3,408 research outputs found
Under-dominance constrains the evolution of negative autoregulation in diploids
Regulatory networks have evolved to allow gene expression to rapidly track
changes in the environment as well as to buffer perturbations and maintain
cellular homeostasis in the absence of change. Theoretical work and empirical
investigation in Escherichia coli have shown that negative autoregulation
confers both rapid response times and reduced intrinsic noise, which is
reflected in the fact that almost half of Escherichia coli transcription
factors are negatively autoregulated. However, negative autoregulation is
exceedingly rare amongst the transcription factors of Saccharomyces cerevisiae.
This difference is all the more surprising because E. coli and S. cerevisiae
otherwise have remarkably similar profiles of network motifs. In this study we
first show that regulatory interactions amongst the transcription factors of
Drosophila melanogaster and humans have a similar dearth of negative
autoregulation to that seen in S. cerevisiae. We then present a model
demonstrating that this fundamental difference in the noise reduction
strategies used amongst species can be explained by constraints on the
evolution of negative autoregulation in diploids. We show that regulatory
interactions between pairs of homologous genes within the same cell can lead to
under-dominance - mutations which result in stronger autoregulation, and
decrease noise in homozygotes, paradoxically can cause increased noise in
heterozygotes. This severely limits a diploid's ability to evolve negative
autoregulation as a noise reduction mechanism. Our work offers a simple and
general explanation for a previously unexplained difference between the
regulatory architectures of E. coli and yeast, Drosophila and humans. It also
demonstrates that the effects of diploidy in gene networks can have
counter-intuitive consequences that may profoundly influence the course of
evolution
Reporting of conflicts of interest in oral presentations at medical conferences : a delegate-based prospective observational study
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Age and size at maturity: sex, environmental variability and developmental thresholds
In most organisms, transitions between different life-history stages occur later and at smaller sizes as growth conditions deteriorate. Day and Rowe recently proposed that this pattern could be explained by the existence of developmental thresholds (minimum sizes or levels of condition below which transitions are unable to proceed). The developmental-threshold model predicts that the reaction norm of age and size at maturity will rotate in an anticlockwise manner from positive to a shallow negative slope if: (i) initial body size or condition is reduced; and/or (ii) some individuals encounter poor growth conditions at increasingly early developmental stages. We tested these predictions by rearing replicated populations of soil mites Sancassania berlesei (Michael) under different growth conditions. High-food environments produced a vertical relationship between age and size at maturity. The slope became increasingly shallow as food was reduced. By contrast, high food in the maternal environment reduced the slope of the reaction norm of age and size at maturity, whereas low food increased it. Overall, the reaction norm of age and size at maturity in S. berlesei was significantly nonlinear and differed for males and females. We describe how growth conditions, mother's environment and sex determine age and size at maturity in S. berlesei
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