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Theses

De la pollinisation à la formation des graines : le cas du châtaignier

Abstract : Sexual reproduction of plants is a key stage in their life cycle, on which the functioning of ecosystems and our food supply depend. For seed plants, sexual reproduction requires successful pollination, i.e. the transport of pollen from the stamens of a flower to the stigmata. This can fail if the quantity of compatible pollen produced is insufficient (pollenizer limitation) or if pollen is not transported successfully (pollinator limitation), both mechanisms resulting in an insufficient quantity or quality of pollen received by the stigmata (pollen limitation). Once the pollen has been delivered, post-pollination mechanisms can result in ovule or seed abortion before maturity. The European chestnut (Castanea sativa, Fagaceae), an important forest and fruit tree, is used to explore and model pollination, from pollen production to fruit set. Using observations (phenology, floral architecture, insects), experiments (insect exclusion with insect-proof nets and emasculation), molecular analyses (characterization of genetic diversity using SNP markers and nuclear and chloroplast microsatellites, and paternity analyses), and a spatially explicit Bayesian model of pollination and fruit set, a detailed account of chestnut pollination is provided. The objective is to identify the key reproductive mechanisms that allow fruit formation in orchards and forests. By preventing insects from accessing to female flowers with insect-proof nets, fruit production collapses, thus proving that chestnut trees are insect-, not wind-pollinated, as was assumed before. Beetles and flies, not bees, are the main pollinators. Some European chestnuts are naturally male-steriles, i.e. they no longer produce pollen, indicating that this species is gynodioecious. Male sterility is shown to be of cytoplasmic origin, with male-sterile trees producing more fruits than male-fertile ones. An emasculation experiment demonstrates the strong negative impact of self-pollination on fruit set, suggesting that one of the main mechanisms behind this sexual polymorphism is self-interference between maternal and paternal functions resulting in ovule discounting in bisexual trees. This mechanism was confirmed by modelling: paternity and pollination success are well explained by a late-acting self-incompatible mechanism causing abortion of self-pollinated seeds. The model also revealed variable and asymmetrical barriers between chestnut species, matching well with observed paternity and seed-set in production orchards. Finally, difficulties encountered in transferring these results to the general public and to farmers are briefly discussed and possible solutions are outlined.
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Submitted on : Thursday, January 20, 2022 - 1:32:10 PM
Last modification on : Friday, January 21, 2022 - 4:11:39 AM

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LARUE_CLEMENT_2021.pdf
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Clément Larue. De la pollinisation à la formation des graines : le cas du châtaignier. Biologie végétale. Université de Bordeaux, 2021. Français. ⟨NNT : 2021BORD0349⟩. ⟨tel-03533842v2⟩

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