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Macro and micro-evolutionary processes within a complex of species, case study of the tropical invasive earthworm : pontoscolex corethrurus

Abstract : Pontoscolex corethrurus is the most widespread earthworm species in the tropical and sub-tropical zones, it is hence one of the most studied earthworm in soil science. Ecological aspects of P. corethrurus, which is known to be present in a wide range of habitats from poor soils of pasture to rich soils of primary forest, were intensively investigated but biological aspects are less addressed. In particular, information on the genetic variation within the morphospecies is scarce except for the finding of two genetically differentiated lineages in São Miguel Island of Azores archipelago in 2014. Moreover, the ploidy degree of the morphospecies is not yet known and its reproduction strategy is not well understood. One of the objectives of this thesis was to understand the mechanisms and characteristics which make P. corethrurus a successful invader. Our second objective was to look for cryptic lineages in the whole world and to describe the phylogenetic relationships between them. A third objective was to identify which lineage was invasive and to characterize its population genetic structure in the native and the introduced ranges. The last objective was to test if the different species of the complex have different ploidy degrees (polyploid complex). This could eventually explain the reproductive isolation among these species. A bibliographic synthesis of 265 studies covering all subjects of knowledge on P. corethrurus showed that the r strategy and plasticity of this earthworm are the key characteristics which make it a successful invader in different habitats. In order to investigate the cryptic diversity within P. corethrurus in a world-wide scale, I examined 792 specimens collected from 25 different countries and islands. These specimens were analyzed using two mitochondrial (COI and 16S rDNA) and two nuclear (internal transcribed spacers 2 and 28S rDNA) markers and a large-scale multilocus sequence data matrix obtained using the Anchored Hybrid Enrichment (AHE) method. In addition, a total of 11 morphological characters, both internal and external, were investigated in all genetically characterized lineages. Four cryptic species (L1, L2, L3 and L4) were found within the P. corethrurus species complex, and four potentially new species within the genus Pontoscolex. The cryptic species were observed in sympatry at several localities, and analyses based on AFLP markers showed no hybridization among L1 and L3. The possibility of reproductive isolation among species of the complex because of different ploidy degrees was investigated by cytogenetic experimentations. Due to different obstacles encountered at different steps of the experimentations, results were just obtained for L4 (2n=70). One of the species belonging to the complex, L1, was particularly widespread per comparison with the others. This species corresponded to topotype specimens (samples from Fritz Müller’s garden where P. corethrurus was first described in 1856). Thus, we focused on this invasive species in a population genetics and phylogeography study. Using COI gene and AFLP markers, we revealed low genetic diversity through the tropical zone, probably due to recent colonization events and asexual reproduction type. Meanwhile, due to weak linkage disequilibrium and relatively high genetic diversity in some populations, sexual reproduction was suggested for L1.To date, this is the first study investigating at a world-wide scale, cryptic species diversity, population genetics and phylogeography of a peregrine earthworm species throughout tropical zone. I produced the first comprehensive review of all ecological and biological aspects of P. corethrurus. Moreover, the taxonomic status of P. corethrurus was clarified as well as its reproduction strategy which is mixed (parthenogenetic and sexual). All these findings represent potentially useful information for future experimentations and researches on species of P. corethrurus complex
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Shabnam Taheri. Macro and micro-evolutionary processes within a complex of species, case study of the tropical invasive earthworm : pontoscolex corethrurus. Environmental Engineering. Université Paris-Est, 2018. English. ⟨NNT : 2018PESC1024⟩. ⟨tel-01972898v2⟩

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