Evolvability of an RNA virus is determined by its mutational neighbourhood

The ubiquity of mechanisms that generate genetic variation has spurred arguments that evolvability, the ability to generate adaptive variation, has itself evolved in response to natural selection. The high mutation rate of RNA viruses is postulated to be an adaptation for evolvability, but the paradox is that whereas some RNA viruses evolve at high rates, others are highly stable. Here we show that evolvability in the RNA bacteriophage phi6 is also determined by the accessibility of advantageous genotypes within the mutational neighbourhood (the set of mutants one or a few mutational steps away). We found that two phi6 populations that were derived from a single ancestral phage repeatedly evolved at different rates and toward different fitness maxima. Fitness measurements of individual phages showed that the fitness distribution of mutants differed between the two populations. Whereas population A, which evolved toward a higher maximum, had a distribution that contained many advantageous mutants, population B, which evolved toward a lower maximum, had a distribution that contained only deleterious mutants. We interpret these distributions to measure the fitness effects of genotypes that are mutationally available to the two populations. Thus, the evolvability of phi6 is constrained by the distribution of its mutational neighbours, despite the fact that this phage has the characteristic high mutation rate of RNA viruses.     

Fringe is a glycosyltransferase that modifies Notch

Notch receptors function in highly conserved intercellular signalling pathways that direct cell-fate decisions, proliferation and apoptosis in metazoans. Fringe proteins can positively and negatively modulate the ability of Notch ligands to activate the Notch receptor. Here we establish the biochemical mechanism of Fringe action. Drosophila and mammalian Fringe proteins possess a fucose-specific beta1,3 N-acetylglucosaminyltransferase activity that initiates elongation of O-linked fucose residues attached to epidermal growth factor-like sequence repeats of Notch. We obtained biological evidence that Fringe-dependent elongation of O-linked fucose on Notch modulates Notch signalling by using co-culture assays in mammalian cells and by expression of an enzymatically inactive Fringe mutant in Drosophila. The post-translational modification of Notch by Fringe represents a striking example of modulation of a signalling event by differential receptor glycosylation and identifies a mechanism that is likely to be relevant to other signalling pathways.     

Evolutionary origins of vertebrate appendicular muscle

The evolution of terrestrial tetrapod species heralded a transition in locomotor strategies. While most fish species use the undulating contractions of the axial musculature to generate propulsive force, tetrapods also rely on the appendicular muscles of the limbs to generate movement. Despite the fossil record generating an understanding of the way in which the appendicular skeleton has evolved to provide the scaffold for tetrapod limb musculature, there is, by contrast, almost no information as to how this musculature arose. Here we examine fin muscle formation within two extant classes of fish. We find that in the teleost, zebrafish, fin muscles arise from migratory mesenchymal precursor cells that possess molecular and morphogenetic identity with the limb muscle precursors of tetrapod species. Chondrichthyan dogfish embryos, however, use the primitive mechanism of direct epithelial somitic extensions to derive the muscles of the fin. We conclude that the genetic mechanism controlling formation of tetrapod limb muscles evolved before the Sarcopterygian radiation.     

Urinary bladder hyporeflexia and reduced pain-related behaviour in P2X3-deficient mice

Extracellular ATP is implicated in numerous sensory processes ranging from the response to pain to the regulation of motility in visceral organs. The ATP receptor P2X3 is selectively expressed on small diameter sensory neurons, supporting this hypothesis. Here we show that mice deficient in P2X3 lose the rapidly desensitizing ATP-induced currents in dorsal root ganglion neurons. P2X3 deficiency also causes a reduction in the sustained ATP-induced currents in nodose ganglion neurons. P2X3-null mice have reduced pain-related behaviour in response to injection of ATP and formalin. Significantly, P2X3-null mice exhibit a marked urinary bladder hyporeflexia, characterized by decreased voiding frequency and increased bladder capacity, but normal bladder pressures. Immunohistochemical studies localize P2X3 to nerve fibres innervating the urinary bladder of wild-type mice, and show that loss of P2X3 does not alter sensory neuron innervation density. Thus, P2X3 is critical for peripheral pain responses and afferent pathways controlling urinary bladder volume reflexes. Antagonists to P2X3 may therefore have therapeutic potential in the treatment of disorders of urine storage and voiding such as overactive bladder.     

The tumour-suppressor genes lgl and dlg regulate basal protein targeting in Drosophila neuroblasts

Drosophila neuroblasts are a model system for studying asymmetric cell division: they divide unequally to produce an apical neuroblast and a basal ganglion mother cell that differ in size, mitotic activity and developmental potential. During neuroblast mitosis, an apical protein complex orients the mitotic spindle and targets determinants of cell fate to the basal cortex, but the mechanism of each process is unknown. Here we show that the tumour-suppressor genes lethal giant larvae (lgl) and discs large (dlg) regulate basal protein targeting, but not apical complex formation or spindle orientation, in both embryonic and larval neuroblasts. Dlg protein is apically enriched and is required for maintaining cortical localization of Lgl protein. Basal protein targeting requires microfilament and myosin function, yet the lgl phenotype is strongly suppressed by reducing levels of myosin II. We conclude that Dlg and Lgl promote, and myosin II inhibits, actomyosin-dependent basal protein targeting in neuroblasts.     

The future of ageing

Advances in our knowledge of age-associated diseases have far outpaced advances in our understanding of the fundamental ageing processes that underlie the vulnerability to these pathologies. If we are to increase human life expectancy beyond the fifteen-year limit that would result if today's leading causes of death were resolved, more attention must be paid to basic research on ageing. Determination of longevity must be distinguished from ageing to take us from the common question of why we age to a more revealing question that is rarely posed: why do we live as long as we do? But if the ability to intervene in ageing ever becomes a reality, it will be rife with unintended and undesirable consequences.