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The witches’ broom (Moniliophthora perniciosa) is considered as one of the main threats for cacao production and, consequently, for chocolate production worldwide. In this work, the genetic diversity and population structure of M. perniciosa were analyzed for 59 isolates collected in five departments of Colombia and using 10 microsatellite markers. Analyses revealed 35 multilocus genotypes and clonal populations structure according to linkage disequilibrium analysis. One of the objectives of this study was to determine whether populations were differentiated by geographic origin or Theobroma cacao host genotype. Analysis of molecular variance, discriminant analysis of principal components, and Bruvo genetic distance suggested that the genetic structure was driven by geographic origin and not by T. cacao genotype. The results of this study were consistent with previous findings obtained in other cocoa-producing countries. Important insights were discussed regarding the dispersal patterns of the pathogen in Colombia and the genetic change of its populations because of different environmental conditions.

Key message: A novel multi-environment competition individual-tree mixed model resulted in better fit, and greater individual narrow- and broad-sense heritabilities than the model without competition, notably for traits showing competition. In multi-environment tests, the proposed model would allow tree breeders to select genotypes with the best performance in both additive direct and competition breeding values, increasing forest productivity. Context: Genetic merit of trees is known to be affected and interact with local competition effects as well as changes across environmental conditions. Recent studies showed that competition genetic effects can affect the genetic variance and bias the tree breeding values, and its covariance with direct breeding values has been variable across traits. Aims: The present paper extends a mixed-model methodology to the problem of accounting for competition in a multi-environment set of forest genetic trials and exploring its impact on genetic variances as well as the multi-environment genetic correlation. Methods: The proposed model is illustrated using data from two full-sib trials of Pinus elliottii var. elliottii × Pinus caribaea var. hondurensis F1. Dispersion parameters and (co)variance of total breeding values were estimated for diameter at breast height, total tree height, and stem straightness at age 10. Results: For traits showing competition effects (diameter at breast height and total tree height), the proposed multi-environment competition model gave better fit than the simpler model. Accounting for competition increased the direct additive variance, reduced the residual variances, and did not change significantly the across-site additive genetic correlation. However, for diameter at breast height, top 5% best genetic rankings showed differences. Conclusion: When traits are strongly affected by inter-tree competition, the use of the proposed model in multi-environment analyses can efficiently identify the phenomenon with general benefits in the fitting of genetic components and open the door to select on the basis of competitiveness.

International audience; Under environmental stress, previously hidden additive genetic variation can be unmasked and exposed to selection. The amount of hidden variation is expected to be higher for life history traits, which strongly correlate to individual fitness, than for morphological traits, in which fitness effects are more ambiguous. However, no consensual pattern has been recovered yet, and this idea is still debated in the literature. Here, we hypothesize that the classical categorization of traits (i.e., life history and morphology) may fail to capture their proximity to fitness. In the desert locust, Schistocerca gregaria, a model organism for the study of insect polyphenism, we quantified changes in additive genetic variation elicited by lifetime thermal stress for ten traits, in which evolutionary significance is known. Irrespective of their category, traits under strong stabilizing selection showed genetic invariance with environmental stress, while traits more loosely associated with fitness showed a marked increase in additive genetic variation in the stressful environment. Furthermore, traits involved in adaptive phenotypic plasticity (growth compensation) showed either no change in additive genetic variance or a change of moderate magnitude across thermal environments. We interpret this mitigated response of plastic traits in the context of integrated evolution to adjust the entire phenotype in heterogeneous environments (i.e., adaptiveness of initial plasticity, compromise of phenotypic compensation with stress, and shared developmental pathway). Altogether, our results indicate, in agreement with theoretical expectations, that environmental stress can increase available additive genetic variance in some desert locust traits, but those closely linked to fitness are largely unaffected. Our study also highlights the importance of assessing the proximity to fitness of a trait on a case-by-case basis and in an ecologically relevant context, as well as considering the processes of canalization and plasticity, involved in the control of phenotypic variation.

Aim Palms are an iconic, diverse and often abundant component of tropical ecosystems that provide many ecosystem services. Being monocots, tree palms are evolutionarily, morphologically and physiologically distinct from other trees, and these differences have important consequences for ecosystem services (e.g., carbon sequestration and storage) and in terms of responses to climate change. We quantified global patterns of tree palm relative abundance to help improve understanding of tropical forests and reduce uncertainty about these ecosystems under climate change. Location Tropical and subtropical moist forests. Time period Current. Major taxa studied Palms (Arecaceae). Methods We assembled a pantropical dataset of 2,548 forest plots (covering 1,191ha) and quantified tree palm (i.e., ≥10cm diameter at breast height) abundance relative to co‐occurring non‐palm trees. We compared the relative abundance of tree palms across biogeographical realms and tested for associations with palaeoclimate stability, current climate, edaphic conditions and metrics of forest structure. Results On average, the relative abundance of tree palms was more than five times larger between Neotropical locations and other biogeographical realms. Tree palms were absent in most locations outside the Neotropics but present in >80% of Neotropical locations. The relative abundance of tree palms was more strongly associated with local conditions (e.g., higher mean annual precipitation, lower soil fertility, shallower water table and lower plot mean wood density) than metrics of long‐term climate stability. Life‐form diversity also influenced the patterns; palm assemblages outside the Neotropics comprise many non‐tree (e.g., climbing) palms. Finally, we show that tree palms can influence estimates of above‐ground biomass, but the magnitude and direction of the effect require additional work. Conclusions Tree palms are not only quintessentially tropical, but they are also overwhelmingly Neotropical. Future work to understand the contributions of tree palms to biomass estimates and carbon cycling will be particularly crucial in Neotropical forests.

Improved performances are partly due to heterosis effects. One of the basis of heterosis is dominance, which cannot be inherited.However, it can be exploited to boost the total genetic merit of the animals. This has a special interest in avian selection schemeswhere commercial animals are crossbred. In this study, we have estimated additive and dominance genetic variances for severalegg quality traits in pure-line layers.Around 10,500 egg quality performances were used, collected from 1,148 female Rhode Island layers, phenotyped at 70 weeksold and genotyped using a 600K high density SNP chip. Five egg quality traits were analysed: egg weight (EW), egg shell color(ESC), egg shell strength (ESS), albumen height (AH) and egg shell shape (ESShape). Additive and dominance genetic varianceswere estimated via EM-REML with univariate models. That included an inbreeding coefficient and an additive and a dominancerandom effect. Dominance variance explained a small fraction of the phenotypic variance (between 2 to 4 % across all traits).However, it represented a relevant fraction of the total genetic variance for some of the traits (16%, 10%, 35%, 2.4% and 15% ofthe total genetic variance for EW, ESC, ESS, AH, ESShape, respectively).Further research will estimate additive and dominance genetic correlations between the traits to maximize the total genetic gainof these traits simultaneously. In addition, a genomic BLUP with dominance effects is envisaged for the joint analyses of purebredand crossbred performances, to evaluate the potential to generate superior crossbred performances.

La régression aléatoire appliquée à l'étude de la dendroplasticité. Plasticité phénotypique, des concepts à l'analyse de données

Q(ST) is a differentiation parameter based on the decomposition of the genetic variance of a trait. In the case of additive inheritance and absence of selection, it is analogous to the genic differentiation measured on individual loci, F(ST). Thus, Q(ST)−F(ST) comparison is used to infer selection: selective divergence when Q(ST) > F(ST), or convergence when Q(ST) < F(ST). The definition of Q-statistics was extended to two-level hierarchical population structures with Hardy–Weinberg equilibrium. Here, we generalize the Q-statistics framework to any hierarchical population structure. First, we developed the analytical definition of hierarchical Q-statistics for populations not at Hardy–Weinberg equilibrium. We show that the Q-statistics values obtained with the Hardy–Weinberg definition are lower than their corresponding F-statistics when F(IS) > 0 (higher when FIS < 0). Then, we used an island model simulation approach to investigate the impact of inbreeding and dominance on the Q(ST)−F(ST) framework in a hierarchical population structure. We show that, while differentiation at the lower hierarchical level (Q(SR)) is a monotonic function of migration, differentiation at the upper level (Q(RT)) is not. In the case of additive inheritance, we show that inbreeding inflates the variance of Q(RT), which can increase the frequency of Q(RT) > F(RT) cases. We also show that dominance drastically reduces Q-statistics below F-statistics for any level of the hierarchy. Therefore, high values of Q-statistics are good indicators of selection, but low values are not in the case of dominance.

Summary Plants need to control transpiration and photosynthesis tightly in order to grow under drought and high temperature. Contrasting genetic-by-environment interactions are exploited by Arabidopsis thaliana to improve stress tolerance., How genetic factors control plant performance under stressful environmental conditions is a central question in ecology and for crop breeding. A multivariate framework was developed to examine the genetic architecture of performance-related traits in response to interacting environmental stresses. Ecophysiological and life history traits were quantified in the Arabidopsis thaliana Ler×Cvi mapping population exposed to constant soil water deficit and high air temperature. The plasticity of the genetic variance–covariance matrix (G-matrix) was examined using mixed-effects models after regression into principal components. Quantitative trait locus (QTL) analysis was performed on the predictors of genotype effects and genotype by environment interactions (G×E). Three QTLs previously identified for flowering time had antagonistic G×E effects on carbon acquisition and the other traits (phenology, growth, leaf morphology, and transpiration). This resulted in a size-dependent response of water use efficiency (WUE) to high temperature but not soil water deficit, indicating that most of the plasticity of carbon acquisition and WUE to temperature is controlled by the loci that control variation of development, size, growth, and transpiration. A fourth QTL, MSAT2.22, controlled the response of carbon acquisition to specific combinations of watering and temperature irrespective of plant size and development, growth, and transpiration rate, which resulted in size-independent plasticity of WUE. These findings highlight how the strategies to optimize plant performance may differ in response to water deficit and high temperature (or their combination), and how different G×E effects could be targeted to improve plant tolerance to these stresses.

Understanding the determinants of phenotypic variation is critical to evaluate the ability of traits to evolve in a changing environment. In trees, the genetic component of the phenotypic variance is most often estimated based on maternal progeny tests. However, the lack of knowledge about the paternal relatedness hampers the accurate estimation of additive genetic and maternal effects. Here, we investigate how different methods accounting for paternal relatedness allow the estimation of heritability and maternal determinants of adaptive traits in a natural population of Fagus sylvatica L., presenting non-random mating. Twelve potentially adaptive functional traits were measured in 60 maternal families in a nursery. We genotyped a subset of offspring and of all the potentially reproductive adults in the population at 13 microsatellite markers to infer paternal relationships and to estimate average relatedness within and between maternal families. This relatedness information was then used in family and animal models to estimate the components of phenotypic variance. All the studied traits displayed significant genetic variance and moderate heritability. Maternal effects were detected for the diameter increment, stem volume and bud burst. Comparison of family and animal models showed that unbalanced mating system led to only slight departures from maternal family assumptions in the progeny trial. However, neglecting the significant maternal effects led to an overestimation of the heritability. Overall, we highlighted the usefulness of relatedness pattern analyses using polymorphic molecular markers to accurately analyse tree sibling designs.

La rapidité du changement climatique observé et prédit soulève la question de la vitesse à laquelle les espèces pourront s'adapter au climat futur. Les populations exposées aux changements de conditions environnementales peuvent s'adapter sur place (sans migration) grâce, dans un premier temps, à la réponse plastique des individus, puis, à long terme, par la réponse à la sélection (adaptation génétique). En situation d'environnement variable à la fois dans l'espace et le temps, les flux de gènes peuvent faciliter la diffusion d'allèles bénéfiques entre populations. L'objectif de cette thèse est d'étudier expérimentalement l'interaction entre adaptation et dispersion pollinique à longue distance sur un gradient altitudinal de hêtre commun (Fagus sylvatica). Cette étude a été conduite à partir de trois populations de hêtre sur le versant Nord du Mont-Ventoux (de 900 m à 1400 m d'altitude). Douze traits fonctionnels (phénologiques, physiologiques et morphologiques) potentiellement adaptatifs ont été mesurés sur 60 descendances maternelles issues de ces populations (20 mères/population, 100 individus/mère) et placées en pépinière. Les résultats de cette thèse montrent que (i) les forts taux de pollen immigrant reçus par les populations (m = 56 %) n'empêchent pas l'émergence de patrons de différenciation adaptative, (ii) la sélection érode la variance génétique des traits, (iii) malgré l’existence de variance génétique pour tous les traits, les covariances génétiques entre traits peuvent affecter les taux et directions d'évolution future, et (iv) les flux de pollen contribuent à augmenter la variance génétique totale au sein des populations. Finalement, les populations à moyenne et hautes altitudes pourraient avoir les capacités de s’adapter au climat futur.

International audience; Multiple mating by social insect queens is a widespread phenomenon. Because of the apparent inclusive fitness benefits of monandry, and the potential costs of polyandry, explanations for the evolution of multiple mating have been frequently sought. Current leading explanations are collectively known as "genetic variance" hypotheses which posit that both queen and colony fitness are increased by an increase in the intracolonial genetic diversity that accrues from multiple mating. However, the precise way in which genetic diversity acts to increase colony fitness is not clear. Furthermore, some of these hypotheses are probably insufficient to explain extreme levels of polyandry observed in the genus Apis.; Évolution de l'accouplement multiple chez le genre Apis . L'accouplement multiple chez les reines d'insectes sociaux est un phénomène largement répandu. À cause des bénéfices apparents de l'adéquation adaptative globale ("inclusive fitness ") dus à la monoandrie et aux coûts potentiels de la polyandrie, on a souvent recherché des explications à l'évolution de l'accouplement multiple. Les explications qui prévalent actuellement sont connues généralement comme hypothèses de "variance génétique ". Elles posent en principe l'hypothèse que la valeur adaptative de la reine et celle de la colonie sont accrues par un accroissement de la diversité génétique intracolonie qui dérive de l'accouplement multiple. On peut répartir ces hypothèses en deux grands groupes : 1) celles relatives aux effets du déterminisme du sexe sur la viabilité du couvain (hypothèses de l'allèle sexuel) et celles relatives au conflit entre ouvrières et reine concernant la répartition optimale des ressources allouées à chaque sexe ; et 2) celles relatives aux bénéfices postulés de la valeur adaptative provenant d'une diversité génétique accrue au sein de la population d'ouvrières. Tandis que nombre de ces hypothèses semble correcte pour expliquer les faibles niveaux d'accouplement multiple, il est plus difficile d'expliquer le grand nombre d'accouplements chez les reines d'Apis. Les effets de l'accouplement multiple sur les variables telles que la parenté des ouvrières et la proportion de mâles diploïdes au sein d'une colonie sont grandement modifiés par les quelques premiers accouplements. Par contre, aux fréquences d'accouplement plus élevées, le bénéfice apporté par chaque accouplement supplémentaire décroît selon une asymptote. Les explications les plus courantes concernant l'évolution de la polyandrie extrême incluent le gain de valeur adaptative par la spécialisation des tâches et la résistance aux parasites et aux agents pathogènes. Nous émettons l'hypothèse que la voie évolutive de la monoandrie à la polyandrie extrême s'est faite en plusieurs étapes. Initialement le coût génétique imposé par le locus sexuel provoquait un passage de la monoandrie à l'oligoandrie. Le conflit reine/ouvrières concernant les sex ratio a renforcé les pressions de sélection vers l'évolution de la polyandrie. Finalement, les pressions des agents pathogènes et des parasites, et le besoins d'une population d'ouvrières aux comportement variés ont pu être la cause première du développement de la polyandrie extrême chez le genre Apis.

The conservation of genetic variability is recognized as a necessary objective for the optimization of selection schemes, particularly when populations are small. Numerous models, differing by the genetic model they rely on, are available to better understand and predict the evolution of genetic variance in a small population undergoing selection. This paper compares three genetic models, treated either analytically or with Monte-Carlo simulations, first in order to validate the predictions provided by a ’full-finite model’ for well-known phenomena (e.g. the effect of population management on genetic variability), and second, to evaluate when and how the assumptions made in the two analytical models induce the departure from the third model. The FFM is shown, first, to be in close agreement with the Gaussian theory when used with a large number of loci, the stochastic approach making it much more flexible than the two algebraic models. In the second part of the study, the infinitesimal model appears to be more robust than the semiinfinitesimal one. Major sources of discrepancy between the deterministic models and the FFM are identified, notably the hypothesis of independence between loci, and then the infinite number of loci or alleles per locus., La conservation de la variabilité génétique est reconnue comme un objectif nécessaire pour l’optimisation des schémas de sélection, notamment pour les petites populations. De nombreux modèles, différant par les hypothèses de déterminisme génétique sur lesquels ils reposent, sont disponibles pour une meilleure compréhension et une meilleure prédiction de l’évolution de la variabilité génétique dans une petite population soumise à sélection. Cet article compare trois modèles génétiques différents, traités soit par la voie analytique soit par des simulations Monte-Carlo, d’une part pour valider les prédictions fournies par un « modèle fini complet » pour des phénomènes connus (comme l’influence de la gestion de la population sur la variabilité génétique), d’autre part pour évaluer quand et comment les hypothèses faites dans les deux modèles analytiques induisent un écart avec le dernier modèle. Le modèle fini complet apparaît, dans un premier temps, en bon accord avec la théorie gaussienne quand il est utilisé avec un grand nombre de locus, l’approche stochastique le rendant de plus beaucoup plus souple que les deux modèles algébriques. Dans la seconde partie de l’étude, le modèle infinitésimal apparaît plus robuste que le modèle semi-infinitésimal. Des sources majeures d’écart entre les modèles déterministes et le modèle fini complet sont identifiées, notamment l’hypothèse de l’indépendance entre les locus, puis le nombre infini de locus et d’allèles par locus.

Human-induced environmental stress may lead to rapid evolutionary processes, and can affect the ability of natural populations to respond to other environmental change or stress. We used quantitative genetics tools, pesticide exposure and a gradient of biotic stress to investigate these questions in the freshwater snail Lymnaea stagnalis. The study focused on the genetic component of variance for life-history traits within populations, and the ability of different lines to respond differently to stress. The effect of parental exposure to a xenobiotic stress on the reaction norm of the progeny to another stress was also estimated (parental non-genetic effect). First, under laboratory conditions, inter-family variance suggested significant heritability for most traits. Second, under outdoor exposure to various pesticides, variation among families was significant for individual growth. Clutch size and hatching rate of the clutches laid in the laboratory after exposure showed similar results, and moreover, family interacted significantly with pesticides. Third, under a gradient of biotic stress (food and competition), inter-family variation was again significant for growth, and a significant interaction with biotic stress was observed for juvenile growth and ultimate size. Family heterogeneity and family × environment interactions indicate the possibility of differential evolutionary responses among lines, through different reaction norms. Stressful conditions did not affect the estimated heritability, and for pesticides, no transgenerational effect was detected on progeny growth in response to the biotic stress. Focused on short-term evolutionary responses, the present study illustrates a possible way of incorporating evolutionary approaches into ecotoxicological risk assessment procedures, for example, by accounting for inter-family variation.

The mechanisms by which nonrandom mating affects selected populations are not completely understood and remain a subject of scientific debate in the development of tractable predictors of population characteristics. The main objective of this study was to provide a predictive model for the genetic variance and covariance among mates for traits subjected to directional selection in populations with nonrandom mating based on the pedigree. Stochastic simulations were used to check the validity of this model. Our predictions indicate that the positive covariance among mates that is expected to result with preferential mating of relatives can be severely overpredicted from neutral expectations. The covariance expected from neutral theory is offset by an opposing covariance between the genetic mean of an individual's family and the Mendelian sampling term of its mate. This mechanism was able to predict the reduction in covariance among mates that we observed in the simulated populations and, in consequence, the equilibrium genetic variance and expected long-term genetic contributions. Additionally, this study provided confirmatory evidence on the postulated relationships of long-term genetic contributions with both the rate of genetic gain and the rate of inbreeding (ΔF) with nonrandom mating. The coefficient of variation of the expected gene flow among individuals and ΔF was sensitive to nonrandom mating when heritability was low, but less so as heritability increased, and the theory developed in the study was sufficient to explain this phenomenon.

International audience; Data were analysed from a divergent selection experiment for an indicator of body composition in the mouse, the ratio of gonadal fat pad to body weight (GFPR). Lines were selected for 20 generations for fat (F), lean (L) or were unselected (C), with three replicates of each. Selection was within full-sib families, 16 families per replicate for the first seven generations, eight subsequently. At generation 20, GFPR in the F lines was twice and in the L lines half that of C. A log transformation removed both asymmetry of response and heterogeneity of variance among lines, and so was used throughout. Estimates of genetic variance and heritability (approximately 50% ) obtained using REML with an animal model were very similar, whether estimated from the first few generations of selection, or from all 20 generations, or from late generations having fitted pedigree. The estimates were also similar when estimated from selected or control lines. Estimates from REML also agreed with estimates of realised heritability. The results all accord with expectations under the infinitesimal model, despite the four-fold changes in mean. Relaxed selection lines, derived from generation 20, showed little regression in fatness after 40 generations without selection.; Analyse de la réponse à la sélection de 20 générations pour la composition corporelle des souris : ajustée aux hypothèses du modèle infinitésimal. Les données provenant d'un programme de sélection divergente ont été analysées pour un indicateur de la composition corporelle des souris : la proportion de tissus adipeux gonadal par rapport au poids corporel (GFPR). Trois répliques de chacune des lignées ont été sélectionnées pendant 20 générations pour l'engraissement (F), la minceur (L), ou non sélectionnées. La sélection fut réalisée dans des familles de plein-frères, 16 familles par réplique durant les sept premières générations et huit pour les suivantes. A la vingtième génération, le GFPR des lignées (F) et (L) étaient respectivement le double et la moitié de celui de (C). Une transformation logarithmique permet de supprimer l'asymétrie de la réponse et l'hétérogénéité des variances entre ces deux lignées. Les estimateurs de la variance génétique et de l'héritabilité (approxi- mativement de 50 % ) obtenus par le REML avec un modèle animal sont semblables à ceux obtenus en utilisant les premières générations de sélection, les 20 générations de sélection ou les dernières en employant l'information sur le pedigree jusqu'à la population de base. De plus, en utilisant les lignées sélectionnées et les lignées de contrôle, les estimateurs sont similaires. Les estimations REML sont conformes à celles de l'héritabilité. Tous les résultats sont conformes à ceux attendus sous un modèle infinitésimal malgré une variation de quatre fois la moyenne. Les lignées soumises à une pression de sélection plus faible à la vingtième génération, montrent peu de diminution en engraissement après 40 générations sans sélection.

Pour compléter une étude expérimentale portant sur l’intérêt, pour l’amélioration des plantes, d’un gène élévateur des fréquences de recombinaison, deux études par simulation ont été réalisées. Dans la première étude, des descendances de 10 individus ont été dérivées par une génération d’autofécondation ou par un plan circulaire de croisement, à partir de quatre populations, dans des cas de fréquences moyennes de recombinaison élevées, intermédiaires et faibles. Les distributions des variances génétiques des descendances issues d’une même population ont été comparées. Dans les deux types de descendances, les distributions des variances génétiques se recouvrent largement, que la population présente un excès d’associations en couplage ou en répulsion. Il risque donc d’être difficile de mettre expérimentalement en évidence l’effet de différences de fréquences de recombinaison sur la variabilité génétique de caractères quantitatifs. Dans la seconde étude, un programme de sélection récurrente a été simulé pour des fréquences moyennes de recombinaison élevées, intermédiaires, et faibles. Pour la fréquence de recombinaison la plus faible, le progrès génétique est plus lent, légèrement moins important, et la variance génétique plus élevée. Par contre, de même que pour la simulation précédente, les fréquences de recombinaison élevées et intermédiaires donnent des résultats similaires du point de vue du progrès génétique et de la variance génétique. Un gène élevant les taux de recombinaison n’est donc intéressant que si le linkage est très étroit ou les croisements difficiles. Sinon, un plan d’intercroisement assure une gestion efficace de la variabilité., Two stimulation studies were carried out to complement an experimental study of the possible interest for plant breeders of an allele increasing recombination rates. In the first simulation study, 4 populations of 10 individuals were simulated in the case of high (45%), medium (19.6%) and low (6.9%) recombination frequencies. Ten-plant populations were derived by one generation, either from selfing or from crossing in a circular mating design, from each of the 4 populations. For each initial population, the distribution of the genetic variances in the populations obtained by one mating system overlapped widely in the 3 cases of recombination rates considered, whether the initial population presented an excess of coupling phase, or an excess of repulsion phase. An experimental assessment of the effect of recombination rate differences on genetic variability of quantitative traits is therefore likely to be difficult. The second study simulated a recurrent selection program in the case of high (45%), medium (19.6%) and low (9%) recombination rates. The mean genetic value increased somewhat more slowly and reached a slightly lower value for low recombination rates, whereas the genetic variance remained the highest after the second selection cycle. The evolutions in genetic mean values and genetic variances were almost identical for medium and high recombination frequencies. Thus, an allele which increases recombination rates will only be of interest in the case of close linkage or difficult crossing. Otherwise, a circular mating design manages efficiently the initial germplasm.

In an Expectation-Maximization type Restricted Maximum Likelihood (REML) procedure, the estimation of a genetic (co-)variance component involves the trace of the product of the inverse of the coefficient matrix by the inverse of the relationship matrix. Computation of this trace is usually the limiting factor of this procedure. In this paper, a method is presented to approximate this trace in the case of an animal model, by using an equivalent model based on the Mendelian sampling effect and by simplifying its coefficient matrix and its inversion. This approximation appeared very accurate for low heritabilities but was downwards biased when the heritability was high. Implemented in a REML procedure, this approximation reduced dramatically the amount of computation, but provided downwards biased estimates of genetic variances. Several examples are presented to illustrate the method., Dans certaines procédures de Maximum de Vraisemblance Restreint (REML), l’estimation des composantes de (co)variance génétique implique le calcul de la trace du produit de l’inverse de la matrice des coefficients par l’inverse de la matrice de parentés, calcul qui constitue généralement le facteur limitant de ce type de procédure. Nous présentons dans cet article une méthode visant à obtenir une valeur approchée de cette trace dans le cadre d’un modèle animal, en utilisant un modèle équivalent basé sur l’aléa de méiose, en simplifiant sa matrice des coefficients et en en calculant une inverse approchée. Cette approximation est très précise lorsque l’héritabilité du caractère est faible mais elle tend à sous-estimer la trace vraie lorsque l’héritabilité est élevée. Intégrée dans une procédure de REML, cette méthode en réduit considérablement le coût mais fournit en général des valeurs sous-estimées de variance génétique. Divers exemples sont présentés à titre d'illustration

10 Pags.- 3 Tabls., [EN] The coefficient of variation of the genetic variance at generation t, C)AV,t(V of an additive trait among replicated unselected small populations has been theoretically shown to be largely due to linkage disequilibrium from sampling. Starting from a population in linkage equilibrium, CA)Vt(V should initially increase, rapidly approaching an asymptotic value. However, when effective population size expands, CV(V At) is expected to decrease. Experiments with Drosophila melanogaster were carried out to check these predictions. Inbred lines were established by brother-sister single pair matings and continued for 3 generations. Each line was then maintained with as many parents as possible up to generation 10. The trait considered was the total number of abdominal bristles on the 5th and the 6th stemites, in generations 0, 4 and 10. A single generation of divergent selection was carried out in each of 75 lines in the same generations. Assuming no differences in environmental variance between lines, CA)Vt(V can then be estimated from the within-line phenotypic variances or from the responses to selection. As predicted by theory, the value of CV(VA!) substantially increased from generation 0 to 4. No reduction was detected afterwards, possibly because the trait was affected by blocks of genes. Other predictions made concerning the redistribution of the genetic variance have been substantiated., [FR] On a montré théoriquement que le coefficient de variation de la variance génétique d’un caractère additif dans la génération t, C)AV(,tV dans un ensemble de lignées non sélectionnées ayant toutes le même effectif génétique, est dû essentiellement au déséquilibre de linkage" généré par échantillonnage. Si la population est initialement en équilibre, CA)V(tV s accroit au fil des générations, tendant rapidement vers une valeur asymptotique. Si, ensuite, l’effectif de la population augmente, CA)V(lV diminuera. Afin de confro"ler ces prédictions, on a réalisé des accouplements consanguins frère x sceur, pendant 3 générations dans un ensemble de lignées de Drosophila melanogaster. Postérieurement chaque lignée a été maintenue jusqu’à la génération 10 avec le plus grand nombre possible de géniteurs. Le caractère considéré a été le nombre total de soies dans les 5Q et 60 segments abdominaux, évalué dans les générations 0, 4 et 10. Parallèlement, et dans ces mêmes générations, on a effectué une sélection divergente pendant une génération en 75 lignées. En supposant que la variance environnementale du caractère soit la même dans toutes les lignées considérées, CA)V(tV peut être évalué à partir des variances phénotypiques intra-lignée ou à partir des réponses à la sélection. La valeur de CA)V(tV a augmenté considérablement en passant de la génération 0 à la 4, ainsi que le prévoyait la théorie. Toutefois, elle ne s’est pas réduite ensuite, ce qui est probablement dû à l’existence de blocs géniques. Les prédictions théoriques relatives à la redistribution de la variance génétique ont été également vérifiées., his work was supported by a grant from Comision Asesora de Investigacion Cientifica y Técnica.

Une décomposition du défaut de fouchaison chez le Pin laricio de Corse a été réalisée sur les verticilles 1983 et 1984 d’un test de descendances maternelles âgé de 9 ans, situé en forêt d’Orléans. La présence de pousses secondaires ou plus précisément d’allongements secondaires sur les branches latérales, bien que sensible à l’effet année, semble être le principal facteur explicatif de l’apparition des 2 types de défauts : fourches et ramicornes. L’utilisation complémentaire d’une note d’angle moyen de branchaison, plus stable dans le temps, permet d’évaluer l’importance des causes de la fourchaison autres que le polycyclisme. Dans la population d’amélioration étudiée, ces 2 caractères s’avèrent être de bons critères prédicteurs de la fourchaison. avec une variabilité génétique intéressante et une héritabilité individuelle assez élevée (0,3-0,4). L’absence de corrélation génétique significative entre hauteur totale et angle de branchaison réduit l’importance de l’antagonisme vigueur-niveau de fourchaison à la seule corrélation génétique positive entre hauteur totale et polycyclisme. Le meilleur compromis au niveau de l’espérance des gains génétiques sera obtenu grâce à la construction d’un indice de sélection combinant les 3 caractères : hauteur totale, polycyclisme et angle de branchaison., Two types of forking defects, fork and ramicorne can seriously affect stem quality of Corsican Pine. The use of a forking score between 0 and 100 as selection criterion seemed not to be very efficient for the breeding population studied (tabl. 1). The purpose of this work is to find through factors explaining forking, one or several good predictors ensuring maximum genetic gain. Measurements were made in fall 1985 on a 9 years old open pollinated progeny test located in Orleans forest. The importance of three already known explicatim factors of forking defects was studied for the years 1983 and 1984 on a sample of 443 forked trees. - Insect attacks (Rhyacionia buoliana) and rust damages, - Polycyclism, through the total number of lateral lammas shoots (PA) and lateral lammas shoots inducing defects (PAD), - Average branch angle with a special notation scale (fig. 1).Genetic parameters (family and residual variances, individual heritabilities, genetic and residual correlations) were computed on 48 half-sib families after an analysis of variance with a hierarchical model (tabl. 2). Insect attacks and rust damages can’t explain forking defects either in 1983 or in 1984. Although it is affected by climatic conditions, polycyclism seems to be the main explanatory forks of (presence of forks and ramicornes) in 1983 and 1984. Meanwhile, defects are rather due to a secondary elongation of some lateral buds, than to real lammas shoots (fig. 3). The branch angle score used, discriminates well the most forking trees (fig. 5) and can take into account factors unducing forks independent of polycyclism. This measure is more stable from one year to another and better describes a characteristic of each tree. In the studied breeding population, polycyclism and branch angle explain a great proportion of the observed variability for the forking score (tabl. 4). Meanwhile, because of their high values of between families variances and individual heritabilities, these 2 traits seem to be more interesting for the breeder than the forking score (tabl. 5). It results, from the non significative genetic correlation between total height and branch angle that mark a simultaneous genetic gain can be obtained on both traits (tabl. 6). This progress is limited by the genetic positive unfavorable correlation between polycyclism and total height and the best selection compromise need the use of selection index combining the 3 characters.

Les techniques actuelles de superovulation, de collecte et de transfert des embryons chez les bovins permettent théoriquement la mise en place de nouveaux programmes de sélection. Dans cet article, on aborde l’étude génétique d’un mode particulier de noyaux de sélection, utilisant le transfert d’oeufs et en vue de la sélection d’un seul caractère : la production laitière. Ces noyaux ont pour fonction et nouveauté essentielles la production systématique de très jeunes mères à taureaux connues sur une lactation suivant 2 collectes d’embryons, qui commence en moyenne après la naissance des produits, de manière à raccourcir l’intervalle de génération sur les voies femelles. La méthode de testage des mâles sur descendance et la méthode de production des pères à taureaux sont identiques à celles utilisées à l’heure actuelle. Elles utilisent notamment le contrôle laitier national. L’évolution génétique des veaux nés dans le noyau est comparée à celle des veaux issus d’accouplements raisonnés dans le cadre d’un schéma classique, avec la contrainte que le nombre annuel des mâles testés et que les procédures de sélection sur la voie père-fils sont les mêmes dans les 2 cas. Le raccourcissement de l’intervalle de génération dans le noyau et l’utilisation de taureaux bien connus sont des facteurs favorables à une accélération du progrès génétique en comparaison d’un schéma classique de référence très efficace (progrès génétique annuel = 0,23 écart-type génétique). Cette accélération est fonction de la taille du noyau et des paramètres techniques du transfert (nombre et taux de survie des embryons). La supériorité est de l’ordre de 20 p. 100 quand la taille du noyau est comparable à l’effectif du nombre de taureaux à tester, pour les paramètres techniques actuels (nombre d’embryons par récolte = 4-5 ; taux de survie des embryons = 40-60 p. 100). La sensibilité de ces schémas à ces paramètres est moins importante que celle des schémas proposés par NICHOLAS & SMITH (MOET) en 1983. Dans le court terme, les valeurs relatives du progrès génétique ne sont guère modifiées quand ont tient compte dans les calculs de facteurs habituellement négligés (déséquilibre de linkage induit par la sélection ; supériorité génétique vraie de reproducteurs apparentés sélectionnés dans une population de taille limitée). En ce qui concerne le long terme, le problème de l’évolution vraie de la variance génétique, notamment dans les schémas de transfert, n’a pas été réellement abordé., With current techniques to induce superovulation and collect and transfer embryos in cattle, the development of new selection schemes is theoretically possible. A genetic study of a special type of selection nuclei using embryo transfer and intended for selection on one trait only, i.e. milk production, is described in this paper. The systematic production of very young bull dams evaluated from their 1st lactation following 2 embryo recoveries and starting, on an average, after birth of the offspring so as to reduce the generation interval along the female paths, represents the main role and novelty of these nuclei. The methods used for the progeny testing of the males and production of bull sires were similar to those applied at present, involving in particular the national milk recording data. The genetic change in calves born within the nuclei was compared to that of calves from planned matings within a conventional scheme. However, there were 2 constraints, i.e. use of the same annual number of tested males and the same selection procedure along the sire-son path in both cases. Reduction of the generation interval within the nuclei and use of well-known bulls favored an acceleration of the annual genetic gain, compared to a very efficient conventional selection scheme (0.23 genetic standard deviation). This acceleration depended on nucleus size and on some technical parameters of transfer such as embryo number and survival rate. There was about a 20 p. 100 improvement when nucleus size was the same as the annual number of bulls to be tested (present parameters : number of embryos per recovery = 4-5 ; embryo survival rate = 40-60 p. 100). The sensitivity of these programs to the above parameters was lower than that of the programs proposed by NICHOLAS & SMITH (MOET). On a short-term basis, the relative values of genetic progress were not modified much when factors such as linkage disequilibria induced by selection and true genetic superiority of apparented breeding animals selected within a population of restricted size (usually neglected) were included. On a long-term basis, the problem of true changes in genetic variance, especially in transfer plans, has not been solved.