Assessing the genetic diversity and the temporal evolution of plant populations for an optimized conservation: the example of the tuberous-rooted chervil.

International audienc

Is there variety × isolate interaction in the polygenic quantitative resistance of carrot to Alternaria dauci ?

Horizontal and polygenic resistance is race-nonspecific and, therefore, more durable, unlike vertical resistance, which is race-specific and unstable. However, this division is perhaps not so obvious since some cultivar × isolate interactions have already been observed for plant species with partial resistance. Carrot is known to be partially resistant to Alternaria dauci, but it is relevant for breeders to study cultivar × isolate interactions in order to develop durable resistant varieties. For this purpose, 12 highly diverse carrot genotypes and one segregating population were inoculated in a tunnel or in a glass house with 11 isolates of A. dauci that also represented a high diversity in terms of geographical origin, aggressiveness and genetic diversity. Disease severity values were assessed three times in the tunnel in a one-year experiment (2002) and twice in the glass house in an experiment over two consecutive years (2011 and 2012). The interaction of isolate with genotype was non-significant in the tunnel, and the same result was obtained in the glasshouse for both years of study except for the first scoring date in 2011, suggesting that the partial resistance of carrot to A. dauci is probably mainly explained by major QTLs that confer resistance to a large number of isolates and, potentially, some minor isolate-specific QTLs as well

Evaluation of different methods for the characterization of carrot resistance to the alternaria leaf blight pathogen (Alternaria dauci) revealed two qualitatively different resistances

Alternaria leaf blight (ALB), caused by Alternaria dauci, is one of the most damaging foliar diseases of carrot worldwide. The aim of this study was to compare different methods for evaluating levels of carrot resistance to ALB. Three techniques were investigated by comparison with a visual disease assessment control: in vivo conidial germination, a bioassay based on a drop-inoculation method, and in planta quantification of fungal biomass by quantitative PCR (Q-PCR). Three carrot cultivars showing different degrees of resistance to A. dauci were used, i.e. a susceptible cultivar (Presto) and two partially resistant genotypes (Texto and Bolero), challenged with an aggressive or a very aggressive isolate of A. dauci. Both partially resistant genotypes produced a higher mean number of germ tubes per conidium (up to 3·42±0·35) than the susceptible one (1·26±0·18). The drop-inoculation results allowed one of the partially resistant genotypes (Bolero, log10(S+1) = 1·34±0·13) to be distinguished from the susceptible one (1·90±0·13). By contrast, fungal growth measured by Q-PCR clearly differentiated the two partially resistant genotypes with log10(I) values of 2·77±0·13 compared to the susceptible cultivar (3·65±0·13) at 15 days post-inoculation. This result was strongly correlated (r2 = 0·91) with the disease severity index scored at the same date. Data obtained with the different assessment methods strongly suggest that the Texto and Bolero genotypes have different genetic resistance sources

Is quantitative resistance qualitative? An example with two Alternaria Leaf Blight resistant carrot genotypes and four resistance assessment techniques

International audienc

Is host range of carrot restricted to Alternaria dauci?

International audienc

QTL mapping of carrot resistance to leaf blight with connected populations: stability across years and consequences for breeding

Combining biparental and multiparental connected population analyses was useful for the identification of 11 QTLs in two new genetic backgrounds of carrot resistance to Alternaria dauci and for breeding recommendations. Leaf blight due to the fungus Alternaria dauci is the major carrot foliar disease worldwide. Some resistance QTLs have been previously identified in one population, but the evaluation of additional genetic backgrounds with higher level of resistance would give opportunities for breeders to combine them by pyramiding. For this purpose, two segregating populations were evaluated twice across 4 years in the same environment (1) to compare the efficiency of the single vs. the connected populations approach for characterizing the new sources of carrot resistance to Alternaria dauci; (2) to evaluate the stability of QTLs over the years; and (3) to give recommendations to breeders for marker-assisted selection. Single and connected analyses were complementary; their combination allowed the detection of 11 QTLs. Connected analyses allowed the identification of common and specific QTLs among the two populations and the most favorable allele at each QTL. Important contrasts between allelic effects were observed with four and five most favorable alleles coming from the two resistant parental lines, whereas two other favorable alleles came from the susceptible parental line. While four QTLs were consistent across years, seven were detected within a single year. The heritabilities for both populations PC2 and PC3 were high (75 and 78 %, respectively), suggesting that the resistance of carrot to A. dauci was little affected by these environmental conditions, but the instability of QTL over years may be due to changing environmental conditions. The complementarity between these parental lines in terms of interesting allelic combinations is also discussed

Evaluating aggressiveness and host range of Alternaria dauci in a controlled environment

The aggressiveness of Alternaria dauci isolates was investigated in greenhouse conditions. Twenty-seven isolates were pre-selected from a large collection to represent high diversity according to geographic or host origins and intergenic spacer (IGS) polymorphism. IGS sequence analysis revealed that isolates were grouped within three different clusters. Eleven isolates were selected and inoculated on a susceptible carrot cultivar. Three criteria (mean lesion number, mean necrotic leaf area and mean disease index) were used to assess the aggressiveness of isolates. Continuous variation in aggressiveness was shown and no clear division into isolate classes was evident. For the host range study, two isolates were inoculated under greenhouse conditions onto nine cultivated Apiaceae species, two wild Daucus species and six cultivated non-Apiaceae species representing six botanical families. Lesions varying in severity were observed on all dicot species (Apiaceae and non-Apiaceae), but no symptoms developed on the two monocots studied (leek and sweetcorn). Plant species were also differentiated on the basis of expanding lesions (cultivated and wild carrot, dill and fennel) or non-expanding lesions (other dicot species). Typical A. dauci conidia were observed after in vitro incubation of leaves with symptoms. Fungal structures were isolated from lesions and A. dauci was confirmed on the basis of conidial morphology and specific conventional PCR results. Genotyping of individual isolates performed with microsatellite markers confirmed the presence of the inoculated isolate. The results clearly showed that, in controlled conditions, the host range of A. dauci is not restricted to carrot

Évolution temporelle de la diversité génétique de Chaerophyllum bulbosum : conséquences sur la gestion des ressources génétiques

Afin d’accroître le pool génétique nécessaire à l’amélioration variétale du cerfeuil tubéreux, apiacée alimentaire en pleine expansion, des prospections de populations sauvages sur deux campagnes successives ont été réalisées dans les bassins du Rhin et de la Weser, en Allemagne. Ces populations, principalement ripariennes, sont susceptibles d’être façonnées par les évolutions des réseaux hydrographiques qui caractérisent leur biotope. L’étude moléculaire de la diversité génétique des populations montre une structuration forte entre ces populations sauvages (GST ∼ 32 %), mais ne met pas en évidence d’effet structurant des réseaux hydrographiques sur cette diversité, ni de phénomène d’érosion génétique global. Une discussion sur la stratégie de maintien de la diversité de cette espèce à long terme est proposée