Impact of ploidy and pathogen life cycle on resistance durability

The breeding of resistant hosts based on the gene-for-gene interaction is crucial to address epidemics of plant pathogens in agroecosystems. Resistant host deployment strategies are developed and studied worldwide to decrease the probability of resistance breakdown and increase the resistance durability in various pathosystems. A major component of deployment host strategies is the proportion of resistant hosts in the landscape. However, the impact of this proportion on resistance durability remains unclear for diploid pathogens with complex life cycles. In this study, we modelled pathogen population dynamics and genetic evolution at the virulence locus to assess the impact of the ploidy (haploid or diploid) and the pathogen's life cycle (with or without host alternation) on resistance durability. Ploidy has a strong impact on evolutionary trajectories, with much greater stochasticity and delayed times of resistance breakdown for diploids. This result emphasizes the importance of genetic drift in this system: as the virulent allele is recessive, positive selection on resistant hosts only applies to homozygous (virulent) individuals, which may lead to population collapses at low frequencies of the virulent allele. We also observed differences in the effect of host deployment depending on the pathogen's life cycle. With host alternation, the probability that the pathogen population collapses strongly increases with the proportion of resistant hosts in the landscape. Therefore, resistance breakdown events occurring at high proportions of resistant hosts frequently amount to evolutionary rescue. Last, life cycles correspond to two selection regimes: without host alternation (soft selection) the resistance breakdown is mainly driven by the migration rate. Conversely, host alternation (hard selection) resembles an all-or-nothing game, with stochastic trajectories caused by the recurrent allele redistributions on the alternate host.

Loss of SNP genetic diversity following population collapse in a recreational walleye (Sander vitreus) fishery

Walleye (Sander vitreus) are in demand as a commercially and recreationally harvested freshwater fish in Canada. Managed populations may exhibit different phenotypic and genetic signatures from their natural counterparts. In Alberta, Canada, this fishery is recovering from population collapses attributed to intensive recreational angling. We hypothesized that historical population collapses would be associated with signatures of reduced genetic diversity. To address this question, we sampled six walleye lakes in northern Alberta, including historical tissue samples for one population, and used genotyping-by-sequencing to characterize 1081 single nucleotide polymorphisms (SNPs). Lakes were identified as unique genetic clusters except for two lakes that unexpectedly exhibited signs of genetic clustering. Using historical DNA samples, 428 homologous SNPs characterized in walleye between pre- and postpopulation collapse exhibited significant reductions in multiple estimates of genetic diversity. Collectively, our results illustrate that genotype-by-sequencing methods that integrate historical and contemporary samples in association with managed populations provide insight into the consequences of harvest pressure causing collapse.

Seneca Collapse for the World’s Human Population?

Most scenarios for the world’s human population predict continued growth into the 22nd century, while some indicate that it could stabilize or begin to fall before 2100. Almost always, decline is seen as not being faster than the preceding growth. Different scenarios are obtained if we consider the human population as a complex system, subject to the general rules that govern complex systems, in particular their tendency to show rapid changes which – in the case of populations – may take the shape of true collapses (defined here as “Seneca Collapses”). The present survey examines a small number of examples of rapid population collapses in the human and in the animal domains. While not pretending to be exhaustive, the data presented here show that biological populations do show rapid “Seneca-style” collapses. So, it is possible that the same phenomenon could occur for the world’s human population.