The mitochondrial genome: so simple yet so complex.

Mitochondria possess a small set of genes that are essential for respiratory function. This review highlights recent advances in our understanding of mitochondrial gene organization and expression. These studies illustrate a remarkable diversity among eukaryotic lineages and an impressive complexity of events needed to achieve nuclear-mitochondrial harmony.     

Circadian rhythms and memory: not so simple as cogs and gears

The influence of circadian rhythms on memory has long been studied; however, the molecular prerequisites for their interaction remain elusive. The hippocampus, which is a region of the brain important for long‐term memory formation and temporary maintenance, shows circadian rhythmicity in pathways central to the memory‐consolidation process. As neuronal plasticity is the translation of numerous inputs, illuminating the direct molecular links between circadian rhythms and memory consolidation remains a daunting task. However, the elucidation of how clock genes contribute to synaptic plasticity could provide such a link. Furthermore, the idea that memory training could actually function as a zeitgeber for hippocampal neurons is worth consideration, based on our knowledge of the entrainment of the circadian clock system. The integration of many inputs in the hippocampus affects memory consolidation at both the cellular and the systems level, leaving the molecular connections between circadian rhythmicity and memory relatively obscure but ripe for investigation.     

Drosophila Melanogaster, Drosophila Simulans: so Similar yet so Different

During the last two decades, the two cosmopolitan species Drosophila melanogaster and Drosophila simulans have been compared with regard to numerous characteristics, ranging from their geographic distribution and ecology to their DNA polymorphism. Various traits have been compared, including morphology, physiology, sexual behavior, allozymes and other proteins, chromosomal inversions, mitochondrial and nuclear DNA, transposable elements, wolbachia etc. Such comparisons reveal similarities and differences between the two species, depending on the trait considered. In most cases, the between-population variability of D. simulans is lower than that of D. melanogaster, but the two species exhibit similar levels of within-population variability. One of the main exceptions is the nucleotide polymorphism of several nuclear regions. Although several hypotheses have been proposed to explain these observations, the evolutionary dynamics of these two species are far from being understood. How have two species sharing a common ancestor in the recent past accumulated so many differences? A brief history of comparisons of the two species, from the first in 1919 by A.H. Sturtevant, and a summary of the hypotheses proposed to explain the similarities and the differences between these species are presented and discussed.     

A Notch so simple influence on T cell development

T cell precursors undergo a series of developmental choices that progressively narrow their ability to give rise to different cell lineages. Evidence accumulated in the last few years suggests that Notch occupies a central place among the signal transduction pathways that regulate many of these choices, including the T/B, alphabeta/gammadelta and CD4/CD8 lineage decisions. Nevertheless the mechanisms by which Notch exerts its effects are not well understood, and in some cases the physiologic role is unclear. In this review we try to present succinctly the experiments and highlight the areas of controversy.     

Not so simple after all: searching for ecological advantages of compound leaves

Leaves come in many sizes and shapes, and the relationships between leaf traits and the environments they occur in are better understood every day. However we still know very little about the ecological consequences of plants having either compound or simple leaves. We attempted to address this knowledge gap by comparing chemical and physical characteristics (leaf area, length:width ratio, water content, leaf mass per area, ‘toughness’ and C:N ratio), as well as rates of herbivory between compound and simple leaves across 34 species in adjacent rainforest, open woodland and wet sclerophyll (tall open forest) vegetation in northeastern Australia. We found C:N ratio to be lower in simple leaves, but this was the only leaf trait that differed significantly between leaf types and did not stand up under phylogenetic analysis. Overall, we found no differences in herbivory between simple and compound leaves. While it remains unclear what the advantages of having one leaf type over another might be, the differences do not seem to lie in construction, or in vulnerability to herbivores, at least in the Australian Wet Tropics.     

Notch signalling: a simple pathway becomes complex

A small number of signalling pathways are used iteratively to regulate cell fates, cell proliferation and cell death in development. Notch is the receptor in one such pathway, and is unusual in that most of its ligands are also transmembrane proteins; therefore signalling is restricted to neighbouring cells. Although the intracellular transduction of the Notch signal is remarkably simple, with no secondary messengers, this pathway functions in an enormous diversity of developmental processes and its dysfunction is implicated in many cancers.     

p23, a simple protein with complex activities

p23 is a small but important cochaperone for the Hsp90 chaperoning pathway. It appears to facilitate the adenosine triphosphate-driven cycle of Hsp90 binding to client proteins. It enters at a late stage of the cycle and enhances the maturation of client proteins. Although this role of p23 is fairly well established, recent studies suggest that it may have additional functions in the cell that merit further exploration.     

Protein translocation at the ER membrane: A complex process becomes more so

Protein transport across or insertion into a membrane is facilitated by multicomponent protein complexes that reside in the bilayer. Current models propose that these complexes mediate translocation and integration by an obligate sequence of interactions between the substrate polypeptide and other components. Recent discoveries extend and complicate these models, but, more importantly, they remind us that our current level of understanding of the actual molecular mechanisms involved is crude and represents only the tip of the iceberg.