Understanding the ecology and evolution of host-parasite interactions across scales

Predicting the emergence, spread and evolution of parasites within and among host populations requires insight to both the spatial and temporal scales of adaptation, including an understanding of within-host up through community-level dynamics. Although there are very few pathosystems for which such extensive data exist, there has been a recent push to integrate studies performed over multiple scales or to simultaneously test for dynamics occurring across scales. Drawing on examples from the literature, with primary emphasis on three diverse host-parasite case studies, we first examine current understanding of the spatial structure of host and parasite populations, including patterns of local adaptation and spatial variation in host resistance and parasite infectivity. We then explore the ways to measure temporal variation and dynamics in host-parasite interactions and discuss the need to examine change over both ecological and evolutionary timescales. Finally, we highlight new approaches and syntheses that allow for simultaneous analysis of dynamics across scales. We argue that there is great value in examining interplay among scales in studies of host-parasite interactions.Peer reviewe

Data from: Discordance between nuclear and mitochondrial genomes in sexual and asexual lineages of the freshwater snail Potamopyrgus antipodarum

The presence and extent of mitonuclear discordance in coexisting sexual and asexual lineages provides insight into 1) how and when asexual lineages emerged, and 2) the spatial and temporal scales at which the ecological and evolutionary processes influencing the evolution of sexual and asexual reproduction occur. Here, we used nuclear single-nucleotide polymorphism (SNP) markers and a mitochondrial gene to characterize phylogeographic structure and the extent of mitonuclear discordance in Potamopyrgus antipodarum. This New Zealand freshwater snail is often used to study the evolution and maintenance of sex because obligately sexual and obligately asexual individuals often coexist. While our data indicate that sexual and asexual P. antipodarum sampled from the same lake population are often genetically similar, suggesting recent origin of these asexuals from sympatric sexual P. antipodarum, we also found significantly more population structure in sexuals vs. asexuals. This latter result suggests that some asexual lineages originated in other lakes and/or in the relatively distant past. When comparing mitochondrial and nuclear population genetic structure, we discovered that one mitochondrial haplotype (‘1A’) was rare in sexuals, but common and widespread in asexuals. Haplotype 1A frequency and nuclear genetic diversity were not associated, suggesting that the commonness of this haplotype cannot be attributed entirely to genetic drift and pointing instead to a role for selection

Interactions Between Stressful Environment and Gene Deletions Alleviate the Expected Average Loss of Fitness in Yeast

The conjecture that the deleterious effects of mutations are amplified by stress or interaction with one another remains unsatisfactorily tested. It is now possible to reapproach this problem systematically by using genomic collections of mutants and applying stress-inducing conditions with a well-recognized impact on metabolism. We measured the maximum growth rate of single- and double-gene deletion strains of yeast in several stress-inducing treatments, including poor nutrients, elevated temperature, high salinity, and the addition of caffeine. The negative impact of deletions on the maximum growth rate was relatively smaller in stressful than in favorable conditions. In both benign and harsh environments, double-deletion strains grew on average slightly faster than expected from a multiplicative model of interaction between single growth effects, indicating positive epistasis for the rate of growth. This translates to even higher positive epistasis for fitness defined as the number of progeny. We conclude that the negative impact of metabolic disturbances, regardless of whether they are of environmental or genetic origin, is absolutely and relatively highest when growth is fastest. The effect of further damages tends to be weaker. This results in an average alleviating effect of interactions between stressful environment and gene deletions and among gene deletions

Mitochondrial (cytochrome b) haplotypes in Potamopyrgus antipodarum (with outgroup)

Sequences of unique haplotypes of mitochondrial cytochrome-b fragment in Potamopyrgus antipodarum. Homologous sequence for related species (P. estuarinus) is included an an outgroup

O R I G I NA L A RT I C L E WIDE VARIATION IN PLOIDY LEVEL AND GENOME SIZE IN A NEW ZEALAND FRESHWATER SNAIL WITH COEXISTING SEXUAL AND ASEXUAL LINEAGES

Asexual animals and plants are usually polyploid, and polyploid animals (and triploid plants) are often asexua

field_experiment

Fecundity and offspring size of Potamopyrgus antipodarum snails in the field experiment. Descriptions of column headings and abbreviations used: A: FAMILY_NUMBER - number of experimental enclosure (cage). B: GENOTYPE_ID - Asexual families were assigned unique clone numbers, "S" - sexual families, "NA" - unresolved genotypes. Allozyme genotypes were obtained from one or two (if available) snails per family. C: PLOIDY - 2- diploid, 3- triploid. D: LINEAGE_TYPE - 1- unresolved, 2- common clone, 3- rare clone, 4 -sexual. We assigned clone as common if the clone was replicated by at least three independent families. E: LENGTH_mm - snail length [mm]

Data from: Wide variation in ploidy level and genome size in a New Zealand freshwater snail with coexisting sexual and asexual lineages

Natural animal populations are rarely screened for ploidy-level variation at a scale that allows detection of potentially important aberrations of common ploidy patterns. This type of screening can be especially important for the many mixed sexual/asexual systems where sexuals are presumed to be dioecious diploids and asexuals are assumed to be triploid and all-female. For example, elevation of ploidy level above triploidy can be a source of genetic variation and raises the possibility of gene flow among ploidy levels and to asexual lineages. We used flow cytometry and mtDNA sequencing to characterize ploidy level and genome size in Potamopyrgus antipodarum, a New Zealand freshwater snail where obligate sexual (presumed diploid and dioecious) and obligate apomictic asexual (presumed triploid and nearly all female) individuals frequently coexist. We documented the widespread occurrence and multiple origins of polyploid males and individuals with >3x ploidy, and find that both are likely to be descended from asexual females. Our survey also revealed extensive variation in genome size. The discovery of widespread variation in ploidy level and genome size in such a well-studied system highlights the importance of broad, extensive, and ecologically representative sampling in uncovering ploidy level and genome size variation in natural populations

Ploidy level, gender and nuclear (SNP) genotypes for Potamopyrgus antipodarum snails from 16 New Zealand lake populations

Ploidy level, gender and nuclear (SNP) genotypes for Potamopyrgus antipodarum snails from 16 New Zealand lake population

Data from: Parasite resistance predicts fitness better than fecundity in a natural population of the freshwater snail Potamopyrgus antipodarum

The cost of males should give asexual females an advantage when in competition with sexual females. In addition, high-fecundity asexual genotypes should have an advantage over low-fecundity clones, leading to reduction in clonal diversity over time. To evaluate fitness components in a natural population, we measured the annual reproductive rate of individual sexual and asexual female Potamopyrgus antipodarum, a New Zealand freshwater snail, in field enclosures that excluded competitors and predators. We used allozyme genotyping to assign the asexual females to particular clonal genotypes. We found that the most fecund asexual clones had similar or higher fecundity as the top 10 % of sexual families, suggesting that fecundity selection, even without the cost of males, would lead to replacement of the sexual population by clones. Consequently, we expected that the clones with the highest fecundity would dominate the natural population. Counter to this prediction, we found that high annual reproductive rates did not correlate with the frequency of clones in the natural population. When we exposed the same clones to parasites in the laboratory, we found that resistance to infection was positively correlated with the frequency of clones in the population. The correlation between fecundity and parasite resistance was negative, suggesting a trade-off between these two traits. Our results thus suggest that parasite resistance is an important short-term predictor of the success of asexual P. antipodarum in this population