The generation of acoustic communication signals is widespread across the animal kingdom, and males of many species, including Drosophilidae, produce patterned courtship songs to increase their chance of success with a female. For some animals, song structure can vary considerably from one rendition to the next; neural noise within pattern generating circuits is widely assumed to be the primary source of such variability, and statistical models that incorporate neural noise are successful at reproducing the full variation present in natural songs. In direct contrast, here we demonstrate that much of the pattern variability in Drosophila courtship song can be explained by taking into account the dynamic sensory experience of the male. In particular, using a quantitative behavioural assay combined with computational modelling, we find that males use fast modulations in visual and self-motion signals to pattern their songs, a relationship that we show is evolutionarily conserved. Using neural circuit manipulations, we also identify the pathways involved in song patterning choices and show that females are sensitive to song features. Our data not only demonstrate that Drosophila song production is not a fixed action pattern, but establish Drosophila as a valuable new model for studies of rapid decision-making under both social and naturalistic conditions.
We study intact and bulging Escherichia coli cells using atomic force microscopy to separate the contributions of the cell wall and turgor pressure to the overall cell stiffness. We find strong evidence of power-law stress-stiffening in the E. coli cell wall, with an exponent of 1.22 ± 0.12, such that the wall is significantly stiffer in intact cells (E = 23 ± 8 MPa and 49 ± 20 MPa in the axial and circumferential directions) than in unpressurized sacculi. These measurements also indicate that the turgor pressure in living cells E. coli is 29 ± 3 kPa.Many cellular-scale processes in biology, such as cell growth, division and motility, necessarily involve mechanical interactions. Recent theoretical work in bacteria has led to a number of physically-realistic models of bacterial cells [1][2][3]. However, in many instances, precise, direct measurements of the mechanical properties of cellular components in live cells are lacking.The cell envelope in most bacteria is made of one or two layers of membrane and a rigid cell wall consisting of a network of peptidoglycan (PG) polymers. These two materials serve different cellular functions. The semipermeable plasma membrane maintains a chemical separation between the cell interior and the surrounding medium. The large concentration of solutes in the cytoplasm generates an osmotic pressure, termed turgor pressure, that pushes the plasma membrane against the cell wall. The cell wall, on the other hand, defines the cell shape and constrains the volume under turgor.The magnitude of the turgor pressure under physiological conditions has been estimated using several techniques: by collapsing gas vesicles in rare species of bacteria [4], by AFM indentation [5, 6], and by calculating the total chemical content of the cytoplasm [7]. The estimated pressure values vary by more than an order of magnitude, from 10 4 to 3 × 10 5 Pa. While mechanical experiments, such as AFM indentation, are the most direct probes, separating the mechanical contributions of the wall and pressure has not been previously possible and thus these experiments may only provide an upper bound on the true turgor pressure.Similarly, the elasticity of the cell wall has been difficult to probe in individual, live cells. Most previous mechanical measurements on the cell wall have been performed using chemically isolated walls, termed sacculi, that may be altered from the native state, or on large bundles of cells [8]. Yao et al. reported an anisotropic elasticity of 25 MPa and 45 MPa in the axial and circumferential directions relative to a cell's rod-shape using single flattened E. coli sacculi adhered to a substrate [9]. Thwaites and coauthors probed the elastic modulus of macroscopic threads of many Bacillus subtilis sacculi in humid air and found that the modulus varied from 10 to 30 MPa depending on the humidity and salt concentration [10][11][12]. Attempts to probe whole-cell elasticity have also been made using AFM indentation of Myxococcus xanthus cells [13] and optical-tweezer bending of Borrelia burgdorf...
Production of healthy gametes requires a reductional meiosis I division in which replicated sister chromatids co-migrate, rather than separating as in mitosis or meiosis II. Fusion of sister kinetochores during meiosis I may underlie sister chromatid co-migration in diverse organisms, but direct evidence for such fusion has been lacking. Here we studied native kinetochore particles isolated from yeast using laser trapping and quantitative fluorescence microscopy. Meiosis I kinetochores formed stronger attachments and carried more microtubule-binding elements than kinetochores isolated from cells in mitosis or meiosis II. The meiosis I-specific monopolin complex was both necessary and sufficient to drive these modifications. Thus, kinetochore fusion directs sister chromatid co-migration, a conserved feature of meiosis that is fundamental to Mendelian inheritance.
Many single-particle tracking and localization-based superresolution imaging techniques use the width of a single lateral fluorescence image to estimate a molecule's axial position. This determination is often done by use of a calibration data set derived from a source adhered to a glass-water interface. However, for sources deeper in solution, aberrations will change the relationship between the image width and the axial position. We analyzed the depth-varying point spread function of a high numerical aperture objective near an index of refraction mismatch at the water-glass interface using an optical trap. In addition to the well-known focal shift, spherical aberrations cause up to 30% relative systematic error in axial position estimation. This effect is nonuniform in depth, and we find that, although molecules below the focal plane are correctly localized, molecules deeper than the focal plane are found to be lower than their actual positions.
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