No abstract

The sigma subunit of procaryotic RNA polymerases is responsible for specific promoter recognition and transcription initiation. In addition to the major sigma factor, 70 , in Escherichia coli, which directs most of the transcription in the cell, bacteria possess multiple, alternative sigma factors that direct RNA polymerase to distinct sets of promoters in response to environmental signals. By activating an alternative sigma factor, gene expression can be rapidly reprogrammed to meet the needs of the cell as the environment changes. Sigma factors are subject to multiple levels of regulation that control their levels and activities. The alternative sigma factor E in Escherichia coli is induced in response to extracytoplasmic stress. Here we demonstrate that E can also respond to signals other than extracytoplasmic stress. E activity increases in a growth phasedependent manner as a culture enters stationary phase. The signaling pathway that activates E during entry into stationary phase is dependent upon the alarmone guanosine 3,5-bispyrophosphate (ppGpp) and is distinct from the pathway that signals extracytoplasmic stress. ppGpp is the first cytoplasmic factor shown to control E activity, demonstrating that E can respond to internal signals as well as signals originating in the cell envelope. ppGpp is a general signal of starvation stress and is also required for activation of the S and 54 alternative sigma factors upon entry into stationary phase, suggesting that this is a key mechanism by which alternative sigma factors can be activated in concert to provide a coordinated response to nutritional stress.Stress responses allow organisms to survive and adapt to changing environments. In bacteria, many stress responses are mediated by alternative sigma factors that can rapidly reprogram gene expression in response to signals by recruiting RNA polymerase to specific subsets of promoters in the cell (16). Pathways that control the activity of these alternative sigma factors are central to the success of this regulatory strategy and ensure that the sigma factors are turned on and off at the appropriate times and with the appropriate kinetics. In general, regulation of alternative sigma factor activity is complex, with several layers of control over the expression of the sigma factor itself, as well as its activity. The alternative sigma factors can be activated both individually by dedicated signaling pathways that respond to specific signals and jointly by global regulatory pathways that activate multiple stress responses at once. These stress responses can also serve multiple functions in the cell, and some alternative sigma factors are activated by several different stresses.In Escherichia coli there are six sigma factors in addition to the major sigma factor, 70 , each of which has a unique role in stress survival and adaptation to environmental conditions. The alternative sigma factor E plays a key role in the response to stress in the cell envelope (1, 4). Cell envelope stresses that activate E are generally t...

Sexual selection arises from competition among individuals for access to mates, resulting in the evolution of conspicuous sexually selected traits, especially when inter-sexual competition is mediated by mate choice. Different sexual selection regimes may occur among populations/subspecies within the same species. This is particularly the case when mate choice is based on multiple sexually selected traits. However, empirical evidence supporting this hypothesis at the among-populations level is scarce. We conducted a meta-analysis of the intensity of sexual selection on the largest database to date for a single species, the barn swallow (Hirundo rustica), relying on quantitative estimates of sexual selection. The intensity of sexual selection was expressed as the strength (effect size) of the relationships between six plumage ornaments (tail length, tail asymmetry, size of white spots on tail, ventral plumage colour, throat plumage colour and throat patch size) and several fitness proxies related to reproduction, parental care, offspring quality, arrival date from spring migration, and survival. The data were gathered for four geographically separated subspecies (H. r. rustica, H. r. erythrogaster, H. r. gutturalis, H. r. transitiva). The overall mean effect size (Z = 0.214; 95% confidence interval = 0.175-0.254; N = 329) was of intermediate magnitude, with intensity of sexual selection being stronger in males than in females. Effect sizes varied during the breeding cycle, being larger before egg deposition, when competition for access to mates reaches its maximum (i.e. in the promiscuous part of the breeding cycle), and decreasing thereafter. In addition, effect sizes from experiments were not significantly larger than those from correlative studies. Finally, sexual selection on different sexually dimorphic traits varied among subspecies. This last result suggests that morphological divergence among populations has partly arisen from divergent sexual selection, which may eventually lead to speciation.

Circannual rhythms often rely on endogenous seasonal photoperiodic timers involving ‘clock’ genes, and Clock gene polymorphism has been associated to variation in phenology in some bird species. In the long-distance migratory barn swallow Hirundo rustica, individuals bearing the rare Clock allele with the largest number of C-terminal polyglutamine repeats found in this species (Q8) show a delayed reproduction and moult later. We explored the association between Clock polymorphism and migration scheduling, as gauged by light-level geolocators, in two barn swallow populations (Switzerland; Po Plain, Italy). Genetic polymorphism was low: 91% of the 64 individuals tracked year-round were Q7/Q7 homozygotes. We compared the phenology of the rare genotypes with the phenotypic distribution of Q7/Q7 homozygotes within each population. In Switzerland, compared to Q7/Q7, two Q6/Q7 males departed earlier from the wintering grounds and arrived earlier to their colony in spring, while a single Q7/Q8 female was delayed for both phenophases. On the other hand, in the Po Plain, three Q6/Q7 individuals had a similar phenology compared to Q7/Q7. The Swiss data are suggestive for a role of genetic polymorphism at a candidate phenological gene in shaping migration traits, and support the idea that Clock polymorphism underlies phenological variation in birds.

Trade-offs select for optimal allocation of resources among competing functions. Parents are selected to maximize production of viable offspring by balancing between progeny number and "quality." Telomeres are nucleoproteins, at the ends of eukaryotic chromosomes, that shorten when cells divide. Because shortening below a certain threshold depresses organismal functioning and rate of shortening depends on environmental conditions, telomeres are good candidates as mediators of trade-offs. We altered brood size of barn swallow Hirundo rustica and found that brood enlargement caused a reduction in relative telomere length (RTL). Reliable signals of offspring quality should evolve that mediate adaptive parental care allocation. Because nestlings with darker coloration receive more care, we analyzed the covariation between RTL and coloration and found that RTL increased with plumage darkness, both within and between broods. Hence, we provide unprecedented evidence that signals relevant to parent-offspring communication reflect telomere length and thus offspring reproductive value.