Polyploidization is an important evolutionary force. It drives sympatric speciation through reproductive isolation of different cytotypes, and often leads to loss of sexual reproduction in polyploid lineages. Polyploidization and asexuality can change how other species engage in ecological interactions with the polyploid lineage and may change coevolutionary dynamics. Here, we quantified the phenotypic divergence in the freshwater oligochaete worm Lumbriculus variegatus, the California blackworm, among its co-occurring sexual diploid (Lineage II) and asexual polyploid (Lineage I) lineages. We further investigated variation in parasite communities and infection prevalence among sympatric and allopatric diploid/polyploid populations. 10 out of 18 populations showed co-existence of both lineages, with 7 populations harbouring only the polyploid lineage. Both worm lineages hosted endoparasitic nematodes, an ectoparasitic rotifer, and one potentially symbiotic gut ciliate. The parasite community similarity and overlapping size range of diploid and polyploid worms points to the ecological similarity of the worm lineages, despite the substantial ploidy and reproductive strategy differentiation. Although parasite prevalence varied independently of worm lineage, the prevalence was associated with the frequency of local cytotypes. Specifically, the rotifer prevalence was highest on the rare local cytotype, and nematode prevalence was highest on the common local cytotype. These results suggest the presence of both positive and negative frequency-dependent parasitism, which may contribute to the co-existence in the L. variegatus species complex.

Genetic drift, gene flow, and natural selection commonly influence population genetic diversity. In populations of self-compatible hermaphrodites, the mating system (e.g., self-fertilization) further reduces individual heterozygosity. Furthermore, selfing, as a form of inbreeding, significantly impacts genetic drift by reducing effective population size (N_e). This can potentially accelerate genetic drift, particularly in small populations where self-fertilization is likely during founder events. To investigate the roles of genetic drift and contemporary mating system in populations of the freshwater snail Lymnaea stagnalis, we examined their effective population sizes (N_e) and Tajima's D values, which reflect genetic drift over extended time periods, as well as estimates of within-population selfing rates and pairwise relatedness reflecting contemporary mating system. We used 4054 SNP markers obtained using restriction site associated DNA (RAD) sequencing from individuals in five snail populations originating from geographically closely located ponds. We found strong population genetic structure and differences in genetic diversity among populations. Covariation between genetic diversity and N_e estimates and Tajima's D values suggested drift being an important determinant of genetic diversity and structure in these populations. However, this effect was independent of the contemporary mating system, as indicated by the similarity of selfing rates and relatedness estimates among populations. Thus, founder events (possibly including historical inbreeding) and/or drift due to small sizes of L. stagnalis populations are likely to explain their genetic structure and limit within-population genetic diversity.

Saccharomyces cerevisiae is occasionally infected by dsRNA totiviruses and their toxin-encoding dsRNA satellite nucleic acids. The autonomous totivirus and its satellite can coexist but with an asymmetric dependence of the satellite on the totivirus for replication and maintenance inside the host cell. Satellites provide their yeast hosts with inhibitory toxins and the necessary self-immunity; loss of the satellite equates to loss of toxin immunity. Because these viral elements lack known extracellular stages, and sex is suspected to be rare, they are assumed to be transmitted vertically, implying that infection states should correlate with host genotypes. However, totivirus-satellite coinfections are rarely examined in natural populations, leaving their associations with host genotypes poorly understood. We screened a multiyear, vineyard-associated population of S. cerevisiae isolates from New Zealand to examine the stability of host-virus associations over time, both within and across genotypes. Over half of the wild isolates harbored infections (55%), but less than half of these (37% of infected) had toxin-encoding satellites. Genotypes that persisted across years typically maintained consistent infection states. However, we also observed stepwise transitions from coinfection through infection to an infection-free state, as well as acquisition of totiviruses and satellites. Genotypes clustered strongly by infection state, and population heterozygosity was significantly lower than expected, supporting vertical transmission while suggesting that outcrossing is not responsible for the acquisition of higher infection states. Despite occasional intragenotypic transitions, genotype clustering by infection state remained intact, suggesting that such transitions are transient and that host genotypes may have optimal infection states with regard to totiviruses and their satellites.

The stone crayfish Austropotamobius torrentium is an endangered species native to the northeastern parts of Switzerland. Since the 19th century its populations have been declining due to habitat deterioration, fragmentation and the devastating crayfish plague that has been introduced by invasive crayfish species. The goal of this study was to analyze the genetic diversity and structure of the remaining stone crayfish populations in the Canton of Thurgau and gain information about conservation measures needed to ensure the long term survival of these populations. For this purpose microsatellite data of twelve stone crayfish populations were analyzed and genetic diversity, effective population size and differentiation among the populations were estimated. Overall low genetic diversity and effective population sizes were observed in most populations whereas differentiation was strong between all population pairs. The genetic structure of the populations was virtually unrelated to drainage structure and I did not observe signficant isolation-by-distance despite the strong overall genetic differentiation. This is characteristic of small and isolated populations in which genetic drift is much more important than gene flow. Even though effective population sizes could only be estimated with high uncertainty, the generally low values suggest rather small effective population sizes for most of the populations. Some estimates are so low that it has to be feared that evolutionary potential has already been lost to a certain amount. Many populations may be too small to even ensure short term survival over five generations. This result is alarming and it is important that conservation measures are initiated as soon as possible. Applicable conservation measures for crafish populations are described in detail in the Swiss Action Plan for crafish. This plan includes proposed genepool sites for translocations and reintroductions. However, my results suggest that these populations (Ruti, Zezikon, Baltschhuuse) are not necessarily better suited than other populations in canton Thurgau, because some showed either low genetic diversity or population density. To increase population size as well as genetic diversity of the populations I recommend to improve the habitat quality by habitat restoration and to reintroduce animals from breeding programs.