Search

UMR 1334 AGAP : Equipe DAVEM Diversité et adaptation de la vigne et des espèces méditerranéennes; The long-term maintenance of specialized mutualisms remains an evolutionary puzzle. Recent focus has been on factors governing the stability of these mutualisms, including sanctions by the host, partner choice, and coevolutionary constraint, that is, the genetic correlation (r(G)) between fitness of both partners. So far these studies have been typically carried out in a single environment. Here, we ask if the genetic correlation between fitness of the host plant Medicago truncatula (Fabaceae) and its bacterial symbiont Sinorhizobium meliloti is affected by the presence/absence of a monoterpene (carvacrol) leached into the soil by Thymus vulgaris-a common plant of the Mediterranean vegetation, often co-occuring with Medicago. We show that the presence of carvacrol in the soil dramatically affects fitness of the rhizobial partner and increases the magnitude of rG between plant and rhizobia fitness (r(G) = 0.02 +/- 0.05 vs. r(G) = 0.57 +/- 0.02). This finding emphasizes the importance of heterogeneity in the biotic environment for understanding the evolution of species interactions.

Here, we evaluate the role of pollen limitation and selfing in the maintenance of labile sex expression in subdioecious plant species.We used a literature survey to explore which factors correlated with a significant occurrence of hermaphrodites in dioecious species. We developed models to explore the selective maintenance of labile sex expression. The models had similar ecological assumptions but differed in the genetic basis of sex lability.We found that a significant frequency of hermaphrodites was associated with animal pollination, and that hermaphrodites were ‘inconstant' males with perfect flowers, suggesting evolution through the gynodioecious pathway. Models showed that a modifier converting pure males into inconstant males could be maintained under a wide range of reduction in both male and female fitness. Pollen limitation and self-fertilization facilitated invasion of the modifier. Depending on the genetics of sex determination, we found pure dioecy, stable subdioecy (trioecy), and situations where inconstant males coexisted with either pure females or pure males. Under selfing and pollen limitation, certain conditions selected for inconstant males which will drive populations to extinction.We discuss our results in relation to the evolution towards, and the breakdown of, dioecy, and the ecological and evolutionary implications of labile sex expression.

Udgivelsesdato: 13 December Abstract AcknowledgementsReferences BackgroundExploiting genetic diversity requires previous knowledge of the extent and structure of the variation occurring in a species. Such knowledge can in turn be used to build a core-collection, i.e. a subset of accessions that aim at representing the genetic diversity of this species with a minimum of repetitiveness. We investigate the patterns of genetic diversity and population structure in a collection of 346 inbred lines representing the breadth of naturally occurring diversity in the Legume plant model Medicago truncatula using 13 microsatellite loci distributed throughout the genome.ResultsWe confirm the uniqueness of all these genotypes and reveal a large amount of genetic diversity and allelic variation within this autogamous species. Spatial genetic correlation was found only for individuals originating from the same population and between neighbouring populations. Using a model-based clustering algorithm, we identified four main genetic clusters in the set of individuals analyzed. This stratification matches broad geographic regions. We also identified a set of "admixed" individuals that do not fit with this population structure scheme.ConclusionThe stratification inferred is discussed considering potential historical events like expansion, refuge history and admixture between neighbouring groups. Information on the allelic richness and the inferred population structure are used to build a nested core-collection. The set of inbred lines and the core collections are publicly available and will help coordinating efforts for the study of naturally occurring variation in the growing Medicago truncatula community.

L'analyse de la liaison statistique entre allèles à 2 locus, ou déséquilibre de liaison (DL), a été abordée simultanément chez le riz, le sorgho, le blé, l'hévéa et Medicago truncatula en utilisant des échantillons tirés de collections de ressources génétiques. Des stratégies utilisant des BACs ont permis de développer des marqueurs dans des zones localisées. L'utilisation des séquences terminales des BACs ciblés a été efficace pour définir des marqueurs microsatellites. Par ailleurs, des amorces ont été développées pour séquencer directement des fragments géniques. Le haut niveau de polymorphisme des marqueurs microsatellites a pu nécessiter des adaptations pour le calcul du DL. La structure génétique observée sur les plantes cultivées génère un fort bruit de fond de DL, tandis que dans les collections d'espèces sauvages autogames, le DL reste faible même à courte distance, décroissant rapidement au delà de 20 kb. A l'intérieur des groupes définis pour les espèces cultivées, le DL est plus faible et décroît assez rapidement, parfois de manière différente selon les groupes, de 100 kb environ pour le riz à 500 kb pour le sorgho. Une bonne connaissance de la diversité de l'espèce apparaît donc nécessaire avant de réaliser des études d'associations qui paraissent dès lors réalisables en utilisant les accessions contenues dans les collections de ressources génétiques.

Correspondance et tirés à part : tbata@daimi.au.dk; National audience; The comparative study of patterns of genetic diversity in crops and their wild ancestors coupled with recent advance in coalescent theory opens the way for making some historical inference on the demographic (and possibly selective) processes that have shaped genetic diversity in crops since domestication. Demographic events affect genome-wide levels of polymorphisms and the spectrum of allelic frequencies. Some forms of selection, in particular recent selective sweeps are expected to reduce diversity locally that is at the target of selection and in surrounding areas in the genome. Although a full reconstruction of the demographic history seems impossible based on patterns of polymorphism alone, contemporary polymorphism data can be used to calibrate simplified demographic models that can account for amounts and patterns of currently observable diversity. We illustrate that approach in two crop species with contrasted histories and mating systems. First, patterns of diversity are studied in the (allo)tetraploid wheat (Triticum turgidum) complex a species with a mixed mating system dominated by high levels of selfing. The intensity of bottlenecks associated with major historical episodes of domestication and subsequent selection are quantified using microsatellite markers. Patterns of microsatellite diversity are also compared with patterns of sequence polymorphism detected at the Gsp-1 locus. We find that domestication and subsequent selection resulted in a massive loss of genetic diversity and estimated a several fold reduction in the long term effective size realized in cultivated wheats relative to their wild ancestors (even after accounting for the probable loss of diversity in wild populations due to extensive habitat loss since Neolithic times). Next we analyze patterns of nucleotide diversity in alfalfa (Medicago sativa) an autotetraploid and outcrossing crop species. Sequence polymorphism among individuals spanning a wide geographic distribution was surveyed and analyzed at two intronic regions. These were located in respectively the alfalfa NADHdependent glutamate synthase and PO146, a pollen pectate lyase-like gene. These genes were chosen while searching in GenBank a public genetic sequence database for nucleotidic sequences of M. sativa and not chosen with any phenotypic à priori. This allowed us to i) quantify the extent of genetic divergence between wild and cultivated populations and thereby assess the impact of domestication on this divergence, ii) assess the originality of the genetic makeup of wild Spanish populations of alfalfa relative to the rest of extant population throughout the species range, iii) determine whether there is likely ongoing (recent) gene-flow among wild and cultivated populations in Spain

National audience

National audience