Annual lipid cycle of the grassland Varanian lizard]

The circannual chronobiology of Varanus exanthematicus exanthematicus Bosc, lacertilian reptile of Senegal, is characterized by a period of rest from January to June (dry season) and a period of activity from July to December (wet season). The study of the lipids metabolism shows a two month deplacement period: as a matter of fact from November to April the lipogenesis prevails, while from May to October it is the lipolysis. These two phases correspond to the periodicity of the development of the gonads, which are at rest from November to April and in activity from May to October. The synchronism of the phases of lipolysis and of gonads muturation argues in favour of the lipids utilization for the growth of the gonads.     

The role of immigration and in-situ radiation in explaining blood parasite assemblages in an island bird clade

Parasite communities on islands are assembled through multiple immigrations and/or in-situ diversification. In this study, we used a phylogenetic approach to investigate the role of such processes in shaping current patterns of diversity in Leucocytozoon, a group of haemosporidian blood parasites infecting whites eyes (Zosterops) endemic to the Mascarene archipelago (south-western Indian Ocean). We found that this parasite community arose through a combination of multiple immigrations and in-situ diversification, highlighting the importance of both processes in explaining island diversity. Specifically, two highly diverse parasite clades appear to have been present in the Mascarenes for most of their evolutionary history and have diversified within the archipelago, while another lineage apparently immigrated more recently, probably with human-introduced birds. Interestingly, the evolutionary histories of one clade of parasites and Indian Ocean Zosterops seem tightly associated with a significant signal for phylogenetic congruence, suggesting that host-parasite co-divergence may have occurred in this system.     

Variable copy number, intra-genomic heterogeneities and lateral transfers of the 16S rRNA gene in Pseudomonas

Even though the 16S rRNA gene is the most commonly used taxonomic marker in microbial ecology, its poor resolution is still not fully understood at the intra-genus level. In this work, the number of rRNA gene operons, intra-genomic heterogeneities and lateral transfers were investigated at a fine-scale resolution, throughout the Pseudomonas genus. In addition to nineteen sequenced Pseudomonas strains, we determined the 16S rRNA copy number in four other Pseudomonas strains by Southern hybridization and Pulsed-Field Gel Electrophoresis, and studied the intra-genomic heterogeneities by Denaturing Gradient Gel Electrophoresis and sequencing. Although the variable copy number (from four to seven) seems to be correlated with the evolutionary distance, some close strains in the P. fluorescens lineage showed a different number of 16S rRNA genes, whereas all the strains in the P. aeruginosa lineage displayed the same number of genes (four copies). Further study of the intra-genomic heterogeneities revealed that most of the Pseudomonas strains (15 out of 19 strains) had at least two different 16S rRNA alleles. A great difference (5 or 19 nucleotides, essentially grouped near the V1 hypervariable region) was observed only in two sequenced strains. In one of our strains studied (MFY30 strain), we found a difference of 12 nucleotides (grouped in the V3 hypervariable region) between copies of the 16S rRNA gene. Finally, occurrence of partial lateral transfers of the 16S rRNA gene was further investigated in 1803 full-length sequences of Pseudomonas available in the databases. Remarkably, we found that the two most variable regions (the V1 and V3 hypervariable regions) had probably been laterally transferred from another evolutionary distant Pseudomonas strain for at least 48.3 and 41.6% of the 16S rRNA sequences, respectively. In conclusion, we strongly recommend removing these regions of the 16S rRNA gene during the intra-genus diversity studies.     

A phosphorylation cycle shapes gradients of the DYRK family kinase Pom1 at the plasma membrane

Concentration gradients regulate many cell biological and developmental processes. In rod-shaped fission yeast cells, polar cortical gradients of the DYRK family kinase Pom1 couple cell length with mitotic commitment by inhibiting a mitotic inducer positioned at midcell. However, how Pom1 gradients are established is unknown. Here, we show that Tea4, which is normally deposited at cell tips by microtubules, is both necessary and, upon ectopic cortical localization, sufficient to recruit Pom1 to the cell cortex. Pom1 then moves laterally at the plasma membrane, which it binds through a basic region exhibiting direct lipid interaction. Pom1 autophosphorylates in this region to lower lipid affinity and promote membrane release. Tea4 triggers Pom1 plasma membrane association by promoting its dephosphorylation through the protein phosphatase 1 Dis2. We propose that local dephosphorylation induces Pom1 membrane association and nucleates a gradient shaped by the opposing actions of lateral diffusion and autophosphorylation-dependent membrane detachment.     

Quantitative protein expression and cell surface characteristics of Escherichia coli MG1655 biofilms

Cell surface physicochemical characterization techniques were combined with quantitative changes in protein expression, to investigate the biological and biophysical changes of Escherichia coli MG1655 cells when grown as a biofilm (BIO). The overall surface charge of BIO cells was found to be less negative, highlighting the need for a lower electrophoretic mobility for attachment to occur. Comparison of the chemical functional groups on the cell surface showed similar profiles, with the absorbance intensity higher for proteins and carbohydrates in the BIO cells. Quantitative proteomic analysis demonstrated that 3 proteins were significantly increased, and 9 proteins significantly decreased in abundance, in cells grown as a BIO compared to their planktonic counterparts, with 7 of these total 12 proteins unique to this study. Proteins showing significant increased or decreased abundance include proteins involved in acid resistance, DNA protection and binding and ABC transporters. Further predictive analysis of the metabolic pathways showed an increased abundance of the amino acid metabolism and tricarboxylic acid (TCA) cycle, with a decrease in expression within the pentose phosphate and glycolysis pathways. It is therefore hypothesized that cells grown as a BIO are still energetically viable potentially using amino acids as an indirect carbon backbone source into the TCA cycle.     

From elaborate to compact seasonal plant epidemic models and back: is competitive exclusion in the details?

Seasonality, or periodic host absence, is a central feature in plant epidemiology. In this respect, seasonal plant epidemic models take into account the way the parasite overwinters and generates new infections. These are termed primary infections. In the literature, one finds two classes of models: high-dimensional elaborate models and low-dimensional compact models, where primary infection dynamics are explicit and implicit, respectively. Investigating a compact model allowed previous authors to show the existence of a competitive exclusion principle. However, the way compact models derive from elaborate models has not been made explicit yet. This makes it unclear whether results such as competitive exclusion extend to elaborate models as well. Here, we show that assuming primary infection dynamics are fast in a standard elaborate model translates into a compact form. Yet, it is not that usually found in the literature. Moreover, we numerically show that coexistence is possible in this original compact form. Reversing the question, we show that the usual compact form approximates an alternate elaborate model, which differs from the earlier one in that primary infection dynamics are density dependent. We discuss to which extent these results shed light on coexistence within soil- and air-borne plant parasites, such as within the take-all disease of wheat and the grapevine powdery mildew cryptic species complexes, respectively.     

Control of M. tuberculosis ESAT-6 secretion and specific T cell recognition by PhoP

Analysis of mycobacterial strains that have lost their ability to cause disease is a powerful approach to identify yet unknown virulence determinants and pathways involved in tuberculosis pathogenesis. Two of the most widely used attenuated strains in the history of tuberculosis research are Mycobacterium bovis BCG (BCG) and Mycobacterium tuberculosis H37Ra (H37Ra), which both lost their virulence during in vitro serial passage. Whereas the attenuation of BCG is due mainly to loss of the ESAT-6 secretion system, ESX-1, the reason why H37Ra is attenuated remained unknown. However, here we show that a point mutation (S219L) in the predicted DNA binding region of the regulator PhoP is involved in the attenuation of H37Ra via a mechanism that impacts on the secretion of the major T cell antigen ESAT-6. Only H37Ra "knock-ins" that carried an integrated cosmid with the wild-type phoP gene from M. tuberculosis H37Rv showed changes in colony morphology, increased virulence, ESAT-6 secretion, and induction of specific T cell responses, whereas other H37Ra constructs did not. This finding established a link between the PhoP regulator and ESAT-6 secretion that opens exciting new perspectives for elucidating virulence regulation in M. tuberculosis.