A Tolman-Bondi-Lemaitre Cell-Model for the Universe and Gravitational Collapse

A piecewise Tolman-Bondi-Lemaitre (TBL) cell-model for the universe incorporating local collapsing and expanding inhomogeneities is presented here. The cell-model is made up of TBL underdense and overdense spherical regions surrounded by an intermediate region of TBL shells embedded in an expanding universe. The cell-model generalizes the Friedmann as well as Einstein-Straus swiss-cheese models and presents a number of advantages over other models, and in particular the time evolution of the cosmological inhomogeneities is now incorporated within the scheme. Important problem of gravitational collapse of a massive dust cloud, such as a cluster of galaxies or even a massive star, in such a cosmological background is examined. It is shown that the collapsing local inhomogeneities in an expanding universe could result in either a black hole, or a naked singularity, depending on the nature of the set of initial data which consists of the matter distribution and the velocities of the collapsing shells in the cloud at the initial epoch from which the collapse commences.Comment: 14 pages, 3 figure

The structure of non-spacelike geodesics in dust collapse

We study here the behaviour of non-spacelike geodesics in dust collapse models in order to understand the casual structure of the spacetime. The geodesic families coming out, when the singularity is naked, corresponding to different initial data are worked out and analyzed. We also bring out the similarity of the limiting behaviour for different types of geodesics in the limit of approach to the singularity.Comment: 23 pages, 6 figures, to appear in PR

The Genome of the Acid Soil-Adapted Strain Rhizobium favelukesii OR191 Encodes Determinants for Effective Symbiotic Interaction With Both an Inverted Repeat Lacking Clade and a Phaseoloid Legume Host

Although Medicago sativa forms highly effective symbioses with the comparatively acid-sensitive genus Ensifer, its introduction into acid soils appears to have selected for symbiotic interactions with acid-tolerant R. favelukesii strains. Rhizobium favelukesii has the unusual ability of being able to nodulate and fix nitrogen, albeit sub-optimally, not only with M. sativa but also with the promiscuous host Phaseolus vulgaris. Here we describe the genome of R. favelukesii OR191 and genomic features important for the symbiotic interaction with both of these hosts. The OR191 draft genome contained acid adaptation loci, including the highly acid-inducible lpiA/acvB operon and olsC, required for production of lysine- and ornithine-containing membrane lipids, respectively. The olsC gene was also present in other acid-tolerant Rhizobium strains but absent from the more acid-sensitive Ensifer microsymbionts. The OR191 symbiotic genes were in general more closely related to those found in Medicago microsymbionts. OR191 contained the nodA, nodEF, nodHPQ, and nodL genes for synthesis of polyunsaturated, sulfated and acetylated Nod factors that are important for symbiosis with Medicago, but contained a truncated nodG, which may decrease nodulation efficiency with M. sativa. OR191 contained an E. meliloti type BacA, which has been shown to specifically protect Ensifer microsymbionts from Medicago nodule-specific cysteine-rich peptides. The nitrogen fixation genes nifQWZS were present in OR191 and P. vulgaris microsymbionts but absent from E. meliloti-Medicago microsymbionts. The ability of OR191 to nodulate and fix nitrogen symbiotically with P. vulgaris indicates that this host has less stringent requirements for nodulation than M. sativa but may need rhizobial strains that possess nifQWZS for N2-fixation to occur. OR191 possessed the exo genes required for the biosynthesis of succinoglycan, which is required for the Ensifer-Medicago symbiosis. However, 1H-NMR spectra revealed that, in the conditions tested, OR191 exopolysaccharide did not contain a succinyl substituent but instead contained a 3-hydroxybutyrate moiety, which may affect its symbiotic performance with Medicago hosts. These findings provide a foundation for the genetic basis of nodulation requirements and symbiotic effectiveness with different hosts

Direct amplification of nodD from community DNA reveals the genetic diversity of Rhizobium leguminosarum in soil

Sequences of nodD, a gene found only in rhizobia, were amplified from total community DNA isolated from a pasture soil. The polymerase chain reaction (PCR) primers used, Y5 and Y6, match nodD from Rhizobium leguminosarum biovar trifolii, R. leguminosarum biovar viciae and Sinorhizobium meliloti. The PCR product was cloned and yielded 68 clones that were identified by restriction pattern as derived from biovar trifolii [11 restriction fragment length polymorphism (RFLP) types] and 15 clones identified as viciae (seven RFLP types). These identifications were confirmed by sequencing. There were no clones related to S. meliloti nodD. For comparison, 122 strains were isolated from nodules of white clover (Trifolium repens) growing at the field site, and 134 from nodules on trap plants of T. repens inoculated with the soil. The nodule isolates were of four nodD RFLP types, with 77% being of a single type. All four of these patterns were also found among the clones from soil DNA, and the same type was the most abundant, although it made up only 34% of the trifolii-like clones. We conclude that clover selects specific genotypes from the available soil population, and that R. leguminosarum biovar trifolii was approximately five times more abundant than biovar viciae in this pasture soil, whereas S. meliloti was rare