Nodulation of legumes by rhizobium

Abstract. The formation of nitrogen-fixing nodules on leguminous plants is the culmination of an integrated development involving many plant and bacterial genes. This review focuses on the structure, function and regulation of the bacterial genes involved in the nodulation process. We attempt to interpret recent observations on these genes in the context of signal exchanges which occur between the macro-and micro-symbionts.     

Remodelling of Neuromuscular Systems During Insect Metamorphosis

During metamorphosis in the hawkmoth, Manduca sexta, the larvalthoracic legs are replaced by a new set of adult legs that includenew sensory neurons and muscles, and participate in new patternsof locomotor activity. Larval leg motoneurons persist to innervatethe new adult leg muscles, but undergo striking changes in dendriticmorphology that are regulated by the insect steroid, 20-hydroxyecdysone.In the periphery, the motor terminals regress as larval musclesdegenerate, and expand as new adult muscles form from myoblasts.Evidence obtained both in vivo and in vitro suggests that theproliferation of myoblasts during metamorphosis is dependentupon innervation.     

Single-nucleotide polymorphism,linkage disequilibrium and geographic structure in the malaria parasite <Emphasis Type="Italic">Plasmodium vivax</Emphasis>: prospects for genome-wide association studies

Background

The ideal malaria parasite populations for initial mapping of genomic regions contributing to phenotypes such as drug resistance and virulence, through genome-wide association studies, are those with high genetic diversity, allowing for numerous informative markers, and rare meiotic recombination, allowing for strong linkage disequilibrium (LD) between markers and phenotype-determining loci. However, levels of genetic diversity and LD in field populations of the major human malaria parasite P. vivax remain little characterized.

Results

We examined single-nucleotide polymorphisms (SNPs) and LD patterns across a 100-kb chromosome segment of P. vivax in 238 field isolates from areas of low to moderate malaria endemicity in South America and Asia, where LD tends to be more extensive than in holoendemic populations, and in two monkey-adapted strains (Salvador-I, from El Salvador, and Belem, from Brazil). We found varying levels of SNP diversity and LD across populations, with the highest diversity and strongest LD in the area of lowest malaria transmission. We found several clusters of contiguous markers with rare meiotic recombination and characterized a relatively conserved haplotype structure among populations, suggesting the existence of recombination hotspots in the genome region analyzed. Both silent and nonsynonymous SNPs revealed substantial between-population differentiation, which accounted for ~40% of the overall genetic diversity observed. Although parasites clustered according to their continental origin, we found evidence for substructure within the Brazilian population of P. vivax. We also explored between-population differentiation patterns revealed by loci putatively affected by natural selection and found marked geographic variation in frequencies of nucleotide substitutions at the pvmdr-1 locus, putatively associated with drug resistance.

Conclusion

These findings support the feasibility of genome-wide association studies in carefully selected populations of P. vivax, using relatively low densities of markers, but underscore the risk of false positives caused by population structure at both local and regional levels.See commentary: http://www.biomedcentral.com/1741-7007/8/90

     

Strain Identification in Rhizobium Using Intrinsic Antibiotic Resistance

The variation in intrinsic resistance to low levels of eight antibiotics was used as an identifying characteristic for 26 Rhizobium leguminosarum strains. The pattern of antibiotic resistance of each strain was a stable property by which rhizobia isolated from root nodules of inoculated Pisum sativum could be recognized. The antibiotic tests for strain identification with R. leguminosarum were applied to R. phaseoli . It was necessary to include reference cultures in tests with this species, as the tests most suitable for the R. leguminosarum strains showed some variability with R. phaseoli .     

Mice prefer rare food

In the first experiment designed to test directly for frequency-dependent selection of food by a mammal, mice were presented with pairs of food in 9: 1 and 1:9 combinations. The foods used were crumbs of laboratory rat cake flavoured vanilla or peppermint and dyed green or brown: vanilla/green was presented with peppermint/brown, and vanilla/brown with peppermint/green. Vanilla was preferred to peppermint and green to brown: peppermint/brown was particularly unacceptable. Each type was preferred more when rare than when common. Preferences changed over the five days of the experiment, the preference for greens becoming less strong. Since the experimental trials were carried out in total darkness, the preferences must have been based on non-visual clues, indicating that the dyes were not odourless and tasteless to mice, as they are to man.     

The occurrence and distribution of actinomycetes in lakes of the English Lake District

Investigation of the occurrence of mesophilic actinomycetes in the lakes of the English Lake District revealed their widespread distribution in the lacustrine environment. Although only low numbers of actinomycetes occurred in the water, high numbers were recovered from all the lake muds. Total numbers of actinomycetes in the muds correlated quite well with the lakes’ productivity status. High numbers of Micro-monospora, Streptomyces and nocardioform actinomycetes were isolated from all the lakes sampled. Low numbers of Streptosporangium were isolated from all the muds but strains of Actinomadura, Actinoplanes, Dactylosporangium, Microbispora and Thermomonospora were only encountered occasionally. Micromonospora was the numerically dominant genus isolated from all the lakes sampled. This dominance was even more striking in deeper layers of mud and this was thought to reflect a more resistant spore stage in Micromonospora than in either of the other two main genera.