Ubiquitous molecular substrates for associative learning and activity-dependent neuronal facilitation.

Recent evidence suggests that many of the molecular cascades and substrates that contribute to learning-related forms of neuronal plasticity may be conserved across ostensibly disparate model systems. Notably, the facilitation of neuronal excitability and synaptic transmission that contribute to associative learning in Aplysia and Hermissenda, as well as associative LTP in hippocampal CA1 cells, all require (or are enhanced by) the convergence of a transient elevation in intracellular Ca2+ with transmitter binding to metabotropic cell-surface receptors. This temporal convergence of Ca2+ and G-protein-stimulated second-messenger cascades synergistically stimulates several classes of serine/threonine protein kinases, which in turn modulate receptor function or cell excitability through the phosphorylation of ion channels. We present a summary of the biophysical and molecular constituents of neuronal and synaptic facilitation in each of these three model systems. Although specific components of the underlying molecular cascades differ across these three systems, fundamental aspects of these cascades are widely conserved, leading to the conclusion that the conceptual semblance of these superficially disparate systems is far greater than is generally acknowledged. We suggest that the elucidation of mechanistic similarities between different systems will ultimately fulfill the goal of the model systems approach, that is, the description of critical and ubiquitous features of neuronal and synaptic events that contribute to memory induction

Shifts in bilateral asymmetry within a distribution range: The case of the chukar partridge

Three major types of bilateral asymmetry (fluctuating asymmetry, directional asymmetry, and antisymmetry) have long been recognized in the literature. Little, however, is known about transitions between asymmetry types, especially in natural populations. It is often assumed that directional asymmetry and antisymmetry have a larger Genetic basis than fluctuating asymmetry. This leads many scientists to exclude traits or populations showing either directional asymmetry or antisymmetry from developmental instability studies, focusing attention on fluctuating asymmetry alone. This procedure may bias the findings and thus our understanding of patterns of bilateral asymmetry and the factors influencing it. To examine changes in bilateral asymmetry across the distribution range of a species, I studied the length of the third toe in I I chukar partridge (Alectoris chukar) populations across a steep environmental gradient of 320 km within the species' range in Israel. This trait was selected due to its adaptive value in the chukar, a species that spends much of its activity walking, and due to its high measurement repeatability. Moving front the core toward the very extreme periphery of the range, the following four trends are detected: ( I) the expression of the directional asymmetry component significantly increases; (2) the frequency of symmetrical individuals in the population significantly decreases, with a sharp decline at the steepest part of the climatic and environmental gradient studied, within the Mediterranean-desert ecotone; (3) mean asymmetry levels, as estimated using the unsigned difference between the right and left toe, significantly increases; and (4) the range of asymmetry increases such that the most asymmetrical individuals originate from the very edge of the range. These findings provide primary evidence that substantial shifts in asymmetry may occur across short geographical distances within a species' distribution range. They show a continuum between asymmetry types and support the notion that all three types of asymmetry can reflect developmental instability. Further studies of developmental instability should be designed so that they enable detection of transitions between asymmetry types across natural populations. Such a procedure may partly resolve some of the contradictions seen in the literature regarding the relationship between bilateral asymmetry and environmental stress

Relationship between heterozygosity and asymmetry: a test across the distribution range

The genetic basis of developmental stability, as measured by bilateral asymmetry, has been debated for over 50 years among developmental and evolutionary biologists. One of the central theories dealing with this relationship suggests that higher levels of genetic diversity, as reflected in heterozygosity, result in increased stability during development and thus in lower asymmetry. In this study, we aimed to test the relationship between asymmetry and heterozygosity at two levels: (1) the population level, where mean heterozygosity within a population is predicted to be negatively correlated with mean population asymmetry and (2) the individual level, where the proportion of heterozygous loci of an individual and its bilateral asymmetry estimates are predicted to be negatively correlated. While previous studies often focused on local populations, work across species ranges can answer the following questions. Are levels of heterozygosity correlated with levels of developmental instability, as estimated by bilateral asymmetry? Are patterns consistent across the distribution range, from the periphery towards the core? Does the relationship between genetic stress and bilateral asymmetry depend on the degree of environmental stress? We tested heterozygosity levels in 26 loci and asymmetry in third toe length in 11 populations of the chukar partridge (Alectoris chukar) across a sharp climatic gradient in Israel from the arid periphery, through the Mediterranean-desert ecotone towards the Mediterranean areas located further away from the range boundaries. Genetic diversity, as estimated using both observed and expected heterozygosity, was not associated with asymmetry at either the population or at the individual level. Whereas heterozygosity showed a hump-shaped pattern, peaking at the ecotone, asymmetry monotonically increased towards the range periphery. We argue that whereas asymmetry may serve as a useful tool for estimating changes in environmental stress, it may not be widely applicable for estimating genetic stress