induce growthform plasticity in A. armata that may be evolutionarily significant given its variability among clones. Current patterns of plasticity displayed by A. armata appear adaptive, moreover, given that a reciprocal transplant of phenotypes between light environments found densely-branched (phalanx-like) phenotypes to have higher relative growth rates than sparsely-branched (guerrilla-like) phenotypes in well-lit patches, but lower relative growth rates than the latter in shaded patches. Using the capacity for rapid growth as a proxy for fitness, multivariate selection analyses identified environment-dependent patterns of directional selection on single traits coupled with linear and nonlinear selection on multi-trait combinations that shape growth-form variation within patches of differing light intensity, thereby reinforcing plasticity across light environments. Quantitative genetic analyses, however, suggest that the modular iteration of genes in morphogenesis may limit further growth-form evolution in A. armata populations exposed to spatial heterogeneity in light by constraining thallus responses to environment-dependent selection. Last, heritable responses to artificial selection on growth-form variation among clonal cell-lineages revealed the surprising capacity for A.armata to circumvent genetic constraints inherent to its development by adapting to environmental change in the absence of sexually-generated variance among clones.

Divergent natural selection for use of locally abundant resources can lead to diversification within and across species. However, the consequences of divergent selection for phenotypic evolution also depend on the development of variation. Because relationships among traits such as shared developmental timing or common involvement in an organismal function can channel variation generated during development, these relationships strongly influence the direction of evolution.During development of the mammalian mandible multiple tissues of distinct developmental origins interact with inputs from the functioning of attached muscles to produce a cohesive and well integrated trait. In soricid shrews, part of the mandible matures late in ontogeny, coinciding with the onset of foraging. In this case, foraging-linked muscle activity should influence the development of the late maturing mandibular region. Here, I show that variation in this late ossifying region reveals the local functional requirements of the jaw and results in an opportunity to decouple internal and external sources of variation (developmental and environmental respectively) in the mandible. Capitalizing on this feature of the Sorex system, I empirically examined the historical persistence of internal and external patterns of variation, the consequences of variation patterning for ecological and morphological diversification across taxa, and differences between early and late ossifying regions in their contribution to local adaptation in mandible morphology.I found that the functional requirements of diet directed mandible development and determined species similarity in both mandible morphology and function. Timing of bone maturation determined the morphological effects of foraging-linked muscle activity, resulting in differential expression of adaptive variation in the late maturing region. Further, I found higher levels of interspecific variation in the late maturing region of the mandible, and showed that interspecific divergence in foraging morphology occurs along the lines delineated by epigenetic inputs of muscle on bone formation during late ontogeny within species. These findings indicate that differences in functional requirements are critical for divergence among taxa in this system. Further, these results suggest that, when external inputs into trait development are indicative of local functional requirements, the same epigenetic mechanism of development can generate diversity both within and among taxa.

Evolutionary processes in any population depend upon patterns of phenotypic variation available to selection and their underlying heritability. In this thesis, I used the filamentous red seaweed Asparagopsis armata, with particular focus on its modularity, to test several key questions underlying its growth-form evolution in heterogeneous environments. I established that experimental manipulations of light quantity and quality mimicking variation in underwater light due to shading or depth induce growthform plasticity in A. armata that may be evolutionarily significant given its variability among clones. Current patterns of plasticity displayed by A. armata appear adaptive, moreover, given that a reciprocal transplant of phenotypes between light environments found densely-branched (phalanx-like) phenotypes to have higher relative growth rates than sparsely-branched (guerrilla-like) phenotypes in well-lit patches, but lower relative growth rates than the latter in shaded patches. Using the capacity for rapid growth as a proxy for fitness, multivariate selection analyses identified environment-dependent patterns of directional selection on single traits coupled with linear and nonlinear selection on multi-trait combinations that shape growth-form variation within patches of differing light intensity, thereby reinforcing plasticity across light environments. Quantitative genetic analyses, however, suggest that the modular iteration of genes in morphogenesis may limit further growth-form evolution in A. armata populations exposed to spatial heterogeneity in light by constraining thallus responses to environment-dependent selection. Last, heritable responses to artificial selection on growth-form variation among clonal cell-lineages revealed the surprising capacity for A.armata to circumvent genetic constraints inherent to its development by adapting to environmental change in the absence of sexually-generated variance among clones.

Doctor of Philosophy / Department of Entomology / Yoonseong Park / Complex life cycles (CLCs), developmental programs in which life-history stages are distinct in morphology, behavior, and physiology, are common throughout the biosphere. However, it is still unclear why and how CLCs evolve. The adaptive decoupling hypothesis (ADH) postulates that CLCs evolve to decouple the developmental processes that underlie traits across ontogeny to allow for independent, stage-specific responses to selection. This ultimately could lead to alternate life-history stages adapting to unique environments, thus optimizing fitness across development. However, few empirical tests of the ADH are available. Detecting genetic and transcriptomic decoupling of thermal hardiness using robust techniques in a model system, D. melanogaster, was the goal of this dissertation. Furthermore, this work illustrates that different life-history stages have the potential to become adapted to unique ecological niches. I performed three primary studies to test the ADH: 1.) estimation of the genetic correlation for cold hardiness between larvae and adults using isogenic lines of D. melanogaster to determine if unique genetic architectures underlie variation in cold stress response using standard quantitative genetic and Genome-Wide Association (GWA) methods, 2.) testing whether developmental acclimation is genetically correlated across stages, and whether acclimation alters cross-stage correlations in cold hardiness, and 3.) analysis of the transcriptional responses of both larvae and adults to extreme cold to determine if stage-specific stress response mechanisms exist across development.