New in vivo and ex vivo models for the experimental study of sheep scrapie: development and perspectives

Sheep scrapie is a prototypical transmissible spongiform encephalopathy (TSE), and the most widespread of these diseases. Experimental study of TSE infectious agents from sheep and other species essentially depends on bioassays in rodents. Transmission of natural sheep scrapie to conventional mice commonly requires one or two years. In an effort to develop laboratory models in which investigations on the sheep TSE agent would be facilitated, we have established mice and cell lines that were genetically engineered to express ovine PrP protein and examined their susceptibility to the infection. A series of transgenic mice lines (tgOv) expressing the high susceptibility allele (VRQ) of the ovine PrP gene from different constructs was expanded. Following intracerebral inoculation with natural scrapie isolates, all animals developed typical TSE neurological signs and accumulated abnormal PrP in their brain. The survival time in the highest expressing tgOv lines ranged from 2 to 7 months, depending on the isolate. It was inversely related to the brain PrP content, and essentially unchanged on further passaging. Ovine PrP transgene expression thus enhanced scrapie disease transmission from sheep to mice. Such tgOv mice may bring new opportunities for analysing the natural variation of scrapie strains and measuring infectivity. As no relevant cell culture models for agents of naturally-occurring TSE exist, we have explored various strategies in order to obtain stable cell lines that would propagate the sheep agent ex vivo without prior adaptation to rodent. In one otherwise refractory rabbit epithelial cell line, a regulable expression of ovine PrP was achieved and found to enable an efficient replication of the scrapie agent in inoculated cultures. Cells derived from sheep embryos or from tgOv mice were also used in an attempt to establish permissive cell lines derived from the nervous system. Cells engineered to express PrP proteins of a specified sequence may thus represent a promising strategy to further explore, at the cellular level, various aspects of TSE diseases.     

Auxin at the Shoot Apical Meristem

Plants continuously generate new tissues and organs through the activity of populations of undifferentiated stem cells, called meristems. Here, we discuss the so-called shoot apical meristem (SAM), which generates all the aerial parts of the plant. It has been known for many years that auxin plays a central role in the functioning of this meristem. Auxin is not homogeneously distributed at the SAM and it is thought that this distribution is interpreted in terms of differential gene expression and patterned growth. In this context, auxin transporters of the PIN and AUX families, creating auxin maxima and minima, are crucial regulators. However, auxin transport is not the only factor involved. Auxin biosynthesis genes also show specific, patterned activities, and local auxin synthesis appears to be essential for meristem function as well. In addition, auxin perception and signal transduction defining the competence of cells to react to auxin, add further complexity to the issue. To unravel this intricate signaling network at the SAM, systems biology approaches, involving not only molecular genetics but also live imaging and computational modeling, have become increasingly important.Plants continuously generate new tissues and organs through the activity of populations of undifferentiated stem cells, called meristems. Because meristems can modulate their activity, they provide the developmental flexibility that allows plants to adapt their development in reaction to the environment (reviews: Lyndon 1998; Traas and Doonan 2001; Aida and Tasaka 2006; Sablowski 2007).Distinct meristems exist. Apical meristems, positioned at the tip of the shoots and roots, initiate aerial and underground organs, respectively. Along the stems and roots, more diffuse secondary meristems are responsible for secondary thickening of these structures.The plant hormone auxin plays an instrumental role in meristem biology and we discuss here its role in a particular meristem, the shoot apical meristem (SAM) that generates all the aerial organs including the floral meristems (Fig. 1A–C). In this context, we limit ourselves to the meristems in angiosperm that have been studied in most detail.Open in a separate windowFigure 1.The shoot apical meristem of Arabidopsis thaliana. (A) Aerial part of a wild-type plant of the Columbia ecotype (Col-0). The SAM is responsible for the production of rosette leaves and, after floral transition, for the production of the stem, cauline leaves, lateral meristems, and flowers of the inflorescence. (B) Details of the tip of the inflorescence, showing the highly organized positioning of flowers around the main axis (a spiral). (C) A dissected inflorescence meristem. Older flowers have been removed to expose the meristem surrounded by young floral buds. (D) Longitudinal section of an inflorescence meristem showing the layered organization (L1, L2, and L3 cell layers). L1 and L2 are also called the tunica and L3 to the corpus. The functional zones are also represented. At the meristem summit the central zone (CZ) contains the stem cells, whereas primordia are initiated in the peripheral zone (PZ). The rib zone (RZ) produces the internal part of the stem.     

Lack of association of CD36 SNPs with early onset obesity: a meta-analysis in 9,973 European subjects

A recent study suggested that four CD36 polymorphisms (namely rs3211867, rs3211883, rs3211908, and rs1527483) were associated with an increased risk of obesity, an increased BMI and percentage of body fat in European adolescents. We first attempted to confirm these results in three independent case-control genome-wide association studies (GWAS) data totaling 3,509 subjects of French and German origin, but we were unable to find any association of these variants with early onset obesity risk. We then genotyped the four CD36 single-nucleotide polymorphisms (SNPs) in a large population-based study of 4,667 Finnish subjects and we did not replicate any of the recently reported associations with BMI. By combining all available data in a meta-analysis (N = 9,973), we found no evidence for an association of the reported four variants in CD36 with increased obesity risk or increased BMI (0.07 ≤ P values ≤ 0.93). Finally, we assessed the contribution of the full CD36 locus gene variation to obesity risk in 3,509 subjects and we did not detect any significant association with obesity after correction for multiple testing. In summary, we were unable to confirm the recently reported association of variants in CD36 with early onset obesity in populations of European ancestry.     

The contrasting effects of short‐ and long‐term habitat drainage on the population dynamics of freshwater turtles in a human‐dominated landscape

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Demographic responses to weather fluctuations are context dependent in a long‐lived amphibian

Weather fluctuations have been demonstrated to affect demographic traits in many species. In long‐lived organisms, their impact on adult survival might be buffered by the evolution of traits that reduce variation in interannual adult survival. For example, skipping breeding is an effective behavioral mechanism that may limit yearly variation in adult survival when harsh weather conditions occur; however, this in turn would likely lead to strong variation in recruitment. Yet, only a few studies to date have examined the impact of weather variation on survival, recruitment and breeding probability simultaneously in different populations of the same species. To fill this gap, we studied the impact of spring temperatures and spring rainfall on survival, on reproductive skipping behavior and on recruitment in five populations of a long‐lived amphibian, the yellow‐bellied toad (Bombina variegata). Based on capture–recapture data, our findings demonstrate that survival depends on interactions between age, population and weather variation. Varying weather conditions in the spring result in strong variation in the survival of immature toads, whereas they have little effect on adult toads. Breeding probability depends on both the individual's previous reproductive status and on the weather conditions during the current breeding season, leading to high interannual variation in recruitment. Crucially, we found that the impact of weather variation on demographic traits is largely context dependent and may thus differ sharply between populations. Our results suggest that studies predicting the impact of climate change on population dynamics should be taken with caution when the relationship between climate and demographic traits is established using only one population or few populations. We therefore highly recommend further research that includes surveys replicated in a substantial number of populations to account for context‐dependent variation in demographic processes.     

Specifying the introgressed regions from H. argophyllus in cultivated sunflower (Helianthus annuus L.) to mark Phomopsis resistance genes

A method based upon targetting of intro-gressed markers in a Phomopsis-resistant line (R) of cultivated sunflower, issuing from a H. argophyllus cross was used to mark the Phomopsis resistance regions. Our study was based upon 203 families derived from a cross between an inbred line susceptible to Phomopsis (S1) and the introgressed resistant line (R). Families were checked for Phomopsis resistance level in a design with replicated plots and natural infection was re-inforced by pieces of contaminated stems. Thirty four primers were employed for RAPD analysis. Out of 102 polymorphic fragments between (S1) and H. argophyllus, seven were still present in (R) suggesting that they marked introgressions of H. argophyllus into (R). The plants were scored for the presence or absence of 19 fragments obtained from five primers, and the relationships between the presence/absence of fragments in plants and Phomopsis resistance/susceptiblity in the progenies was determined by using an analysis of variance. We found that at least two introgressed regions, as well as favourable factors from sunflower, contributed to the level of Phomopsis resistance in cultivated sunflower. Received: 28 June 1996 / Accepted: 5 July 1996     

Multi-event models reveal the absence of interaction between an invasive frog and a native endangered amphibian

Among the multiple factors involved in the decline of amphibians, competition with alien species remains understudied. In Western Europe, non-native marsh frogs (Pelophylax ridibundus and other sister species) have been widely imported for gastronomic purposes and have then often escaped from captivity. Apart from closely related species, the impact of the subsequent colonization of non-native species on native amphibians is still unknown. In this study, we analysed the response of a threatened species, the yellow-bellied toad (Bombina variegata), faced with the presence of alien frogs in mountain rivers in France. We studied the co-occurrence pattern of the native toad and the alien frog, while taking into account co-dependence of the detection rates in both species. We tested three main scenarios to explain a non-random pond occurrence for each species and to establish their respective habitat preferenda: (1) a pond-area-dependent scenario, predicting different responses from each species for a given pond size, (2) a fish-dependent scenario, predicting different responses from each species given the presence of fish in ponds, and (3) a floodplain-width-dependent scenario, predicting different responses from each species given the geomorphological characteristics of the floodplain. Taking into account site-specific covariates, we concluded that introduced frogs do not currently have an impact on the native population of the yellow-bellied toad.