Intra-clonal variation and a similarity threshold for identification of clones: application to Salix exigua using AFLP molecular markers.

Although molecular methods are a major advance over phenological or root connectivity studies in the identification of clonal plants, there is still a level of ambiguity associated with two types of error: misidentification of genetically similar seedlings as clones and misidentification of dissimilar fingerprints from clones as genetically distinct individuals. We have addressed the second of these error types by determining the level of variation for AFLP fingerprints in Salix exigua, and then by developing a threshold value of Jaccard's similarity index for assigning individuals to clones or to siblings. Variation in AFLP banding patterns among clones was partitioned into three potential sources; clones, stems within-clones and foliage within-stems. Most of the variation was attributable to clones and then to stems within-clones. To provide an objective means of identifying clones, we developed a method for establishing a threshold similarity index to assign individuals to the same clone. Our method yielded a Jaccard similarity threshold of 0.983 that resulted in a potential pairwise error rate of 8.1% putative clone assigned to siblings and 1.5% sibling assigned to clones. The method was tested on independent clonal and sibling individuals resulting in the same threshold value and similar error rates. We applied our method to assign individuals to clones in a population of S. exigua along the Cosumnes River, California. A total of 11 clones were identified, with one clone including 43% of the individuals sampled. Our results show that this approach can be useful in the accurate identification of clones.

Land barriers and open oceans: effects on gene diversity and population structure in Avicennia germinans L. (Avicenniaceae).

Avicennia germinans L. is a widespread mangrove species occupying the west coast of Africa and the Atlantic and Pacific coasts of the Americas from the Bahamas to Brazil and Baja California to Peru. An amplified fragment length polymorphism (AFLP) molecular analysis was carried out to assess genetic architecture within this species and to evaluate the effects of the Atlantic Ocean and the Central American Isthmus (CAI) on population and regional genetic diversity and differentiation. In total, 349 polymorphic AFLP fragments were identified among 144 individuals from 14 populations from the east Atlantic, west Atlantic and east Pacific. Levels of genetic diversity varied considerably among populations, but were generally higher in populations from the east Atlantic. Regional differentiation between the Pacific coast and Atlantic populations was greater than between east and west Atlantic populations, suggesting that the CAI has had an important influence on population genetic structure in this species. The lower level of divergence of east Atlantic from west Atlantic populations suggests some dispersal across the Atlantic Ocean, although migration rates are probably low; Nm from GST equal to 0.41 and accumulation of private and rare alleles in the east Atlantic. Population differentiation did not appear to follow an isolation by distance model and has probably resulted from complex patterns of population bottlenecks, and founder events due to landscape changes during the Pleistocene, particularly in the west Atlantic. The molecular data provide no support for the treatment of east Atlantic populations as a separate species A. africana.

The epidemiology of aircraft fire in commuter and air taxi crashes.

BACKGROUND: Aircraft fire is recognized as an important risk factor for occupant fatality in aviation crashes, but its epidemiology has not been adequately studied. OBJECTIVE: This study examines factors related to the occurrence of aircraft fire, ignition sources, and factors influencing rescue and firefighting in commuter and air taxi crashes. METHODS: Commuter and air taxi crashes in 1983-88 that involved aircraft fire were analyzed using National Transportation Safety Board (NTSB) data, including NTSB supplemental forms on fire and on rescue and firefighting. RESULTS: Aircraft fire was recorded in 17% of the 888 commuter and air taxi crashes. Factors associated with increased likelihood of fire in these crashes were non-airport location (adjusted relative risk 2.7, 95% confidence interval 1.8-4.1), twin-engine aircraft (RR 2.5, 95% Cl 1.6-3.9), and nighttime (RR 1.7, 95% Cl 1.1-2.5). Fire was more common in Beech 18's than other twin-engine airplanes. The majority (59%) of fires occurred during impact sequence, and an additional 17% occurred after the aircraft came to rest. The engine was identified as the ignition source in 59% of the crashes, followed by hot surface (18%), short circuit (9%), and sparks (9%). Fire sensing and extinguishing systems were installed in only 19% of the aircraft. Rescue and firefighting efforts were hampered most commonly by terrain (48%), weather (47%), and darkness (38%). CONCLUSIONS: Typically induced by impact and started at the engine, aircraft fire is most likely when a crash occurs at night, in a non-airport location, and in instrument meteorological conditions. These high-risk circumstances also often thwart rescue and firefighting efforts.

Crashes of instructional flights.

BACKGROUND: A pilot's basic flight training influences his or her entire flying career. Instructional flights are involved in more than 300 crashes annually and in 36% of all midair collisions. Research was undertaken to identify the circumstances of crashes of instructional flights and to describe related factors. METHODS: We analyzed NTSB data types and 2-page descriptive briefs for 638 crashes that occurred during 1989 and 1991 and that involved a student pilot or a flight for instructional purposes. RESULTS: Loss of control on landing characterized 36% of all crashes, and crosswinds contributed to 28%. Stalls occurred in 15% of the series and 46% of all fatal crashes. Trainees on solo comprised 56% of the series; 193 of the 360 students who crashed on solo did so due to loss of control on landing or takeoff. Touch-and-go landings accounted for 22% of all crashes on solo. Among the 84 crashes on cross-country solos, 26 (31%) were due to running out of fuel. Instructors were present in 50% of crashes from stalls and 32% of crashes from fuel starvation. Simulated emergencies ended in 49 crashes. The NTSB identified instructor factors as contributory to one-third of cases. CONCLUSIONS: Greater emphasis during flight training needs to be placed on avoiding stalls and midair collisions, managing crosswinds, and understanding the dynamics of takeoffs and landings prior to solo touch-and-go practice. Instructors contribute to crashes both directly during dual instruction and indirectly through failure to successfully monitor their students and convey the elements necessary for safe and proficient flight.

The cost-effectiveness of air medical helicopter crash survival enhancements. An evaluation of the costs, benefits and effectiveness of injury prevention interventions.

OBJECTIVE: This study evaluates EMS helicopter-injury reduction interventions and associated costs for survivable crashes. Specific injury categories evaluated include head injuries, spinal injuries and thermal injuries. The benefits and costs of the preventive interventions are evaluated through cost-effectiveness analyses that provide a basis for informed safety-enhancement decisions for EMS helicopter operators, based on the most cost-efficient interventions. METHODS: The incidence and type of injuries experienced by EMS helicopter occupants were determined, and future risk of injury was estimated. Then the costs of those injuries, as well as the costs of the preventive interventions, were determined so estimates could be made of the cost benefits of the injuries prevented. Estimates were made regarding current levels of injury prevention interventions already in the field and their effectiveness in preventing injury. RESULTS: Improvements can be made to reduce the risk of injury to medical crew and pilots in survivable crashes. Nomex uniforms, helmets, and energy-absorbing seats (EAS) for medical crew members all prove cost-effective in reducing preventable injuries in survivable crashes. CONCLUSION: Emergency medical service helicopter occupants should wear fire-resistant uniforms and helmets, and medical crew members should have EAS systems when available. These EAS systems also are recommended for pilots, although they were not shown to be cost-effective based on the projections developed in this study.

Human factors in crashes of commuter airplanes.

Pilots and frequent travelers on commuter aircraft are exposed to higher risks of death or injury than those on major air carriers. To provide a better understanding of the circumstances of crashes of scheduled commuter airplanes, National Transportation Safety Board data were analyzed for all cases of death, serious injury, or major damage involving commuter airplanes during 1983-88, when 172 people were killed and 207 injured in 118 events. Three-fourths of cases involved inadequate pilot performance, notably poor handling of emergencies and improper instrument flying procedures. Pilot errors occurred disproportionately in bad weather, which played a role in 30% of crashes. Aircraft malfunctions were involved in 42% of crashes. Certain airplanes were overinvolved in gear-up landings or in crashes due to fuel mismanagement. Greater priority should be given to applying known preventive measures to the problem of commuter crashes.