Fragmentation of metastable SF6−* ions with microsecond lifetimes in competition with autodetachment

Fragmentation of metastable SF6-* ions formed in low energy electron attachment to SF₆has been investigated. The dissociation reaction SF6-*?SF5-+F has been observed ~ 1.5–3.4 µs and ~ 17–32 µs after electron attachment in a time-of-flight and a double focusing two sector field mass spectrometer, respectively. Metastable dissociation is observed with maximum intensity at ~ 0.3 eV between the SF6-* peak at zero and theSF5- peak at ~ 0.4 eV. The kinetic energy released in dissociation is low, with a most probable value of 18 meV. The lifetime of SF6-* decreases as the electron energy increases, but it is not possible to fit this decrease with statistical Rice–Ramsperger–Kassel/quasiequilibrium theory. Metastable dissociation of SF6-* appears to compete with autodetachment of the electron at all electron energies.

High levels of genetic structuring as a result of population fragmentation in the tropical tree species Caesalpinia echinata Lam.

We have investigated levels of genetic diversity within and among seven remnant populations of Caesalpinia echinata Lam., an endangered species found as fragmented populations in three major areas around the coastal regions of Brazil. Using amplified fragment length polymorphism (AFLP) genetic markers, we detected levels of within-population genetic diversity ranging from 0.092 to 0.163, with the lowest values generally being found in the smallest populations. Estimates of between-population genetic differentiation were strongly correlated with geographical distance ( r = 0.884, p <0.001), which, along with a neighbour-joining phylogenetic analysis, strongly suggested high levels of genetic isolation by distance. Over half (62%) of the total genetic diversity was partitioned between populations, further highlighting the genetic distinctness of individual populations. Taken together, these results suggest that fragmentation has led to an increase in population differentiation between fragments of C. echinata. These formations will be of great value in the development of conservation plans for species exhibiting high levels of genetic differentiation due to fragmentation, such as indication of conservation unit size, which populations should be chosen as priority in conservation plans and which samples should be introduced in areas with a low number of individuals of brazilwood.

Temporal patterns of territory settlement and detectability in mated and unmated Nightingales Luscinia megarhynchos

In many bird species the sex ratio of adults is male-biased, which is likely to have consequences for the ecology as well as for the conservation of a species. For example, when some males remain unpaired in a population, there should be strong selection on behavioural traits that enhance pairing success. A surplus of males is also likely to have important implications for the interpretation of breeding bird survey data. In our study population of Nightingales Luscinia megarhynchos, about half of the males stayed unpaired, suggesting that the number of males encountered singing was greater than the number of breeding pairs. Furthermore, the detectability (the probability of encountering a male singing) of mated males was only two-thirds that of unmated males when censused in the morning or late in the breeding season. The relative detectability was more similar early in the season and during the twilight periods before sunrise and after sunset. Males that arrived earlier on the breeding grounds were more successful in attracting a mate than males arriving later. Some of the unmated males deserted their territories and prospected areas up to 4000 m distant, whereas others settled on the study site only late in the season and may actually have changed territories. We suggest that adult sex ratios and the time of the census should be taken into account when interpreting the results of breeding bird surveys.

The day after: effects of vocal interactions on territory defence in nightingales

Models on territory acquisition and tenure predict that territorial animals benefit by adjusting territorial defence behaviour to previous challenges they had experienced within the socially complex environment of communication networks.

Fragmentation and plasmid strand breaks in pure and gold-doped DNA irradiated by beams of fast hydrogen atoms

The results of an investigation into the damage caused to dry plasmid DNA after irradiation by fast (keV) hydrogen atoms are presented. Agarose gel electrophoresis was used to assess single and double strand break yields as a function of dose in dry DNA samples deposited on a mica substrate. Damage levels were observed to increase with beam energy. Strand break yields demonstrated a considerable dependence on sample structure and the method of sample preparation. Additionally, the effect of high-Z nanoparticles on damage levels was investigated by irradiating DNA samples containing controlled amounts of gold nanoparticles. In contrast to previous (photonic) studies, no enhancement of strand break yields was observed with the particles showing a slight radioprotective effect. A model of DNA damage as a function of dose has been constructed in terms of the probability for the creation of single and double strand breaks, per unit ion flux. This model provides quantitative conclusions about the effects of both gold nanoparticles and the different buffers used in performing the assays and, in addition, infers the proportion of multiply damaged fragments.

A study of DNA fragmentation patterns in cells irradiated with charged particles: evidence for non-random distributions

Many studies have shown that the effectiveness of radiations of varying LET is similar when yields of dsb have been measured, despite large differences in biological response. Recent evidence has suggested however, that current techniques underestimate the yields of dsb. By monitoring the fragmentation of DNA over a wide range of fragment sizes ( 6 Mbp) by pulsed field electrophoresis, RBE values greater than 1.0 for radiations of around 100 keV/mm have been determined. The data provide evidence for the production of correlated breaks produced within cells as particle tracks traverse the nucleus. The highly ordered structure of DNA within mammalian cells may lead to clustering of breaks over distances related to the repeating unit structures of the chromatin. As well as these regionally damaged sites, a major contributor to radiation effectiveness will be the localised clustering of damage in the 1 - 20 bp region. A major effort is required to elucidate the relative importance of these levels of clustering and their importance in biological response.