Emergence of layer IV barrel cytoarchitecture is delayed in somatosensory cortex of GAP-43 deficient mice following delayed development of dendritic asymmetry

The emergence of barrel cytoarchitecture in mouse somatosensory cortex is extremely well defined. However, mechanisms underlying the development of this cellular organization are not completely understood. While it is generally accepted that hollows emerge via passive displacement of cortical cells by dense thalamocortical afferent clusters in barrel centers, it is not known what causes cellular segregation of barrel sides and septa. Here, we hypothesized that the emergence of sides and septa is related to the progressive asymmetry of dendrites from the cells of the barrel side toward the barrel hollow during development. We tested this hypothesis in the barrel cortex of growth-associated protein-43 heterozygous mice (GAP43 (+/?) mice) that display a 2-day delay in retraction of septally oriented dendrites compared to (+/+) littermates. We predicted that this delayed retraction would result in a subsequent 2-day delay in the emergence of barrel sides and septa. Using cresyl violet staining of barrel cortex, we found that initial emergence of hollows was not different between GAP43 (+/?) mice and (+/+) littermate controls. However, the emergence of sides and septa was delayed by 2 days, supporting our hypothesis that the emergence of barrel sides and septa is related to, and perhaps reliant upon, the developmental step of dendritic orientation toward barrel hollows. This process, which is mechanistically distinct from the emergence of barrel hollows, is likely due to both active and passive events resulting from asymmetric cell orientation.     

Enhanced hippocampus‐dependent memory and reduced anxiety in mice over‐expressing human catalase in mitochondria

Oxidative stress (OS) and reactive oxygen species (ROS) play a modulatory role in synaptic plasticity and signaling pathways. Mitochondria (MT), a major source of ROS because of their involvement in energy metabolism, are important for brain function. MT‐generated ROS are proposed to be responsible for a significant proportion of OS and are associated with developmental abnormalities and aspects of cellular aging. The role of ROS and MT function in cognition of healthy individuals is relatively understudied. In this study, we characterized behavioral and cognitive performance of 5‐ to 6‐month‐old mice over‐expressing mitochondrial catalase (MCAT). MCAT mice showed enhancements in hippocampus‐dependent spatial learning and memory in the water maze and contextual fear conditioning, and reduced measures of anxiety in the elevated zero maze. Catalase activity was elevated in MCAT mice in all brain regions examined. Measures of oxidative stress (glutathione, protein carbonyl content, lipid peroxidation, and 8‐hydroxyguanine) did not significantly differ between the groups. The lack of differences in these markers of oxidative stress suggests that the differences observed in this study may be due to altered redox signaling. Catalase over‐expression might be sufficient to enhance cognition and reduce measures of anxiety even in the absence of alteration in levels of OS.     

SEASONALITY MAINTAINS ALTERNATIVE LIFE‐HISTORY PHENOTYPES

Many organisms express discrete alternative phenotypes (polyphenisms) in relation to predictable environmental variation. However, the evolution of alternative life‐history phenotypes remains poorly understood. Here, we analyze the evolution of alternative life histories in seasonal environments by using temperate insects as a model system. Temperate insects express alternative developmental pathways of diapause and direct development, the induction of a certain pathway affecting fitness through its life‐history correlates. We develop a methodologically novel and holistic simulation model and optimize development time, growth rate, body size, reproductive effort, and adult life span simultaneously in both developmental pathways. The model predicts that direct development should be associated with shorter development time (duration of growth) and adult life span, higher growth rate and reproductive effort, smaller body size as well as lower fecundity compared to the diapause pathway, because the two generations divide the available time unequally. These predictions are consistent with many empirical data. Our analysis shows that seasonality alone can explain the evolution of alternative life histories.     

Chironomid midges: a forgotten model of developmental biology research

For more than a century, embryologists have been exploring various model systems to gain insights into developmental processes. This article presents an overview of the role of chironomid midges in embryology research since their introduction as model organisms in the 19th century. We present the vestiges of bibliography since the days of Weismann (1834–1914), who raised preliminary queries to unravel many unique features of insect embryogenesis using midges as a crucible. Unfortunately, over the years, chironomid midges got lost into obscurity as a model for developmental biology, which is evident from the paucity of developmental biology–related literature on midges in the past decades. Through this essay, the authors intend to share reminiscences of the heydays of chironomid research with the wider community of zoologists with an aim of reviving chironomid embryology. Midges not only possess the basic qualities essential for an ideal model system, but being one of the ancestral dipteran stocks, they can also prove an excellent test system for evo‐devo, transgenetic, and embryogenomic investigations that utilize methodologies at the interface of developmental biology and high‐throughput molecular genetic and genomics approach. An introspection of re‐introducing chironomid midgesas model system will be rewarding for the contemporary developmental biologists.     

Early-life exposure to a herbicide has enduring effects on pathogen-induced mortality

Exposure to stressors at formative stages in the development of wildlife and humans can have enduring effects on health. Understanding which, when and how stressors cause enduring health effects is crucial because these stressors might then be avoided or mitigated during formative stages to prevent lasting increases in disease susceptibility. Nevertheless, the impact of early-life exposure to stressors on the ability of hosts to resist and tolerate infections has yet to be thoroughly investigated. Here, we show that early-life, 6-day exposure to the herbicide atrazine (mean ± s.e.: 65.9±3.48 µg l⁻¹) increased frog mortality 46 days after atrazine exposure (post-metamorphosis), but only when frogs were challenged with a chytrid fungus implicated in global amphibian declines. Previous atrazine exposure did not affect resistance of infection (fungal load). Rather, early-life exposure to atrazine altered growth and development, which resulted in exposure to chytrid at more susceptible developmental stages and sizes, and reduced tolerance of infection, elevating mortality risk at an equivalent fungal burden to frogs unexposed to atrazine. Moreover, there was no evidence of recovery from atrazine exposure. Hence, reducing early-life exposure of amphibians to atrazine could reduce lasting increases in the risk of mortality from a disease associated with worldwide amphibian declines. More generally, these findings highlight that a better understanding of how stressors cause enduring effects on disease susceptibility could facilitate disease prevention in wildlife and humans, an approach that is often more cost-effective and efficient than reactive medicine.     

A heuristic model on the role of plasticity in adaptive evolution: plasticity increases adaptation,population viability and genetic variation

An ongoing new synthesis in evolutionary theory is expanding our view of the sources of heritable variation beyond point mutations of fixed phenotypic effects to include environmentally sensitive changes in gene regulation. This expansion of the paradigm is necessary given ample evidence for a heritable ability to alter gene expression in response to environmental cues. In consequence, single genotypes are often capable of adaptively expressing different phenotypes in different environments, i.e. are adaptively plastic. We present an individual-based heuristic model to compare the adaptive dynamics of populations composed of plastic or non-plastic genotypes under a wide range of scenarios where we modify environmental variation, mutation rate and costs of plasticity. The model shows that adaptive plasticity contributes to the maintenance of genetic variation within populations, reduces bottlenecks when facing rapid environmental changes and confers an overall faster rate of adaptation. In fluctuating environments, plasticity is favoured by selection and maintained in the population. However, if the environment stabilizes and costs of plasticity are high, plasticity is reduced by selection, leading to genetic assimilation, which could result in species diversification. More broadly, our model shows that adaptive plasticity is a common consequence of selection under environmental heterogeneity, and hence a potentially common phenomenon in nature. Thus, taking adaptive plasticity into account substantially extends our view of adaptive evolution.     

Assaying Predatory Feeding Behaviors in Pristionchus and Other Nematodes

This protocol provides multiple methods for the analysis and quantification of predatory feeding behaviors in nematodes. Many nematode species including Pristionchus pacificus display complex behaviors, the most striking of which is the predation of other nematode larvae. However, as these behaviors are absent in the model organism Caenorhabditis elegans, they have thus far only recently been described in detail along with the development of reliable behavioral assays ¹. These predatory behaviors are dependent upon phenotypically plastic but fixed mouth morphs making the correct identification and categorization of these animals essential. In P. pacificus there are two mouth types, the stenostomatous and eurystomatous morphs ², with only the wide mouthed eurystomatous containing an extra tooth and being capable of killing other nematode larvae. Through the isolation of an abundance of size matched prey larvae and subsequent exposure to predatory nematodes, assays including both "corpse assays" and "bite assays" on correctly identified mouth morph nematodes are possible. These assays provide a means to rapidly quantify predation success rates and provide a detailed behavioral analysis of individual nematodes engaged in predatory feeding activities. In addition, with the use of a high-speed camera, visualization of changes in pharyngeal activity including tooth and pumping dynamics are also possible.