Temporal sampling helps unravel the genetic structure of naturally occurring populations of a phytoparasitic nematode. 2. Separating the relative effects of gene flow and genetic drift

Abstract

Studying wild pathogen populations in natural ecosystems offers the opportunity to better understand the evolutionary dynamics of biotic diseases in crops and to enhance pest control strategies. We used simulations and genetic markers to investigate the spatial and temporal population genetic structure of wild populations of the beet cyst nematode Heterodera schachtii on a wild host plant species, the sea beet (Beta vulgaris spp. maritima), the wild ancestor of cultivated beets. Our analysis of the variation of eight microsatellite loci across four study sites showed that (i) wild H. schachtii populations displayed fine‐scaled genetic structure with no evidence of substantial levels of gene flow beyond the scale of the host plant, and comparisons with simulations indicated that (ii) genetic drift substantially affected the residual signals of isolation‐by‐distance processes, leading to departures from migration–drift equilibrium. In contrast to what can be suspected for (crop) field populations, this showed that wild cyst nematodes have very low dispersal capabilities and are strongly disconnected from each other. Our results provide some key elements for designing pest control strategies, such as decreasing passive dispersal events to limit the spread of virulence among field nematode populations.

Impact of ploidy and pathogen life cycle on resistance durability

“…Therefore, the avr allele is poorly selected, and variations in its frequency are mostly driven by genetic drift, which induces high stochasticity among replicates. This effect should be strengthened in species with small effective population sizes, such as in cyst nematodes (Gracianne et al, 2016;Montarry et al, 2019). As a consequence, we observed lower frequencies of avr fixation and higher extinction rates for diploid individuals, independently of the life cycle and the host deployment strategy.…”

Genetic drift during the spread phase of a biological invasion

“…Gene flow, when limited in distance relative to the sampled area, results in isolation‐by‐distance patterns in both equilibrium and nonequilibrium situations (Slatkin, ). Therefore, IBD on its own does not report whether gene flow is currently occurring, or occurred, for instance during a colonization process, and stopped afterwards (Gracianne et al, ). We estimated current dispersal rates ( m e ) between ponds and marshes, using the maximum likelihood method implemented in mlne 1.0 (Wang & Whitlock, ).…”

Impact of ploidy and pathogen life cycle on resistance durability

“…At this state, the avr allele cannot be selected, and variations in its frequency are only driven by genetic drift, which induces high stochasticity among replicates. This effect should be strengthened in species with small effective population sizes, such as in cyst nematodes (Gracianne et al 2016; Montarry et al 2019).…”