| Titre : | Disentangling ongoing hybridization and cryptic species of the submerged macrophyte genus Ruppia, using ecological and molecular insights |
| Auteurs : | Lise Beirinckx, Auteur |
| Type de document : | Thèse |
| Editeur : | Brussels, Belgium : Vrije Universiteit, 2022 |
| Format : | 313 |
| Langues: | Anglais |
| Mots-clés : | Mer Méditerranée ; Mer Baltique ; Ruppiaceae ; Ruppia maritima ; Hybridation ; Génétique |
| Résumé : | Aquatic plants are an ecologically and taxonomically diverse group that plays key role in continental and coastal aquatic habitats worldwide. However, despite their ecological importance in aquatic ecosystems, they are relatively poorly studied. Aquatic plant genera are generally assumed to have only few species, but they occupy broad geographical and ecological ranges. Only recently, evidence with different types of molecular markers suggested that macrophytes are taxonomically more complex than previously considered, due to either cryptic species or hybridization and polyploidization. The salt-tolerant macrophyte genus Ruppia is one of these aquatic genera with a simple and slender morphology and difficult species identification. It occurs in brackish to saline waterbodies all over the world that are characterized by variations in water level and salinity. Taxonomy within this genus has always been difficult because it combines high morphological variability with the lack of unambiguous identification criteria. Two European species are generally recognized, R. spiralis and R. maritima, although they are sometimes difficult to distinguish morphologically. As a result of wrong species identification, our ecological understanding of these species remained limited. In this thesis we used a combination of molecular markers and germination experiments to unravel locally co-existing Ruppia lineages. First, microsatellite markers combined with chloroplast and nuclear sequence data were used to detect and delineate different Ruppia lineages that co-exist within a Mediterranean and a North-sea wetland area. Second, seed germination experiments were carried out to understand how these lineages can locally co-exist. We found that R. maritima and R. spiralis are two well-diverged species that can be easily distinguished genetically and ecologically. R. spiralis is a perennial, outcrossing species. Seeds either germinate under freshwater conditions at summer temperatures, or saltwater conditions under early spring temperatures. This broad germination niche coincides with the wide environmental distribution in different types of (semi-)permanent waterbodies. R. maritima on the other hand is a diploid, selfing species. Seeds only germinated under low salinities, which partly explains its very restricted habitat of temporary ditches in the studied areas. However, genetic markers also revealed that Ruppia taxonomy is complicated by several hybrid lineages that could not be distinguished morphologically. A Mediterranean hybrid lineage of an ancient origin may present an emerging species and occupies a distinct pond type. This lineage likely results from a cross between a currently unknown taxon and R. maritima, probably followed by frequent backcrossings with R. maritima as the pollen donor. This led to chloroplast capture, a phenomenon where the nuclear and chloroplast DNA have different origins. In case of these ancient Mediterranean hybrids, the chloroplast DNA -which is inherited maternally- probably descends from the unknown taxon and is distinct form any other known European Ruppia species, whereas the nuclear DNA is resembles R. maritima. In addition, more recent lineages of a hybrid origin were detected, involving R. spiralis and this ancient Mediterranean hybrid lineage, where the latter acted as the pollen donor. In wetlands along the North-sea however, recent R. spiralis x R. maritima hybrids were encountered. The detection of several morphologically similar but genetically and ecologically diverged Ruppia lineages, suggests that we must reconsider Ruppia species diversity and distributions. Our findings support the hypothesis that macrophyte genera might be more diverse and complex than previously thought. Considering the high threats aquatic ecosystems currently face, this complexity should be more thoroughly investigated, both taxonomically and ecologically |
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