Measurement of Ultra-Low Potassium Contaminations with Accelerator Mass Spectrometry

Levels of trace radiopurity in active detector materials is a subject of major concern in low-background experiments. Among the radio-isotopes, \k40 is one of the most abundant and yet whose signatures are difficult to reject. Procedures were devised to measure trace potassium concentrations in the inorganic salt CsI as well as in organic liquid scintillator (LS) with Accelerator Mass Spectrometry (AMS), giving, respectively, the \k40-contamination levels of $\sim 10^{-10}$ and $\sim 10^{-13}$ g/g. Measurement flexibilities and sensitivities are improved over conventional methods. The projected limiting sensitivities if no excess of potassium signals had been observed over background are $8 \times 10^{-13}$ g/g and $3 \times 10^{-17}$ g/g for the CsI and LS, respectively. Studies of the LS samples indicate that the radioactive contaminations come mainly in the dye solutes, while the base solvents are orders of magnitude cleaner. The work demonstrate the possibilities of measuring naturally-occurring isotopes with the AMS techniques.Comment: 18 pages, 4 figures, 3 table

Ants Sow the Seeds of Global Diversification in Flowering Plants

Background: The extraordinary diversification of angiosperm plants in the Cretaceous and Tertiary periods has produced an estimated 250,000–300,000 living angiosperm species and has fundamentally altered terrestrial ecosystems. Interactions with animals as pollinators or seed dispersers have long been suspected as drivers of angiosperm diversification, yet empirical examples remain sparse or inconclusive. Seed dispersal by ants (myrmecochory) may drive diversification as it can reduce extinction by providing selective advantages to plants and can increase speciation by enhancing geographical isolation by extremely limited dispersal distances. Methodology/Principal Findings: Using the most comprehensive sister-group comparison to date, we tested the hypothesis that myrmecochory leads to higher diversification rates in angiosperm plants. As predicted, diversification rates were substantially higher in ant-dispersed plants than in their non-myrmecochorous relatives. Data from 101 angiosperm lineages in 241 genera from all continents except Antarctica revealed that ant-dispersed lineages contained on average more than twice as many species as did their non-myrmecochorous sister groups. Contrasts in species diversity between sister groups demonstrated that diversification rates did not depend on seed dispersal mode in the sister group and were higher in myrmecochorous lineages in most biogeographic regions. Conclusions/Significance: Myrmecochory, which has evolved independently at least 100 times in angiosperms and is estimated to be present in at least 77 families and 11 000 species, is a key evolutionary innovation and a globally important driver of plant diversity. Myrmecochory provides the best example to date for a consistent effect of any mutualism on largescale diversification

Ants cannot account for interpopulation dispersal of the arillate pea Daviesia triflora

• Estimating distances and rates of seed dispersal, especially long-distance dispersal (LDD), is critical for understanding the dynamics of patchily distributed populations and species’ range shifts in response to environmental change. • Daviesia triflora (Papilionaceae) is an ant-dispersed shrub. The ant Rhytidoponera violacea was recorded dispersing its seeds to a maximum distance of 4.7 m, and in more intensive trials seeds of a related species from the study area, to a maximum of 8.1 m. • Microsatellite DNA markers and population assignment tests identified interpopulation immigrants among 764 plants on 23 adjacent dunes bearing D. triflora, and 13 interpopulation seed dispersal (LDD) events (1.7%) were inferred. The distance between source and sink populations ranged from 410 m to 2350 m (mean 1260 m). These distances exceed ant dispersal distances by two to three orders of magnitude but are comparable with previous measurements of LDD for two co-occurring wing-seeded (wind-dispersed) species from the same system. • The observed distances of seed dispersal in this arillate species demonstrate the significance of nonstandard dispersal mechanisms in LDD and the independence of these from primary dispersal syndromes. The likely role of emus in dispersal of the many ‘ant-dispersed’ species in Australia is discussed