Genetic Changes to a Transcriptional Silencer Element Confers Phenotypic Diversity within and between Drosophila Species

The modification of transcriptional regulation has become increasingly appreciated as a major contributor to morphological evolution. However, the role of negative-acting control elements (e.g. silencers) in generating morphological diversity has been generally overlooked relative to positive-acting “enhancer” elements. The highly variable body coloration patterns among Drosophilid insects represents a powerful model system in which the molecular alterations that underlie phenotypic diversity can be defined. In a survey of pigment phenotypes among geographically disparate Japanese populations of Drosophila auraria, we discovered a remarkable degree of variation in male-specific abdominal coloration. In testing the expression patterns of the major pigment-producing enzymes, we found that phenotypes uniquely correlated with differences in the expression of ebony, a gene required for yellow-colored cuticle. Assays of ebony’s transcriptional control region indicated that a lightly pigmented strain harbored cis-regulatory mutations that caused correlated changes in its expression. Through a series of chimeric reporter constructs between light and dark strain alleles, we localized function-altering mutations to a conserved silencer that mediates a male-specific pattern of ebony repression. This suggests that the light allele was derived through the loss of this silencer’s activity. Furthermore, examination of the ebony gene of D. serrata, a close relative of D. auraria which secondarily lost male-specific pigmentation revealed the parallel loss of this silencer element. These results demonstrate how loss-of-function mutations in a silencer element resulted in increased gene expression. We propose that the mutational inactivation of silencer elements may represent a favored path to evolve gene expression, impacting morphological traits

Mercury's Surface Magnetic Field Determined from Proton-Reflection Magnetometry

Solar wind protons observed by the MESSENGER spacecraft in orbit about Mercury exhibit signatures of precipitation loss to Mercury's surface. We apply proton-reflection magnetometry to sense Mercury's surface magnetic field intensity in the planet's northern and southern hemispheres. The results are consistent with a dipole field offset to the north and show that the technique may be used to resolve regional-scale fields at the surface. The proton loss cones indicate persistent ion precipitation to the surface in the northern magnetospheric cusp region and in the southern hemisphere at low nightside latitudes. The latter observation implies that most of the surface in Mercury's southern hemisphere is continuously bombarded by plasma, in contrast with the premise that the global magnetic field largely protects the planetary surface from the solar wind

Mercury's Magnetopause and Bow Shock from MESSENGER Magnetometer Observations

We have established the average shape and location of Mercury's magnetopause and bow shock from orbital observations by the MESSENGER Magnetometer. We fit empirical models to midpoints of boundary crossings and probability density maps of the magnetopause and bow shock positions. The magnetopause was fit by a surface for which the position R from the planetary dipole varies as [1 + cos(theta)]-alpha, where theta is the angle between R and the dipole-Sun line, the subsolar standoff distance Rss is 1.45 RM (where RM is Mercury's radius), and the flaring parameter alpha = 0.5. The average magnetopause shape and location were determined under a mean solar wind ram pressure PRam of 14.3 nPa. The best fit bow shock shape established under an average Alfvén Mach number (MA) of 6.6 is described by a hyperboloid having Rss = 1.96 RM and an eccentricity of 1.02. These boundaries move as PRam and MA vary, but their shapes remain unchanged. The magnetopause Rss varies from 1.55 to 1.35 RM for PRam in the range of 8.8-21.6 nPa. The bow shock Rss varies from 2.29 to 1.89 RM for MA in the range of 4.12-11.8. The boundaries are well approximated by figures of revolution. Additional quantifiable effects of the interplanetary magnetic field are masked by the large dynamic variability of these boundaries. The magnetotail surface is nearly cylindrical, with a radius of ~2.7 RM at a distance of 3 RM downstream of Mercury. By comparison, Earth's magnetotail flaring continues until a downstream distance of ~10 Rss

Characteristics of the plasma distribution in Mercury's equatorial magnetosphere derived from MESSENGER Magnetometer observations

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96228/1/jgra22273.pd

Low‐degree structure in Mercury's planetary magnetic field

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96330/1/jgre3134.pd

Plasma pressure in Mercury's equatorial magnetosphere derived from MESSENGER Magnetometer observations

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95264/1/grl28621-sup-0002-txts01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/95264/2/grl28621.pd

Constraints on the secular variation of Mercury's magnetic field from the combined analysis of MESSENGER and Mariner 10 data

Observations of Mercury's internal magnetic field from the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have revealed a dipole moment of 190 nT R M3 offset about 480 km northward from the planetary equator, where R M is Mercury's radius. We have reanalyzed magnetic field observations acquired by the Mariner 10 spacecraft during its third flyby of Mercury (M10‐III) in 1975 to constrain the secular variation in the internal field over the past 40 years. With the application of techniques developed in the analysis of MESSENGER data, we find that the dipole moment that best fits the M10‐III data is 188 nT R M3 offset 475 km northward from the equator. Our results are consistent with no secular variation, although variations of up to 10%, 16%, and 35%, respectively, are permitted in the zonal coefficients g 10, g 20, and g 30 in a spherical harmonic expansion of the internal field

Solar wind forcing at Mercury: WSA‐ENLIL model results

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97237/1/jgra50070.pd

MESSENGER observations of induced magnetic fields in Mercury's core

Orbital data from the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft allow investigation of magnetic fields induced at the top of Mercury's core by time‐varying magnetospheric fields. We used 15 Mercury years of observations of the magnetopause position as well as the magnetic field inside the magnetosphere to establish the presence and magnitude of an annual induction signal. Our results indicate an annual change in the internal axial dipole term, g10, of 7.5 to 9.5 nT. For negligible mantle conductivity, the average annual induction signal provides an estimate of Mercury's core radius to within ±90 km, independent of geodetic results. Larger induction signals during extreme events are expected but are challenging to identify because of reconnection‐driven erosion. Our results indicate that the magnetopause reaches the dayside planetary surface 1.5–4% of the time