Light masking in the field: an experiment with nocturnal and diurnal spiny mice under semi-natural field conditions

Light masking has been studied almost exclusively in the laboratory. The authors populated four field enclosures with locally coexisting nocturnal Acomys cahirinus and diurnal A. russatus, and monitored their body temperatures (T(b)) using implanted temperature-sensitive radio transmitters. A 3-h light pulse was initiated at the beginning of two consecutive nights; preceding nights were controls. A. cahirinus T(b) and calculated activity levels decreased significantly during the light pulse, demonstrating a negative light masking response (light effect on T(b): -0.32 °C?±?0.15 °C; average calculated activity records during the light pulse: 7?±?1.53, control: 9.8?±?1.62). Diurnal A. russatus did not respond to the light pulse. We conclude that light masking is not an artifact of laboratory conditions but represents a natural adaptive response in free-living populations.     

The cucurbit pathogenic bacterium Acidovorax citrulli requires a polar flagellum for full virulence before and after host-tissue penetration

Acidovorax citrulli causes seedling blight and bacterial fruit blotch of cucurbits. Previous reports demonstrated the contribution of type IV pili (T4P) to A. citrulli virulence and to systemic infection of melon seedlings. Microfluidic flow-chamber assays demonstrated the involvement of T4P in surface adhesion and biofilm formation, whereas polar flagella did not appear to contribute to either of these features. On the other hand, a transposon mutant impaired in the biosynthesis of polar flagella was identified in screens for reduced virulence of an A. citrulli mutant library. Further characterization of polar flagellum mutants confirmed that A. citrulli requires a polar flagellum for full virulence on melon plants. Foliage and stem inoculation experiments revealed that polar flagella contribute to A. citrulli virulence and growth in planta at both pre- and post-host-tissue penetration. Interestingly, light microscope observations revealed that almost all A. citrulli wild-type cells extracted from the xylem sap of stem-inoculated melon seedlings remained motile, supporting the importance of this organelle in virulence and colonization of the host vascular system. We also report a negative effect of polar flagellum impairment on T4P-mediated twitching motility of A. citrulli and discuss a possible co-regulation of these two motility machineries in this bacterium.     

Adaptive thermoregulation in golden spiny mice: the influence of season and food availability on body temperature

We studied the effect of food supplementation during summer and winter in seminatural field conditions on thermoregulation of a desert rodent, the golden spiny mouse Acomys russatus. We hypothesized that (a) under natural food availability (control conditions), mice will use less precise thermoregulation (i.e., an increase in the variance of body temperature [T(b)]) during winter because of low ambient temperatures (T(a)'s) and low food availability and during summer because of low food and water availability; (b) food supplementation will result in more precise thermoregulation during winter, but the effect will be smaller during summer because variation in T(b) in summer is also driven by water availability during that period. We found that under natural food availability, spiny mice thermoregulated more precisely during summer than during winter. They spent more time torpid during summer than during winter even when food was supplemented (although summer nights are shorter), allowing them to conserve water. Supplementing food resulted in more precise thermoregulation in both seasons, and mice spent less time torpid. In summer, thermoregulation at high T(a)'s was less precise, resulting in higher maximum T(b)'s in summer than in winter and when food was supplemented, in accord with the expected effect of water shortage on thermoregulation. Our results suggest that as expected, precise thermoregulation is beneficial when possible and is abandoned only when the costs of homeothermy outweigh the benefits.     

Biophysical modeling of the temporal niche: from first principles to the evolution of activity patterns

Most mammals can be characterized as nocturnal or diurnal. However infrequently, species may overcome evolutionary constraints and alter their activity patterns. We modeled the fundamental temporal niche of a diurnal desert rodent, the golden spiny mouse, Acomys russatus. This species can shift into nocturnal activity in the absence of its congener, the common spiny mouse, Acomys cahirinus, suggesting that it was competitively driven into diurnality and that this shift in a small desert rodent may involve physiological costs. Therefore, we compared metabolic costs of diurnal versus nocturnal activity using a biophysical model to evaluate the preferred temporal niche of this species. The model predicted that energy expenditure during foraging is almost always lower during the day except during midday in summer at the less sheltered microhabitat. We also found that a shift in summer to foraging in less sheltered microhabitats in response to predation pressure and food availability involves a significant physiological cost moderated by midday reduction in activity. Thus, adaptation to diurnality may reflect the "ghost of competition past"; climate-driven diurnality is an alternative but less likely hypothesis. While climate is considered to play a major role in the physiology and evolution of mammals, this is the first study to model its potential to affect the evolution of activity patterns of mammals.     

A new method for measuring scouring efficiency of natural fibers based on the cellulose-binding domain-beta-glucuronidase fused protein

Cellulose-binding domains (CBDs) are characterized by their ability to strongly bind to different forms of cellulose. This study examined the use of a recombinant CBD fused to the reporter enzyme beta-glucuronidase (CBD-GUS) to determine the extent of removal of the water-repellent waxy component of cotton fiber cuticles following hydrolytic treatment, i.e., scouring. The CBD-GUS test displayed higher sensitivity and repeatability than conventional water absorb techniques applied in the textile industry. Increases in the levels of CBD-GUS bound to the exposed cellulose correlated to increases in the fabric's hydrophilicity as a function of the severity of the scouring treatment applied, clearly indicating that the amount of bound enzyme increases proportionally with the amount of available binding sites. The binding of CBD-GUS also gave measurable and repeatable results when used on untreated or raw fabrics in comparison with conventional water drop techniques. The quantitative response of the reaction as bound enzyme activity was optimized for fully wettable fabrics. A minimal free enzyme concentration-to-swatch weight ratio of 75:1 was found to be necessary to ensure enzyme saturation (i.e., a linear response), corresponding to a free enzyme-to-bound enzyme ratio of at least 3:5.     

The role and regulation of the preRC component Cdc6 in the initiation of premeiotic DNA replication

In all eukaryotes, the initiation of DNA replication is regulated by the ordered assembly of DNA/protein complexes on origins of DNA replication. In this report, we examine the role of Cdc6, a component of the prereplication complex, in the initiation of premeiotic DNA replication in budding yeast. We show that in the meiotic cycle, Cdc6 is required for DNA synthesis and sporulation. Moreover, similarly to the regulation in the mitotic cell cycle, Cdc6 is specifically degraded upon entry into the meiotic S phase. By contrast, chromatin-immunoprecipitation analysis reveals that the origin-bound Cdc6 is stable throughout the meiotic cycle. Preliminary evidence suggests that this protection reflects a change in chromatin structure that occurs in meiosis. Using the cdc28-degron allele, we show that depletion of Cdc28 leads to stabilization of Cdc6 in the mitotic cycle, but not in the meiotic cycle. We show physical association between Cdc6 and the meiosis-specific hCDK2 homolog Ime2. These results suggest that under meiotic conditions, Ime2, rather than Cdc28, regulates the stability of Cdc6. Chromatin-immunoprecipitation analysis reveals that similarly to the mitotic cell cycle, Mcm2 binds origins in G1 and meiotic S phases, and at the end of the second meiotic division, it is gradually removed from chromatin.     

Replication and/or separation of some human telomeres is delayed beyond S-phase in pre-senescent cells

Cultured primary human cells, which lack telomerase, enter a state of replicative senescence after a characteristic number of population doublings. During this process telomeres shorten to a critical length of approximately 5-7 kb. The mechanistic relationship between advanced cell passage, cellular senescence and telomeric function has yet to be fully elucidated. In the study described here, we investigated the relationship between changes in telomeric replication timing and/or sister chromatid separation at telomeric regions and advanced cell passage. Using fluorescence in situ hybridization, we analyzed the appearance of double hybridization signals (doublets), which indicate that the region of interest has replicated and the replicated products have separated sufficiently to be resolved as two distinct signals. The results showed that the replication and separation of several telomeric regions occurs during the second half of S-phase and that a delay in replication and/or separation of sister chromatids at these regions occurs in pre-senescent human fibroblasts. Surprisingly, in a significant percentage of pre-senescent cells, several telomeric regions did not hybridize as doublets even in metaphase chromosomes. This delay was not associated with extensive changes in methylation levels at subtelomeric regions and was circumvented in human fibroblasts expressing ectopic telomerase. We propose that incomplete replication and/or separation of telomeric regions in metaphase may be associated with proliferative arrest of senescent cells. This cell growth arrest may result from the activation of a mitotic checkpoint, or from chromosomal instability consequent to progression in the cell cycle despite failure to replicate and/or separate these regions completely.     

The intracellular nucleotide‐binding leucine‐rich repeat receptor (SlNRC4a) enhances immune signalling elicited by extracellular perception

Plant recognition and defence against pathogens employs a two‐tiered perception system. Surface‐localized pattern recognition receptors (PRRs) act to recognize microbial features, whereas intracellular nucleotide‐binding leucine‐rich repeat receptors (NLRs) directly or indirectly recognize pathogen effectors inside host cells. Employing the tomato PRR LeEIX2/EIX model system, we explored the molecular mechanism of signalling pathways. We identified an NLR that can associate with LeEIX2, termed SlNRC4a (NB‐LRR required for hypersensitive response‐associated cell death‐4). Co‐immunoprecipitation demonstrates that SlNRC4a is able to associate with different PRRs. Physiological assays with specific elicitors revealed that SlNRC4a generally alters PRR‐mediated responses. SlNRC4a overexpression enhances defence responses, whereas silencing SlNRC4 reduces plant immunity. Moreover, the coiled‐coil domain of SlNRC4a is able to associate with LeEIX2 and is sufficient to enhance responses upon EIX perception. On the basis of these findings, we propose that SlNRC4a acts as a noncanonical positive regulator of immunity mediated by diverse PRRs. Thus, SlNRC4a could link both intracellular and extracellular immune perceptions.     

Ontogeny constrains phenology: opportunities for activity and reproduction interact to dictate potential phenologies in a changing climate

As global warming has lengthened the active seasons of many species, we need a framework for predicting how advances in phenology shape the life history and the resulting fitness of organisms. Using an individual‐based model, we show how warming differently affects annual cycles of development, growth, reproduction and activity in a group of North American lizards. Populations in cold regions can grow and reproduce more when warming lengthens their active season. However, future warming of currently warm regions advances the reproductive season but reduces the survival of embryos and juveniles. Hence, stressful temperatures during summer can offset predicted gains from extended growth seasons and select for lizards that reproduce after the warm summer months. Understanding these cascading effects of climate change may be crucial to predict shifts in the life history and demography of species.