Thèse--Sc. nat.--Paris XI, 1981. / Bibliogr. p. 143-151.

Manipulation de la recombinaison chez une plante cultivée, le riz.L’accroissement prévisible de la population mondiale ainsi que les conséquences du changement climatique obligent les sélectionneurs à créer de nouvelles variétés plus productives et plus résilientes. Les nouvelles combinaisons d’allèles favorables sont issues de la recombinaison génétique entre chromosomes homologues dont le siège est la prophase de première division de méiose. De récentes avancées chez la plante modèle Arabidopsis ont montré que l'inactivation de certains gènes permet de manipuler la méiose pour abolir ou au contraire augmenter très significativement la recombinaison. Les mécanismes de la méiose étant relativement bien conservés chez les eucaryotes, l’objectif de cette thèse était de transposer ces avancées chez une plante cultivée importante, le riz. Abolir la recombinaison méiotique permettrait de propager de façon clonale par grain des formules variétales hybrides F1 dont le rendement est de 20% supérieur à celui des lignées pures chez le riz mais dont les semences restent peu utilisées par les riziculteurs de subsistance. Les travaux réalisés dans une première partie de la thèse ont montré que le cumul de trois mutations Ososd1, pair1 et Osrec8, permettait d’obtenir des gamètes clonaux diploïdes mâles et femelles. Le phénotype apoméiotique obtenu, appelé MiMe (Mitosis instead of meiosis) chez Arabidopsis, peut être utilisé pour tester différentes stratégies d’induction de la parthénogenèse afin de produire des grains formant des plantes diploïdes clonales apomictiques. Une optimisation du mécanisme permettrait d'envisager l'utilisation de l'apomixie pour fixer l'hétérosis dans les semences hybrides F1. Par ailleurs, une augmentation globale ou locale de la recombinaison méiotique est recherchée car elle permettrait de diminuer la taille des populations de sélection et de réduire la taille des segments chromosomiques introduits dans les variétés élite de riz. Nous avons montré dans une seconde partie, que la mutation du gène OsRECQl4 codant pour une hélicase permet d'augmenter le taux de recombinaison d'un facteur de 3,3 fois faisant passer la taille de la carte génétique de 1670 cM à 5538 cM sans affecter la fertilité de la plante ni le déroulement de la méiose. Chez les plantes affectées dans la fonction d’une autre hélicase, OsFANCM, le taux de recombinaison a été également augmenté mais dans une moindre mesure (x 2,2). L’augmentation de la recombinaison s’opère sur l'ensemble des bras chromosomiques sauf au niveau des centromères. Ces résultats confirment ceux obtenus chez A. thaliana qui ont montré le rôle de régulateur négatif des crossing-overs (CO) des protéines RECQ4 et FANCM. La combinaison en cours de ces mutations entre elles ou avec celle affectant l’AAA-ATPase FIGL1 permet d’espérer une augmentation de la recombinaison encore supérieure. Ces résultats ouvrent la voie à l'utilisation des gènes anti-COs pour augmenter de façon globale le nombre de recombinants dans les croisements chez le riz et sans doute chez les autres céréales. Pour offrir une possibilité concrète aux sélectionneurs d'utiliser les gènes anti-CO, nous avons montré que la technologie CRISPR/cas9 permet d'éteindre l'expression de OsFANCM OsRECQl4 et OsFIGL1. / Manipulation of recombination in a crop, rice.The forecasted increase of world population as well as the consequences of global climate change oblige plant breeders to develop new varieties that are both more productive and resilient. Novel combinations of favourable alleles are generated through genetic recombination between homologous chromosomes, which occurs during the prophase of the first division of meiosis. Recent advances in the model plant Arabidopsis have demonstrated that the inactivation of some genes allows meiosis manipulation resulting in either an abolishment or in contrast, a significant enhancement of meiotic recombination. The meiosis mechanisms being relatively conserved across eucaryotes, the overall objective of this thesis was to transfer these advances to a crop of crucial importance, rice. To abolish meiotic recombination would allow the clonal propagation by seeds of F hybrids, which exhibit a 20% yield enhancement compared to that of pure lines in rice but remain rarely used in subsistence farming. In a first part, we showed that rice plants cumulating 3 mutations inactivating Ososd1, pair1 and Osrec8, formed clonal diploid male and female gametes. This apomeiotic phenotype, called MiMe (Mitosis instead of meiosis) in Arabidopsis, can serve as material to assay several strategies of parthenogenetic induction that would result in seed forming diploid clonal plants. Further optimization of the mechanisms would allow the use of apomixis to fix heterosis in hybrid seeds. Global and local enhancement of recombination is another desirable goal since it would allow a reduction in breeding population size and a downsizing of the introgressed chromosomal segments in elite plant materials. In a second part, we showed that mutation in the DNA helicase gene OsRECQl4 conducted to a 3.3 fold increase of recombination and inflated the genetic map size from de 1670 cM to 5538 cM, without altering plant fertility nor meiosis progression. Plants altered in a second DNA helicase, OsFANCM, exhibited a more modest 2.2 fold recombination enhancement. Recombination increase operated along the whole chromosome arms except at the centromere level. These results confirms the negative regulator role of RECQ4 and FANCM on crossing overs (CO), previously reported in Arabidopsis. On going combination of these mutations together with that altering the l’AAA-ATPase FIGL1 should conduct to an even higher recombination enhancement. These results pave the way to the use of anti-CO genes to enhance recombinant recovery in crosses of rice and possibly of other cereals. To provide breeders with a workable anti-CO system, we eventually showed that the CRISPR/Cas9 technology can be used to abolish OsFANCM, OsRECQl4 and OsFIGL1 expression.

The main drivers of microevolution in the genus Sorbus are interspecific hybridisation and polyploidy. The fate of new hybrid and polyploid taxa is determined by their mode of reproduction. Especially apomixis could be very advantageous for these new taxa. The S. aria agg. (subg. Aria) plays an important role within the genus since its members are involved in all hybridisation events and thereby is responsible for the substantial part of variation of the genus. Flow cytometry, molecular markers and multivariate morphological analyses were employed to evaluate the processes generating the variability in the S. aria group. Three ploidy levels were detected among species from subg. Aria in the Czech Republic. All of them could be found in the South Moravia, whereas only tetraploids occur in the Bohemia region. Moreover, most of the Czech taxa (5 out of 7) grow also only in the South Moravia which is therefore considered as a centre of diversity of the genus Sorbus in the Czech Republic. Flow cytometry seed screen revealed 7 modes of reproduction among the individuals from S. aria agg. A wide range of sexual and apomictic types of reproduction including reduced and unreduced gametes was detected. All of the diploid individuals are completely sexual. Among polyploid taxa, most of the species are...

Apomixis (asexual reproduction by seeds) has the considerable potential in agriculture and crop breeding due to its ability to produce genetically identical progenies in a form of persistent propagules, i.e. seeds. However, the processes laying behind the origin of apomixis and connected molecular mechanisms are still unknown. Despite the fact, that apomicts are considered to be an evolutionary dead-ends, they are often more widely distributed than their sexual relatives (this phenomenon is commonly referred to as geographical parthenogenesis (GP). Although this phenomenon is studied for decades, its causes are still not fully understood. Importantly, several recent studies pointed out that apomicts with limited genetic variability can at some extent react to changing environment through changes in gene transcription by epigenetic modifications. It is generally assumed, that hybridisation and polyploidization played a crucial role in the emergence of apomixis. For this reason, presented studies test the putative origin of selected polyploid apomicts and if the interspecific hybridisation between diploid sexuals triggers polyploidization using predominantly polyploid and apomictic genus Hieracium s. str. Surprisingly, the presented study demonstrates the hybrid origin of almost all investigated...

The mode of reproduction can greatly influence the demography and the evolutionary success of the taxon. In the case of autonomous asexual formation seeds are apomictic taxa fully independent of pollinators and compatible partners. For sexual taxa with strict autoincompatibility it is the opposite, i.e. sexual taxa need pollinators and compatible partners for birth of offspring. Therefore, in marginal population and for more extreme situation with lower pollinating activity will have apomictic taxa a higher level of reproductive assurance than sexual taxa vascular plants. This hypothesis was tested in natural populations of apomictic and sexual taxa. In the diploma thesis we therefore investigate the method mode of reproduction and reproductive assurance of 52 taxa of the genus Hieracium s. str. (family Asteraceae) in Europe. Of these, 12 were diploid sexually diploid taxa and 42 polyploid apomictic reproductive taxa. From these taxa we harvested seeds from fully developed capitulum and we determined the potential (total number of seeds in the capitulum) and the realized (the percentage of well-developed seeds at the capitulum). The ploidy of the offspring (the embryos and the seedling) and method origins of seeds we examined using flow cytometry. The results show that the plants of diploid species...

Les systèmes reproducteurs déterminent comment le matériel génétique est transmis d’une génération à la suivante […]. Les espèces qui combinent de la reproduction sexuée et asexuée/clonale sont très répandues [… mais] l’effet de leur système reproducteur sur leur évolution reste énigmatique et discuté.L’objectif de cette thèse est de modéliser la dynamique des fréquences génotypiques d’une population avec une combinaison de reproduction sexuée et/ou clonale dans des cycles de vie principalement diploïdes [. … Un] modèle du type chaine de Markov avec temps et états discrets sert de base mathématique pour décrire [leurs] changements […] au cours du temps.Les résultats montrent que la reproduction partiellement asexuée peut en effet modifier la dynamique de la diversité génomique par rapport à une reproduction strictement sexuée ou strictement asexuée. […] L’histoire démographique a un rôle important pour les organismes partiellement clonaux et doit être prise en compte dans toute analyse […].Cette thèse fait des recommandations pour la collecte des données et une hypothèse de base pour l’interprétation des données de génétique/génomique […]. Ces résultats ont des retombées dans plusieurs domaines, allant de la recherche fondamentale […] à des applications en agriculture […], pêche […] et protection de la nature […]. / Reproductive systems determine how genetic material is passed from one generation to the next, making them an important factor for evolution. Organisms that combine sexual and asexual/clonal reproduction are very widespread [… yet] the effects of their reproductive system on their evolution are still controversial and poorly understood.The aim of this thesis was to model the dynamics of genotype frequencies under combined sexual/clonal reproduction in dominantly diploid life cycles [. … A] state and time discrete Markov chain model served as the mathematical basis to describe [their] changes […] through time.The results demonstrate that partial clonality may indeed change the dynamics of genomic diversity compared to either exclusively sexual or exclusively clonal populations. […] Time has a crucial role in partially clonal populations and needs to be taken into account in any analysis of their genomic diversity.This thesis provides recommendations for data collection and a null hypothesis for the interpretation of population genetic/genomic data […]. Moreover, it includes new methods for the analysis of genotype-based population genetic Markov chain models. These results have a high potential relevance in several areas, ranging from basic research […] to applications in agriculture […], fisheries […] and nature conservation […].