A comparison of estimators of the population recombination rate

Three new estimators of the population recombination rate C = 4Nr are introduced. These estimators summarize the data using the number of distinct haplotypes and the estimated minimum number of recombination events, then calculate the value of C that maximizes the likelihood of obtaining the summarized data. They are compared with a number of previously proposed estimators of the recombination rate. One of the newly proposed estimators is generally better than the others for the parameter values considered here, while the three programs that calculate maximum-likelihood estimates give conflicting results.     

Detecting ancient admixture in humans using sequence polymorphism data

A debate of long-standing interest in human evolution centers around whether archaic human populations (such as the Neanderthals) have contributed to the modern gene pool. A model of ancient population structure with recent mixing is introduced, and it is determined how much information (i.e., sequence data from how many unlinked nuclear loci) would be necessary to distinguish between different demographic scenarios. It is found that approximately 50-100 loci are necessary if plausible parameter estimates are used. There are not enough data available at the present to support either the "single origin" or the "multiregional" model of modern human evolution. However, this information should be available in a few years.     

New bioactive aromatic compounds from Vismia guianensis

Five benzophenones, vismiaguianones A-E, and two benzocoumarins, vismiaguianins A and B were isolated from the CHCl3 extract of the roots of Vismia guianensis by bioassay-directed fractionation using the DNA strand-scission assay and KB cell line. Of the isolates obtained, vismiaguianone B exhibited DNA strand-scission activity, whereas vismiaguianones D and E and vismiaguianin A were found to be significantly cytotoxic.     

Growth of elaborate microbial pinnacles in Lake Vanda,Antarctica

Microbial pinnacles in ice‐covered Lake Vanda, McMurdo Dry Valleys, Antarctica, extend from the base of the ice to more than 50 m water depth. The distribution of microbial communities, their photosynthetic potential, and pinnacle morphology affects the local accumulation of biomass, which in turn shapes pinnacle morphology. This feedback, plus environmental stability, promotes the growth of elaborate microbial structures. In Lake Vanda, all mats sampled from greater than 10 m water depth contained pinnacles with a gradation in size from <1‐mm‐tall tufts to pinnacles that were centimeters tall. Small pinnacles were cuspate, whereas larger ones had variable morphology. The largest pinnacles were up to ~30 cm tall and had cylindrical bases and cuspate tops. Pinnacle biomass was dominated by cyanobacteria from the morphological and genomic groups Leptolyngbya, Phormidium, and Tychonema. The photosynthetic potential of these cyanobacterial communities was high to depths of several millimeters into the mat based on PAM fluorometry, and sufficient light for photosynthesis penetrated ~5 mm into pinnacles. The distribution of photosynthetic potential and its correlation to pinnacle morphology suggests a working model for pinnacle growth. First, small tufts initiate from random irregularities in prostrate mat. Some tufts grow into pinnacles over the course of ~3 years. As pinnacles increase in size and age, their interiors become colonized by a more diverse community of cyanobacteria with high photosynthetic potential. Biomass accumulation within this subsurface community causes pinnacles to swell, expanding laminae thickness and creating distinctive cylindrical bases and cuspate tops. This change in shape suggests that pinnacle morphology emerges from a specific distribution of biomass accumulation that depends on multiple microbial communities fixing carbon in different parts of pinnacles. Similarly, complex patterns of biomass accumulation may be reflected in the morphology of elaborate ancient stromatolites.     

Spatial and temporal habitat partitioning by calliphorid blowflies

Calliphorid blowflies perform an essential ecosystem service in the consumption, recycling and dispersion of carrion nutrients and are considered amongst the most important functional groups in an ecosystem. Some species are of economic importance as facultative agents of livestock myiasis. The interspecific ecological differences that facilitate coexistence within the blowfly community are not fully understood. The aim of this work was to quantify differences in habitat use by calliphorid species. Thirty traps were distributed among three habitats at two sites in southwest England for collections made during March–August 2016. A total of 17 246 specimens were caught, of which 2427 were Lucilia sericata, 51 Lucilia richardsi, 6580 Lucilia caesar, 307 Lucilia ampullacea, 4881 Calliphora vicina and 2959 Calliphora vomitoria (all: Diptera: Calliphoridae). Lucilia sericata was the dominant species in open habitats, whereas L. caesar was the most abundant species in shaded habitats. Calliphora specimens were more abundant in the cooler months. These findings suggest that Calliphora and Lucilia species show strong temporal segregation mediated by temperature, and that species of the genus Lucilia show differences in the use of habitats that are likely to be driven by differences in humidity tolerance and light intensity. These factors in combination result in effective niche partitioning.     

Protein-induced conformational changes in 16 S ribosomal RNA during the initial assembly steps of the Escherichia coli 30 S ribosomal subunit

The mechanism of 16 S ribosomal RNA folding into its compact form in the native 30 S ribosomal subunit of Escherichia coli was studied by scanning transmission electron microscopy and circular dichroism spectroscopy. This approach made it possible to visualize and quantitatively analyze the conformational changes induced in 16 S rRNA under various ionic conditions and to characterize the interactions of ribosomal proteins S4, S8, S15, S20, S17 and S7, the six proteins known to bind to 16 S rRNA in the initial assembly steps. 16 S rRNA and the reconstituted RNA-protein core particles were characterized by their mass, morphology, radii of gyration (RG), and the extent and stability of 16 S rRNA secondary structure. The stepwise binding of S4, S8 and S15 led to a corresponding increase of mass and was accompanied by increased folding of 16 S rRNA in the core particles, as evident from the electron micrographs and from the decrease of RG values from 114 A and 91 A. Although the binding of S20, S17 and S7 continued the trend of mass increase, the RG values of these core particles showed a variable trend. While there was a slight increase in the RG value of the S20 core particles to 94 A, the RG value remained unchanged (94 A) with the further addition of S17. With subsequent addition of S7 to the core particles, the RG values showed an increase to 108 A. Association with S7 led to the formation of a globular mass cluster with a diameter of about 115 A and a mass of about 300 kDa. The rest of the mass (about 330 kDa) remained loosely coiled, giving the core particle a "medusa-like" appearance. Morphology of the 16 S rRNA and 16 S rRNA-protein core particles, even those with all six proteins, does not resemble the native 30 S subunit, contrary to what has been reported by others. The circular dichroism spectra of the 16 S rRNA-protein complexes and of free 16 S rRNA indicate a similarity of RNA secondary structure in the core particles with the first four proteins, S4, S8, S15, S20. The circular dichroism melting profiles of these core particles show only insignificant variations, implying no obvious changes in the distribution or the stability of the helical segments of 16 S rRNA. However, subsequent binding of proteins S17 and S7 affected both the extent and the thermal stability of 16 S rRNA secondary structure.(ABSTRACT TRUNCATED AT 400 WORDS)