Symbiotic bacteria often navigate complex environments before colonizing privileged sites in their host organism. Chemical gradients are known to facilitate directional taxis of these bacteria, guiding them toward their eventual destination. How-ever, less is known about the role of physical features in shaping the path the bacteria take and defining how they traverse a given space. The flagellated marine bacterium Vibrio fischeri, which forms a binary symbiosis with the Hawaiian bobtail squid, Euprymna scolopes, must navigate tight physical confinement during colonization, squeezing through a tissue bottleneck constricting to-2 mm in width on the way to its eventual home. Using microfluidic in vitro experiments, we discovered that V. fischeri cells alter their behavior upon entry into confined space, straightening their swimming paths and promoting escape from confinement. Using a computational model, we attributed this escape response to two factors: reduced directional fluctuation and a refractory period be-tween reversals. Additional experiments in asymmetric capillary tubes confirmed that V. fischeri quickly escape from confined ends, even when drawn into the ends by chemoattraction. This avoidance was apparent down to a limit of confinement approaching the diameter of the cell itself, resulting in a balance between chemoattraction and evasion of physical confinement. Our findings demon-strate that nontrivial distributions of swimming bacteria can emerge from simple physical gradients in the level of confinement. Tight spaces may serve as an additional, crucial cue for bacteria while they navigate complex environments to enter specific habitats.

The Hawaiian bobtail squid, Euprymna scolopes, harvests its luminous symbiont, Vibrio fischeri, from the surrounding seawater within hours of hatching. During embryogenesis, the host animal develops a nascent light organ with ciliated fields on each lateral surface. We hypothesized that these fields function to increase the efficiency of symbiont colonization of host tissues. Within minutes of hatching from the egg, the host's ciliated fields shed copious amounts of mucus in a non-specific response to bacterial surface molecules, specifically peptidoglycan (PGN), from the bacterioplankton in the surrounding seawater. Experimental manipulation of the system provided evidence that nitric oxide in the mucus drives an increase in ciliary beat frequency (CBF), and exposure to even small numbers of V. fischeri cells for short periods resulted in an additional increase in CBF. These results indicate that the light-organ ciliated fields respond specifically, sensitively, and rapidly, to the presence of nonspecific PGN as well as symbiont cells in the ambient seawater. Notably, the study provides the first evidence that this induction of an increase in CBF occurs as part of a thus far undiscovered initial phase in colonization of the squid host by its symbiont, i.e., host recognition of V. fischeri cues in the environment within minutes. Using a biophysics-based mathematical analysis, we showed that this rapid induction of increased CBF, while accelerating bacterial advection, is unlikely to be signaled by V. fischeri cells interacting directly with the organ surface. These overall changes in CBF were shown to significantly impact the efficiency of V. fischeri colonization of the host organ. Further, once V. fischeri has fully colonized the host tissues, i.e., about 12-24h after initial host-symbiont interactions, the symbionts drove an attenuation of mucus shedding from the ciliated fields, concomitant with an attenuation of the CBF. Taken together, these findings offer a window into the very first interactions of ciliated surfaces with their coevolved microbial partners.

We conducted a search for very short-period transiting objects in the publicly available Kepler data set. Our preliminary survey has revealed four planetary candidates, all with orbital periods less than 12 hr. We have analyzed the data for these candidates using photometric models that include transit light curves, ellipsoidal variations, and secondary eclipses to constrain the candidates' radii, masses, and effective temperatures. Even with masses of only a few Earth masses, the candidates' short periods mean that they may induce stellar radial velocity signals (a few m s(-1)) detectable by currently operating facilities. The origins of such short-period planets are unclear, but we discuss the possibility that they may be the remnants of disrupted hot Jupiters. Whatever their origins, if confirmed as planets, these candidates would be among the shortest-period planets ever discovered. Such planets would be particularly amenable to discovery by the planned TESS mission.

To date, most simulations of the final accretion of the terrestrial planets have assumed that all collisions lead to mergers. Recent hydrodynamic simulations of impacts between planetary mass bodies (Leinhardt, Z.M., Stewart, S.T. [2012]. Astrophys. J. 745,79; Genda, H., Kokubo, E., Ida, S. [2012]. Astrophys J. 744, 137) have parameterized the outcome of planetary collisions in terms of the masses and velocities of the colliding bodies. Using these results, it is now possible to simulate late-stage planetary growth using a more realistic model for collisions. Here, we describe results of eight N-body simulations of terrestrial planet formation that incorporate collisional fragmentation and hit-and-run collisions. The results are compared to simulations using identical initial collisions in which all collisions were assumed to result in mergers (Chambers, J.E. [2001]. Icarus 152, 205-224). The new simulations form 3 to 5 terrestrial planets moving on widely spaced orbits with growth complete by 400 My. The mean time for Earth-like planets to reach half their final mass is 17 My, comparable to the time in simulations without fragmentation. However, the prolonged sweep up of collision fragments lengthens the mean time required for Earth analogues to become fully formed to 159 My. The final planets have somewhat smaller masses m and eccentricities e when fragmentation is included. Masses are particularly reduced in the region now occupied by Mars. The final distributions of m, e and semi-major axis are similar to the terrestrial planets of the Solar System, but the strong concentration of mass in the narrow zone occupied by Earth and Venus is not reproduced. Collisional fragmentation is likely to preferentially eject silicate-rich mantle material leaving a target enriched in iron-rich core material. However, large bodies often reaccrete silicate-rich mantle fragments at a later time, leaving their final composition largely unchanged. The final core mass fractions of all but one planet formed in the simulations lie in the range 0.25-0.37 assuming an initial mass fraction of 0.3. (C) 2013 Elsevier Inc. All rights reserved.