A hyperparasite affects the population dynamics of a wild plant pathogen.

Assessing the impact of natural enemies of plant and animal pathogens on their host's population dynamics is needed to determine the role of hyperparasites in affecting disease dynamics, and their potential for use in efficient control strategies of pathogens. Here, we focus on the long-term study describing metapopulation dynamics of an obligate pathogen, the powdery mildew (Podosphaera plantaginis) naturally infecting its wild host plant (Plantago lanceolata) in the fragmented landscape of the Åland archipelago (southwest Finland). Regionally, the pathogen persists through a balance of extinctions and colonizations, yet factors affecting extinction rates remain poorly understood. Mycoparasites of the genus Ampelomyces appear as good candidates for testing the role of a hyperparasite, i.e. a parasite of other parasites, in the regulation of their fungal hosts' population dynamics. For this purpose, we first designed a quantitative PCR assay for detection of Ampelomyces spp. in field-collected samples. This newly developed molecular test was then applied to a large-scale sampling within the Åland archipelago, revealing that Ampelomyces is a widespread hyperparasite in this system, with high variability in prevalence among populations. We found that the hyperparasite was more common on leaves where multiple powdery mildew strains coexist, a pattern that may be attributed to differential exposure. Moreover, the prevalence of Ampelomyces at the plant level negatively affected the overwinter survival of its fungal host. We conclude that this hyperparasite may likely impact on its host population dynamics and argue for increased focus on the role of hyperparasites in disease dynamics.

Genetic evidence for male-biased dispersal in the three-spined stickleback (Gasterosteus aculeatus).

Sex-biased dispersal is capable of generating population structure in nonisolated populations and may affect adaptation processes when selective conditions differ among populations. Intrasexual competition for local resources and/or mating opportunities predicts a male-biased dispersal in polygynous species and a female bias in monogamous species. The patterns of sex-biased dispersal in birds and mammals are well explained by their respective mating systems, but the picture emerging from fish studies is still mixed. Using neutral genetic markers, we investigated whether there is any evidence for sex-biased dispersal among Baltic Sea populations of the three-spined stickleback (Gasterosteus aculeatus). The null hypothesis of non sex-biased dispersal was rejected in favour of male-biased dispersal in this species. As the three-spined stickleback has a polygynous mating system, the observed male bias in dispersal is consistent with the hypothesis that local mate competition might drive the observed pattern. Although more research both on the proximate and ultimate causes behind the observed pattern is needed, our results serve as a first step towards understanding patterns of sex-biased dispersal in this species.

Identifying footprints of directional and balancing selection in marine and freshwater three-spined stickleback (Gasterosteus aculeatus) populations.

Natural selection is expected to leave an imprint on the neutral polymorphisms at the adjacent genomic regions of a selected gene. While directional selection tends to reduce within-population genetic diversity and increase among-population differentiation, the reverse is expected under balancing selection. To identify targets of natural selection in the three-spined stickleback (Gasterosteus aculeatus) genome, 103 microsatellite and two indel markers including expressed sequence tags (EST) and quantitative trait loci (QTL)-associated loci, were genotyped in four freshwater and three marine populations. The results indicated that a high proportion of loci (14.7%) might be affected by balancing selection and a lower proportion (2.8%) by directional selection. The strongest signatures of directional selection were detected in a microsatellite locus and two indel markers located in the intronic regions of the Eda-gene coding for the number of lateral plates. Yet, other microsatellite loci previously found to be informative in QTL-mapping studies revealed no signatures of selection. Two novel microsatellite loci (Stn12 and Stn90) located in chromosomes I and VIII, respectively, showed signals of directional selection and might be linked to genomic regions containing gene(s) important for adaptive divergence. Although the coverage of the total genomic content was relatively low, the predominance of balancing selection signals is in agreement with the contention that balancing, rather than directional selection is the predominant mode of selection in the wild.

Mitochondrial DNA phylogeography of the three-spined stickleback (Gasterosteus aculeatus) in Europe-evidence for multiple glacial refugia.

An analysis of mitochondrial DNA sequence variation in 172 three-spined sticklebacks (Gasterosteus aculeatus) sampled across the European distribution range revealed three major evolutionary lineages occupying relatively large and separate geographic areas. The trans-Atlantic lineage comprised of populations spanning from the East Coast of USA to the continental Europe and was basal group to the other European lineages in the phylogeny. The European lineage included populations located in the Western and Eastern Europe, British Isles, Scandinavia as well as some parts of the Mediterranean region. The third lineage was specific to the Black Sea drainages. The within lineage structure was characterized by significant excess of low frequency haplotypes and star-like mtDNA genealogies, which suggest a recent population expansions to the formerly glaciated marine and freshwater environments. A coalescent-based method dated the splits between the major lineages to have occurred during the Saalian and Weichselian glaciations in the late Pleistocene, depending on the molecular clock calibration. The coalescent simulations further indicate high degree of genetic diversity within the lineages and a substantial increase in the genetic diversity in the European lineage relative to the ancestral level. In addition to the three major lineages, the freshwater populations in R. Neretva and L. Skadar in the Adriatic Sea coast region harboured unique and highly divergent haplotypes suggesting long independent histories of these populations. Evidence from mtDNA analyses suggests that these populations deserve a status of an evolutionary significant unit.

Genetic relationships among marine and freshwater populations of the European three-spined stickleback (Gasterosteus aculeatus) revealed by microsatellites.

To assess the population genetic structure of the three-spined stickleback, Gasterosteus aculeatus, variability at 18 microsatellite loci was examined in 1724 individuals from 74 locations covering most of the species distribution range in Europe. The results revealed high overall degree of differentiation (F(ST) = 0.21) but contrasting level of divergence and genetic variability between habitat types. Marine populations were genetically relatively uniform even across great geographical distances as compared to substantial differentiation among freshwater populations. Analysis of molecular variance indicated low but significant (2.7%) variation in allele frequencies between geographical regions, but a negligible effect of habitat type (0.2%). The phylogenetic pattern was not explained by habitat type, but a weak signal of populations clustering according to geographical or water system origin was found. The results support the view that three-spined stickleback marine ancestors colonized northern European fresh waters during the postglacial marine submergence c. 10,000 years ago, whereas in the Mediterranean region colonization probably dates back to the Pleistocene. The independent origins of river and lake populations indicate that they originate from multiple colonizations rather than sharing common ancestry. In the continuous marine environment, the low degree of differentiation among populations can be explained by gene flow among subpopulations and large effective population size buffering divergence in neutral markers. In contrast, among postglacially established freshwater populations differentiation appears to be driven by genetic drift and isolation. The stepwise mutations appear to have contributed to the population differentiation in the southern part of the three-spined stickleback distribution range.

Microsatellite variation in ringed seals (Phoca hispida): genetic structure and history of the Baltic Sea population.

Genetic variability and population structure of Baltic ringed seals and an Arctic reference population were assessed using eight microsatellite loci. Ringed seals colonized the Baltic Sea basin soon after deglaciation 11 500 years ago and are supposed to have remained largely isolated from the main Arctic stock since then, approximately 1000 generations. In the 1900s the Baltic population declined rapidly, and is now confined to three distinct breeding areas, with N < 6000 seals altogether. Microsatellite heterozygosity in ringed seals was higher than that in the closely related, boreal harbour seal and grey seal, for which the markers were initially developed. This is plausibly attributed to an overall greater population (species) size of ringed seals during the Quaternary. Allele frequency differentiation between the Baltic and Arctic ringed seals, conventionally treated as different subspecies, was weak. Assuming complete isolation, the divergence (FST=0.023) would imply a notably high postglacial effective population size, approximately 20 000 for the Baltic population. The isolation assumption however, seems unrealistic in the light of the data: a coalescent-based simulation approach to the likelihood of alternative demographic histories clearly favoured a scenario with recurrent gene flow to the Baltic, over one of complete isolation (drift only). Within the Baltic Sea, no differentiation was found between the Gulf of Finland and the Gulf of Bothnia breeding areas; the recent population decline and split have not yet affected the inbreeding levels of the disjunct breeding stocks.