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The human vestibulo-ocular reflex (VOR) is known to undergo major plastic modification in coping with sensory conflict brought about by optical reversing prisms. The first phase of this study proposes a simple model to account for this modifiability in terms of known neurophysiology. To study the phenomenon further two series of experiments were conducted on long-term vision-reversed cats. For this a new computer technique was developed to analyse oculomotor responses. The first series studied the time course and amplitude dependence of adaptation; the second, the frequency response of the fully adapted system. The adaptive process exhibited strict limitations, the functional effectiveness of which is quantitatively defined. The findings as a whole suggest that adaptive mechanisms other than the VOR are at play and that far more complex interactions exist between vestibulo- and visual-motor mechanisms than originally envisaged.

Most stroke survivors recover some degree of lost function after an ischemic event. Recovery however, is negatively affected by comorbid conditions such as diabetes. Successful recovery is dependent on the ability of adjacent surviving cortical tissue and functionally related areas to take over functions lost by the stroke. Recently our lab has shown that diabetes interferes with the remapping of sensory function to peri-infarct areas after photothrombotic stroke. Given this result, it is crucial to understand how diabetes affects the structure of neurons following stroke, particularly at the level of dendritic spines, which receive the vast majority of excitatory synaptic inputs. Type I diabetes was pharmacologically induced in transgenic mice expressing yellow fluorescent protein (YFP) in a subset of cortical neurons 4 weeks prior to receiving unilateral photothrombotic stroke in the forelimb area of the primary somatosensory cortex (FLS1). Spine density measurements were made on the apical and basilar dendrites of layer-5 pyramidal neurons at 1 and 6 weeks after stroke. Our analysis indicated that diabetes was associated with fewer apical and basilar dendritic spines in the peri-infarct region 1 week after stroke. At 6 weeks of recovery, peri-infarct dendritic spine density in both control and diabetic animals returned to baseline levels. These changes were specific to the peri-infarct cortex, as spine density in distant cortical areas such as the forelimb sensorimotor region of the contralateral hemisphere, were not affected by stroke. In order to relate changes in spine density to the recovery of forepaw function, we re-analyzed data from a previous study that employed the forepaw adhesive-tape-removal test (Sweetnam et al 2012). This analysis revealed that diabetes significantly increased the latency of tape removal from the impaired forepaw (when normalized to the unaffected paw) at 1 but not 6 weeks of recovery. Collectively, these findings indicate that diabetes exacerbates forepaw impairments and basilar spine loss initially after stroke, but does not affect the ability of the brain to replace lost spines over weeks of recovery. / Graduate / 0317

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

Diss. (sammanfattning) Göteborg : Göteborgs universitet, 2007. / Härtill 4 uppsatser.

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.

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