A Phytoplankton Invasion: Population Genetics, Phylogeography, and Invasion Success of Gonyostomum semen

Abstract: nvasive species is a major threat to ecosystems and biodiversity. Invasions by macroorganisms have been intensively studied, but little is known concerning microbial invasions. In aquatic environments, phytoplankton, i.e. autotrophic microbes, have a key role as primary producers. In my thesis, I focused on the population genetic structure and phylogeography of the freshwater raphidophyte Gonyostomum semen, which is nowadays considered invasive in Northern Europe, as it has increased in abundance and occurence during the last decades. Phytoplankton blooms were for a long time believed to be monoclonal, however recent studies have shown that phytoplankton blooms are highly diverse. G. semen can form blooms in lakes for extended periods of time. In the first study, I investigated the population genetic structure and genetic diversity of G. semen in one lake over time, covering the different stages of the bloom using Amplified Fragment Lenght Polymorphism (AFLP). The genetic diversity of G. semen increased over time, especially at the beginning of the bloom. This increase was likely due to the recruitment of cysts from the sediment bringing new genotypes to the population. The blooms from two consecutive years formed a single genetic population, although a significant differentiation was detected. The results of this study highlight the importance of life cycle characteristics for the intraspecific genetic diversity of partially asexual organisms. In the second study, I investigated the population genetic structure of G. semen in 11 lakes in Northern Europe using AFLP. I found that all the strains isolated from 11 lakes belong to a single genetic population, although significant differentiation was dectected between the sampling locations. The differentiation between locations was likely caused by founder effect following the colonization of the lakes. In addition, low genetic diversity was measured at each location, which could reflect the recent establishment of G. semen in the lakes. These results suggest that the colonization of G. semen in these lakes is relatively recent, and that G. semen has colonized new lakes. In addition, I performed a phylogeographic analysis with strains from 15 Northern European lakes, and two Japanese strains, by sequencing four DNA regions. All the strains from Northern Europe have identical sequences for the four markers. For one marker, the mitochondrial gene cox1, clear differences were detected between the sequences of the Japanese strains and the Northern European ones. In this study the low diversity observed in Northern Europe also support the hypothesis of recent expansion of G. semen. The results of these two studies are coherent with monitoring data suggesting recent invasion of new lakes by G. semen in Northern Europe. Several factors can potentially facilitate the invasion by a species. The absence of a grazer for instance might facilitate the formation of a dense population of an invader. In G. semen, due to its large cell size and mucilage production, the grazing pressure was expected to be low. I performed grazing experiments to determine which potential grazers could feed on G. semen. I found that only the large cladoceran Daphnia magna was able to feed on G. semen. The large cell size of G. semen was likely limiting the grazing by smaller cladocerans, furtermore the mucilage production and expulsion of trichocsyts were likely limiting the feeding of copepods. Field data revealed that the large cladoceran D. magna do not co-exist in G. semen lakes. Thus the grazing pressure on G. semen is likely restricted, allowing the formation of dense populations, and potentially the establishment after colonization of new lakes.