Genetic Factors Affecting Hybrid Male Sterility Leading to Speciation

The process whereby speciation occurs can come about through the evolution of barriers to gene flow. One of these barriers to gene flow can be an incompatibility, which leaves hybrids dead or sterile. Two theories underlie the work of this experiment, Haldane’s Rule and the large X effect. Haldane’s Rule is the observation that unisexual inviability or sterility among species’ hybrids is almost always found in the heterogametic sex. The large X effect is the observation that substitution of one species’ X-chromosome for another’s has a disproportionately large effect on hybrid fitness compared to similar substitution of an autosome. For Drosophila, the cause of the large X effect has been identified as density dependent for the number of sterility-inducing incompatibilities on the Xchromosome. In this project we are using Drosophila simulans and Drosophila mauritiana. We are using introgressed lines of flies that make use of physical markers that can be used to track the progress of genetic material throughout crosses. The visible markers that we are using affect eye color and express fluorescent protein, allowing us to determine the regions on the recombinant chromosomes that contain the factors leading to hybrid male sterility. Males that carry the recombinant X-chromosomes are sterile unless the sterility factors have been removed via recombination. Flies that are fertile will be genotyped using Real-Time PCR. Genetic mapping will then allow us to determine the location of the sterility-causing gene in question. At this time we have generated a number of recombinant genotypes and through the genotyping of these samples we have narrowed our candidate region. At the start of this project the region was approximately 300kb in length and we have shortened that segment of interest to 100kb. By shortening this segment we have narrowed our search area for this sterility-causing gene that is of interest to us

A recurrent neural network approach to quantitatively studying solar wind effects on TEC derived from GPS; preliminary results

This paper attempts to describe the search for the parameter(s) to represent solar wind effects in Global Positioning System total electron content (GPS TEC) modelling using the technique of neural networks (NNs). A study is carried out by including solar wind velocity (Vsw), proton number density (Np) and the Bz component of the interplanetary magnetic field (IMF Bz) obtained from the Advanced Composition Explorer (ACE) satellite as separate inputs to the NN each along with day number of the year (DN), hour (HR), a 4-month running mean of the daily sunspot number (R4) and the running mean of the previous eight 3-hourly magnetic A index values (A8). Hourly GPS TEC values derived from a dual frequency receiver located at Sutherland (32.38° S, 20.81° E), South Africa for 8 years (2000–2007) have been used to train the Elman neural network (ENN) and the result has been used to predict TEC variations for a GPS station located at Cape Town (33.95° S, 18.47° E). Quantitative results indicate that each of the parameters considered may have some degree of influence on GPS TEC at certain periods although a decrease in prediction accuracy is also observed for some parameters for different days and seasons. It is also evident that there is still a difficulty in predicting TEC values during disturbed conditions. The improvements and degradation in prediction accuracies are both close to the benchmark values which lends weight to the belief that diurnal, seasonal, solar and magnetic variabilities may be the major determinants of TEC variability

Towards a GPS-based TEC prediction model for Southern Africa with feed forward networks

In this paper, first results from a national Global Positioning System (GPS) based total electron content (TEC) prediction model over South Africa are presented. Data for 10 GPS receiver stations distributed through out the country were used to train a feed forward neural network (NN) over an interval of at most five years. In the NN training, validating and testing processes, five factors which are well known to influence TEC variability namely diurnal variation, seasonal variation, magnetic activity, solar activity and the geographic position of the GPS receivers were included in the NN model. The database consisted of 1-min data and therefore the NN model developed can be used to forecast TEC values 1 min in advance. Results from the NN national model (NM) were compared with hourly TEC values generated by the earlier developed NN single station models (SSMs) at Sutherland (32.38°S, 20.81°E) and Springbok (29.67°S, 17.88°E), to predict TEC variations over the Cape Town (33.95°S, 18.47°E) and Upington (28.41°S, 21.26°E) stations, respectively, during equinoxes and solstices. This revealed that, on average, the NM led to an improvement in TEC prediction accuracy compared to the SSMs for the considered testing periods

Breaking the Rocket Equation: How Refuellable Spacecraft Change the Dynamics of Space Transportation

One of the chief tenets of space transportation has been the immutability of the Tsiolkovsky rocket equation, placing great emphasis on the specific impulse of a thruster to determine the propellant mass. For applications where all propellant must be carried from the start, this drives most vehicle designs to the highest possible ISP. The compromise is the typical tradeoff between ISP and thrust magnitude. Thus, transfers which require either impulsive maneuvers or a tight timeline will favor low-ISP platforms. If adequate infrastructure is provided to allow for a refuellable spacecraft, a smaller and lighter vehicle can be used. This vehicle will have a payload mass fraction more in line with an equivalent system with many multiples higher specific impulse. As effective specific impulse increases to values approaching the highest performance electric propulsion systems, the time to destination remains of the same order of magnitude as an impulsive orbital maneuver. Further work must be done to optimize for the positioning and design of fuel depot infrastructure, especially bearing in mind both interactions with high radiation in the Van Allen belts, and ideal orbital planes to seed with these refuel depots

Reconstructing ionospheric TEC over South Africa using signals from a regional GPS network

Radio signals transmitted by GPS satellites orbiting the Earth are modulated as they propagate through the electrically charged plasmasphere and ionosphere in the near-Earth space environment. Through a linear combination of GPS range and phase measurements observed on two carrier frequencies by terrestrial-based GPS receivers, the ionospheric total electron content (TEC) along oblique GPS signal paths may be quantified. Simultaneous observations of signals transmitted by multiple GPS satellites and observed from a network of South African dual frequency GPS receivers, constitute a spatially dense ionospheric measurement source over the region. A new methodology, based on an adjusted spherical harmonic (ASHA) expansion, was developed to estimate diurnal vertical TEC over the region using GPS observations over the region. The performance of the ASHA methodology to estimate diurnal TEC and satellite and receiver differential clock biases (DCBs) for a single GPS receiver was first tested with simulation data and subsequently applied to observed GPS data. The resulting diurnal TEC profiles estimated from GPS observations compared favourably to measurements from three South African ionosondes and two other GPS-based methodologies for 2006 solstice and equinox dates. The ASHA methodology was applied to calculating diurnal two-dimensional TEC maps from multiple receivers in the South African GPS network. The space physics application of the newly developed methodology was demonstrated by investigating the ionosphere’s behaviour during a severe geomagnetic storm and investigating the long-term ionospheric stability in support of the proposed Square Kilometre Array (SKA) radio astronomy project. The feasibility of employing the newly developed technique in an operational near real-time system for estimating and dissimenating TEC values over Southern Africa using observations from a regional GPS receiver network, was investigated

Application of neural networks to South African GPS TEC modelling

The propagation of radio signals in the Earth’s atmosphere is dominantly affected by the ionosphere due to its dispersive nature. Global Positioning System (GPS) data provides relevant information that leads to the derivation of total electron content (TEC) which can be considered as the ionosphere’s measure of ionisation. This paper presents part of a feasibility study for the development of a Neural Network (NN) based model for the prediction of South African GPS derived TEC. The South African GPS receiver network is operated and maintained by the Chief Directorate Surveys and Mapping (CDSM) in Cape Town, South Africa. Vertical total electron content (VTEC) was calculated for four GPS receiver stations using the Adjusted Spherical Harmonic (ASHA) model. Factors that influence TEC were then identified and used to derive input parameters for the NN. The well established factors used are seasonal variation, diurnal variation, solar activity and magnetic activity. Comparison of diurnal predicted TEC values from both the NN model and the International Reference Ionosphere (IRI-2001) with GPS TEC revealed that the IRI provides more accurate predictions than the NN model during the spring equinoxes. However, on average the NN model predicts GPS TEC more accurately than the IRI model over the GPS locations considered within South Africa

Reuseable Orbital Transfer Vehicles: Why it is the Future for LEO, Cislunar Space and Beyond

The emerging Low-Earth-Orbit commercial economy brings exciting opportunities for technological innovation in space at unprecedented speed. This vibrant new commercial space age is disrupting the status quo, driving down costs, and revolutionizing general accessibility and sustained presence in LEO. Today, however, there is a reluctance in the space industry to embrace reusable spacecraft because of the perception of increased mission risk for no additional return on investment. This hesitation appears similar to the industry’s initial caution in adopting reusable launch vehicles in the early 2000s. In this work we share our vision of the future: reusable orbital service vehicles (OSVs) will transform the space economy and grow the emerging on-orbit servicing sector. A reusable OSV serves as a satellite’s ‘connecting flight’ that provides multiple on-orbit destinations, analogous to the airline industry. Development of a reliable, reusable OSV will expand the utility of a single satellite, allowing for plane changes, escape trajectories, multi-orbit missions and more. OSVs further enable payload upgrades, satellite constellation maintenance, deorbiting at the end of operational life, and orbital debris removal. These added capabilities differentiate reusable OSVs from single orbit mission alternatives and increase the on-orbit economic opportunity. Once a network of OSVs has been established in LEO, rendezvous and transfers can be scheduled efficiently to minimize on-orbit wait times between connections. This poster emphasizes current trends in the industry and presents an OSV architecture and logistics model that enables expanded access to space. Safety and reliability aspects are considered, and a satellite constellation design reference mission discussed. Future work will leverage 6 decades of hard-won lessons and insights from the commercial airline industry with its hub and spoke carrier models to improve OSV reliability and cost effectiveness. Just like reusable launch vehicles drastically reduced launch costs, and well-placed hub airports transformed the costs and safety of commercial air travel, reusable OSVs and orbital infrastructure have the potential to increase space asset safety and return on investment. These vehicles have utility in LEO and cislunar space, as well as far reaching deep space missions to Mars and beyond. Ultimately, more affordable, sustained access to space will enable the next deep space telescope, space station, or other similarly bold and impactful endeavors to be built and deployed in orbit at a fraction of the cost of a terrestrially built equivalent

Development of a regional GPS-based ionospheric TEC model for South Africa

Advances in South African space physics research and related disciplines require better spatial and time resolution ionospheric information than was previously possible with the existing ionosonde network. A GPS-based, variable degree adjusted spherical harmonic (ASHA) model was developed for near real-time regional ionospheric total electron content (TEC) mapping over South Africa. Slant TEC values along oblique GPS signal paths are quantified from a network of GPS receivers and converted to vertical TEC by means of the single layer mapping function. The ASHA model coefficients and GPS differential biases are estimated from vertical TEC at the ionospheric pierce points and used to interpolate TEC at any location within the region of interest. Diurnal TEC variations with one minute time resolution and time-varying 2D regional TEC maps are constructed. In order to validate the ASHA method, simulations with an IRI ionosphere were performed, while the ASHA results from actual data were compared with two independent GPS-based methodologies and measured ionosonde data

Comparison of the ability of anthropometric indices to predict the risk of diabetes mellitus in South African males: Sanhanes-1

This study aimed to assess the sensitivity of body mass index (BMI) to predict the risk of diabetes mellitus (DM) and whether waist circumference (WC), waist-to-hip (WHR) and waist-toheight (WHtR) ratios are better predictors of the risk of DM than BMI in South African men aged 20 years and older. Data from the first South African National Health and Nutrition Examination Survey (SANHANES-1) were used. Overall, 1405 men who had valid HbA1c outcomes were included. The sensitivity, specificity, and optimal cut-off points for predicting DM were determined using the receiver operating characteristic (ROC) curve analysis. A total of 34.6% percent of the study participants were overweight/obese, while 10.5%, 10.4%, 36.6% and 61.0% had HbA1c, WC, WHR and WHtR above the normal reference ranges, respectively