Variabilité de la vitesse de développement chez le sorgho cultivé (Sorghum bicolor (L.) Moench) et relation avec le photopériodisme

The rate of development of highly photoperiodic and consequently late sorghums adapted to the West-African savannas is reduced during the phases of jointing and panicle development. Slow development, expressed by a long phyllochron or time between the appearances of two successive leaves, is related to the limited size of the panicles of late sorghums. Therefore, variation for phyllochron during plant development could be a way of increasing the potential grain yield of this type of sorghum. Variability for this trait was assessed in the Cirad (Centre de coopération internationale en recherche agronomique pour le développement) sorghum mini core-collection but none was detected. On the contrary, the rates of development appear to be homogeneous within the species. There was no significant variation observed for the initial phyllochron, following emergence. Although there was large variation in the secondary phyllochron expressed during the jointing period, there were no significant differences among varieties of a similar maturity. Secondary phyllochron was strongly correlated with vegetative phase duration and thus appears to be one of the functions affected by the photoperiod-sensitive response

Past, present and future criteria to breed crops for water-limited environments in West Africa

Asia's Green Revolution of the 1960s and 1970s has largely bypassed West Africa, and "modern" (high-yielding, input responsive) germplasm for staple crops has found comparatively little adoption, except for systems that are have good access to markets and sufficient water resources. It is unlikely, however, that breeding objectives conserving traditional crop characteristics as found in extensive systems would have been more successful. The authors identify systems caught in the agricultural transition from subsistence to intensified, market-oriented production as the most important target for crop improvement, and provide examples of new breeding objectives for cowpea, sorghum and upland rice. In each of these cases, breeders, with the help of physiologists, have developed innovative plant-type concepts that combine improved yield potential and input responsiveness with specific traditional crop characteristics that remain essential during the agricultural transition. In the case of cowpea, dual-purpose varieties were developed that produce a good grain yield due to an erect plant habit, then produce new leaves enabling a second harvest of green foliage. For upland rice systems that are limited by labour (mainly needed to control weeds that abound due to shortened fallow periods), a weed competitive plant type was developed from Oryza sativa × Oryza glaberrima crosses. Lastly, sorghum breeders who had previously deselected photoperiod sensitivity are now re-inserting sensitivity into plants having "modern" architecture, in order to allow for flexible sowing dates while maintaining an agro-ecologically optimal time of flowering near the end of the wet season. The ecophysiological basis of these plant types, their place in current and future cropping systems, as well as the problem of under-funding for their realisation, are discussed

Surprising flowering response to photoperiod: Preliminary characterization of West and Central African pearl millet germplasm

Pearl millet (Pennisetum glaucum) is considered to be a short-day species that flowers, or flowers earlier, when day lengths are short. A few studies with two to six planting dates and few selected entries have been conducted in USA (Burton 1965), Senegal (Ramond 1968), and India (Patil et al. 1978, Das 1991). However, there is no known research on the flowering response of pearl millet to photoperiod changes over the entire year. Likewise, knowledge about the photoperiod-sensitivity in West and Central African pearl millets is insufficient

Genetic risk and a primary role for cell-mediated immune mechanisms in multiple sclerosis.

Multiple sclerosis is a common disease of the central nervous system in which the interplay between inflammatory and neurodegenerative processes typically results in intermittent neurological disturbance followed by progressive accumulation of disability. Epidemiological studies have shown that genetic factors are primarily responsible for the substantially increased frequency of the disease seen in the relatives of affected individuals, and systematic attempts to identify linkage in multiplex families have confirmed that variation within the major histocompatibility complex (MHC) exerts the greatest individual effect on risk. Modestly powered genome-wide association studies (GWAS) have enabled more than 20 additional risk loci to be identified and have shown that multiple variants exerting modest individual effects have a key role in disease susceptibility. Most of the genetic architecture underlying susceptibility to the disease remains to be defined and is anticipated to require the analysis of sample sizes that are beyond the numbers currently available to individual research groups. In a collaborative GWAS involving 9,772 cases of European descent collected by 23 research groups working in 15 different countries, we have replicated almost all of the previously suggested associations and identified at least a further 29 novel susceptibility loci. Within the MHC we have refined the identity of the HLA-DRB1 risk alleles and confirmed that variation in the HLA-A gene underlies the independent protective effect attributable to the class I region. Immunologically relevant genes are significantly overrepresented among those mapping close to the identified loci and particularly implicate T-helper-cell differentiation in the pathogenesis of multiple sclerosis

Comprehensive prediction of chromosome dimer resolution sites in bacterial genomes

<p>Abstract</p> <p>Background</p> <p>During the replication process of bacteria with circular chromosomes, an odd number of homologous recombination events results in concatenated dimer chromosomes that cannot be partitioned into daughter cells. However, many bacteria harbor a conserved dimer resolution machinery consisting of one or two tyrosine recombinases, XerC and XerD, and their 28-bp target site, <it>dif</it>.</p> <p>Results</p> <p>To study the evolution of the <it>dif/</it>XerCD system and its relationship with replication termination, we report the comprehensive prediction of <it>dif </it>sequences <it>in silico </it>using a phylogenetic prediction approach based on iterated hidden Markov modeling. Using this method, <it>dif </it>sites were identified in 641 organisms among 16 phyla, with a 97.64% identification rate for single-chromosome strains. The <it>dif </it>sequence positions were shown to be strongly correlated with the GC skew shift-point that is induced by replicational mutation/selection pressures, but the difference in the positions of the predicted <it>dif </it>sites and the GC skew shift-points did not correlate with the degree of replicational mutation/selection pressures.</p> <p>Conclusions</p> <p>The sequence of <it>dif </it>sites is widely conserved among many bacterial phyla, and they can be computationally identified using our method. The lack of correlation between <it>dif </it>position and the degree of GC skew suggests that replication termination does not occur strictly at <it>dif </it>sites.</p

Yield of Photoperiod-sensitive Sorghum Hybrids Based on Guinea-race Germplasm under Farmers’ Field Conditions in Mali

The first sorghum [Sorghum bicolor (L.) Moench] hybrids based on West African Guinea-race-derived parents were created to enhance farmer’s food security and income through increased yields. To assess their performance, eight hybrids, six experimental pure-line cultivars, one pure-line check (Lata), and a highly adapted landrace cultivar (Tieble) were evaluated in 27 farmer-managed and two on-station yield trials in Mali, West Africa, from 2009 to 2011. The hybrids were confirmed to have photoperiod sensitivity similar to the well-adapted Guinea landrace check cultivar. Genotypic differences for on-farm grain yield were highly significant and genotype × environment crossover interactions were limited. The yield superiorities of individual hybrids, relative to the landrace check, ranged from 17 to 37% over the 27 on-farm trials. The three top yielding hybrids showed 30% yield advantages across productivity levels, with absolute yield advantages averaging 380 kg ha−1 under lower (1.0–1.5 t ha−1) and 660 kg ha−1 under higher (2.0–3.5 t ha−1) productivity conditions. A mean male-parent (better parent) heterosis of 26% was observed for the four hybrids having Lata as a male parent. As the hybrids studied here were obtained with a low intensity of selection using a limited number of parents, even greater yield superiorities may be attained with development of distinct parental pools and scaled-up hybrid breeding

Genetic Structure, Linkage Disequilibrium and Signature of Selection in Sorghum: Lessons from Physically Anchored DArT Markers

Population structure, extent of linkage disequilibrium (LD) as well as signatures of selection were investigated in sorghum using a core sample representative of worldwide diversity. A total of 177 accessions were genotyped with 1122 informative physically anchored DArT markers. The properties of DArTs to describe sorghum genetic structure were compared to those of SSRs and of previously published RFLP markers. Model-based (STRUCTURE software) and Neighbor-Joining diversity analyses led to the identification of 6 groups and confirmed previous evolutionary hypotheses. Results were globally consistent between the different marker systems. However, DArTs appeared more robust in terms of data resolution and bayesian group assignment. Whole genome linkage disequilibrium as measured by mean r2 decreased from 0.18 (between 0 to 10 kb) to 0.03 (between 100 kb to 1 Mb), stabilizing at 0.03 after 1 Mb. Effects on LD estimations of sample size and genetic structure were tested using i. random sampling, ii. the Maximum Length SubTree algorithm (MLST), and iii. structure groups. Optimizing population composition by the MLST reduced the biases in small samples and seemed to be an efficient way of selecting samples to make the best use of LD as a genome mapping approach in structured populations. These results also suggested that more than 100,000 markers may be required to perform genome-wide association studies in collections covering worldwide sorghum diversity. Analysis of DArT markers differentiation between the identified genetic groups pointed out outlier loci potentially linked to genes controlling traits of interest, including disease resistance genes for which evidence of selection had already been reported. In addition, evidence of selection near a homologous locus of FAR1 concurred with sorghum phenotypic diversity for sensitivity to photoperiod