Ecological and evolutionary dynamics in natural populations of co-existing sexual and asexual lineages
Theory predicts that asexually reproducing organisms should have a two-fold reproductive advantage over their sexual counterparts, which invest half of their reproductive potential into male offspring. The implications are that the descendants of a single asexual mutant can potentially replace a coexisting sexual population within tens of generations. On the other hand, asexual lineages are expected to be short-lived on evolutionary time scales because they lack mechanisms that rapidly generate new genetic diversity and are thus limited in their ability to adapt to changing environments. In this thesis, I focused on understanding 1) the spatial and temporal scales at which ecological and evolutionary processes favouring sexual reproduction occur and 2) the evolutionary origins and diversity of asexual lineages. As a model system I used a freshwater snail Potamopyrgus antipodarum that is characterized by frequent coexistence between diploid sexual and polyploid asexual individuals in its native New Zealand. Previous research on the P. antipodarum system has established that negative frequency-dependent selection by parasites provides a selective advantage to rare (e.g., sexually produced) genotypes and almost certainly plays an important role in the maintenance of sex and the coexistence of sexual and asexual individuals. To assess the extent to which asexual P. antipodarum actually realize the predicted two-fold reproductive advantage, I measured reproductive output of sexual families and multiple different asexual lineages in a natural lake population using experimental enclosures. I found that the reproductive output of several asexual lineages is as high as the best sexual families, which implies that sexuals do pay the full “cost of sex” relative to some asexual lineages and that the coexistence of sexual and asexual lineages in the populations where sexuals and asexuals coexist and where asexual lineage fitness is high must involve factors giving sexuals a short-term selective advantage. To assess the variation in ploidy level and genome size among asexual lineages, which were presumed to be triploid and nearly all female, an extensive survey aimed at characterizing ploidy-level variation was performed at a large geographic scale. The widespread occurrence and multiple origins of polyploid males and individuals with higher than triploid ploidy level have been found, and the results indicate that they are likely to be descended from asexual females. The survey also suggested the existence of extensive variation in genome size. These results highlight the importance of considering ploidy level variation while investigating the origins and diversity of asexual lineages. I addressed the questions of how the diversity of asexual lineages is generated by examining the evolutionary history of both mitochondrial and nuclear genetic markers across 16 lakes across the native New Zealand range of the P. antipodarum While the nuclear data suggest that asexual lineages are usually derived from coexisting sexual populations within lakes, the mitochondrial data is largely discordant with this pattern. In particular, the presence of a very common and widespread mitochondrial haplotype strongly associated with asexuality and paired with many different nuclear backgrounds indicates it had spread in parthenogenetic population due to selective advantage, and implies a more complex scenario of origin of new asexual lineages. Finally, I examined the spatial variation and temporal dynamics of the genetic structure in a mixed sexual/asexual P. antipodarum population featuring a cline in the intensity of parasitism between two time points over a four-year interval. In habitats where parasite pressure is high, the genetic structure of asexual population (identities and frequencies of clones) changed significantly, while in the habitat where parasite pressure is low, it did not. The genetic structure of the sexual population also remained stable. These results are consistent with the Red Queen hypothesis for the maintenance of sex, which predicts that parasite mediated selection should disproportionately affect asexual genotypes when they become common, and give advantage to rare genotypes, therefore maintaining sex. Overall, my results emphasize the importance of linking ecological, genetic and phylogeographic concepts and approaches when evaluating the evolution of reproductive modes in nature.
Systemische Risiken: Charakterisierung, Management und Integration in eine aktive Nachhaltigkeitspolitik
Ortwin Renn et al. behandeln in ihrem Beitrag das Management und die Integration von systemischen Risiken in eine aktive Nachhaltigkeitspolitik. Systemische Risiken sind komplexe Risiken mit schwer abschätzbaren Breiten- und Langzeitwirkungen und erheblichen politischen und sozialen Nebenfolgen (Beispiele: Atomenergie und grüne Gentechnik). Bei systemischen Risiken geht es um den Erhalt und den (notwendigen) Wandel zur Sicherung der Kontinuität menschenwürdiger Lebensbedingungen unter den Bedingungen von Ungewissheit, teils auch Ambiguität. In der diskursiven Bewältigung dieser Risiken sehen sie elementare Verbindungen zwischen der Nachhaltigkeits- und der Risikopolitik.
Reproductive health of brown trout inhabiting Swiss rivers with declining fish catch
In recent years, brown trout catches have been declining in many Swiss rivers. One hypothesis is that this declining catch is linked to environmental estrogens, known to have the potential to adversely affect fish reproduction. In order to assess if the reproductive health of brown trout is impaired, we sampled fish at three sites along four rivers with a well documented catch decline. These rivers are affected by inputs of treated sewage effluent. The sampling was conducted during two years; we measured plasma vitellogenin (Vtg) concentrations and surveyed gonadal histology. Analysis of plasma Vtg revealed elevated concentrations (up to 8 µg/mL) in only 10 out of 197 males. Furthermore, there were no site specific patterns to this induction. These results indicate that the exposure to estrogenic compounds is low. Also the incidence of ovarian atresia was low and we found no male intersex fish. In contrast to males, females caught along two rivers had spermatogenic activity in ovarian tissue. However, this intersex condition does not appear to be connected to exposure to environmental estrogens. At one of 12 sites there was a high incidence of gonadal parasites in ovarian tissue, which may affect reproductive output but was not a general problem across sites. In conclusion, the exposure to estrogenic compounds does not appear to significantly affect the reproductive parameters we investigated in Swiss brown trout.