- Dr Tanja Scwander, Université de Lausanne, Department of Ecology and Evolution, Rapportrice
- Dr John Pannell, Université de Lausanne, Department of Ecology and Evolution, Rapporteur
- Dr Julie Jaquiéry, UMR IGEEP, Inra Rennes, Examinatrice
- Pr Christophe Destombe, UMI Station biologique de Roscoff, Président du Jury
- Dr Susana Coelho, UMR8227 Station biologique de Roscoff, supervisor
-Dr Mark Cock, UMR8227 Station biologique de Roscoff, supervisor.
Although sexual reproduction predominates in eukaryotes, several hundred lineages have undergone the transition from sexuality to asexuality. Transitions between sexual and asexual reproduction are believed to have important evolutionary and ecological consequences, yet the molecular, genetic, and cytological foundations of such transitions remain elusive. One type of asexual reproduction is parthenogenesis, i.e., the development of an adult organism directly from gametes in the absence of fertilisation. Although many eukaryotes are capable of reproducing by parthenogenesis, we know very little about its genetic basis, and the evolutionary causes and consequences of transitions to asexuality are poorly understood. The brown algae are a group of multicellular eukaryotes, that show an extraordinary diversity of types of life cycle, sexual systems, modes of reproduction, and they provide excellent models to look at the origins, evolution and mechanisms underlying parthenogenesis. In this thesis, we have used a wide array of genomic and cell biology tools available for the model brown alga Ectocarpus to identify and characterize loci involved in parthenogenesis, shedding light on the causes and consequences of parthenogenesis at the organism level. Our results highlight the key role of the sex chromosome as a major regulator of asexual reproduction, together with two autosomal loci. Importantly, we identify several negative effects of parthenogenesis on male fitness, but also different fitness effects between parthenogenesis and life cycle generations, supporting the idea that parthenogenesis may be under both sexual selection and generation/ploidally-antagonistic selection (Chapter 2). Zygotic growth was significantly affected by the parthenogenetic capacity of the male parent and the putative role of mitochondrial inheritance patterns on the fitness of sporophytes was also investigated (Chapter 2 and 3). This work revealed an unusual transmission pattern of mitochondria specifically in Ectocarpus species 7 (Chapter 3). Finally, the QTL analysis (Chapter 2) required the construction of a genetic map for Ectocarpus siliculosus and a comparison with Ectocarpus species 7 genetic map (reference genome sequenced in 2010) showed that the synteny was highly conserved between the two species (Chapter 4). By investigating parthenogenesis in a multicellular organism that has independently evolved from plants and animals, the work presented in this thesis has helped to assess the diversity of evolutionary mechanisms that lead to parthenogenesis.
Keywords: Ectocarpus, parthenogenesis, mitochondria inheritance, quantitative genetics, development