Host heterogeneity mitigates virulence evolution

Abstract

Parasites often infect genetically diverse host populations, and the evolutionary trajectories of parasite populations may be shaped by levels of host heterogeneity. Mixed genotype host populations, compared to homogeneous host populations, can reduce parasite prevalence and potentially reduce rates of parasite adaptation due to trade-offs associated with adapting to specific host genotypes. Here, we used experimental evolution to select for increased virulence in populations of the bacterial parasite Serratia marcescens exposed to either heterogeneous or homogeneous populations of Caenorhabditis elegans . We found that parasites exposed to heterogeneous host populations evolved significantly less virulence than parasites exposed to homogeneous host populations over several hundred bacterial generations. Thus, host heterogeneity impeded parasite adaptation to host populations. While we detected trade-offs in virulence evolution, parasite adaptation to two specific host genotypes also resulted in modestly increased virulence against the reciprocal host genotypes. These results suggest that parasite adaptation to heterogeneous host populations may be impeded by both trade-offs and a reduction in the efficacy of selection as different host genotypes exert different selective pressures on a parasite population.

An experimental test of parasite adaptation to common versus rare host genotypes

“…These heterogeneous treatments are a subset of treatments in two larger experimental evolution studies [17,18]. These two studies demonstrated that Sm2170 can adapt to attack these four host genotypes: parasites selected in homogeneous host populations of each genotype evolved higher virulence than parasites selected in the absence of hosts (electronic supplementary material, figure S2, table S1).…”

Genetic slippage after sex maintains diversity for parasite resistance in a natural host population

“…Due to dominance and epistasis in the genetic architecture of resistance, this reshuffling resets the clock to the time before selection acted, rendering the response to selection zero. Although this is an extreme case of genetic slippage in response to sex, it is a powerful agent to maintain genetic diversity, which is a hallmark of resistance in natural populations of Daphnia and other animals, plants and bacteria (Altermatt and Ebert 2008;Desai and Currie 2015;Zhao et al 2016;Cabalzar et al 2019;Broniewski et al 2020;Sallinen et al 2020;White et al 2020). As climatic seasonality seems to determine the dynamics of parasite resistance in our host population, and given the known impact of climate change on epidemics in the D. magna-P. ramosa system (Auld and Brand 2017), one may speculate that the dynamics in our study population may change in response to the predicted changes in climatic conditions and seasonality.…”

“…Indeed, the role of selection by parasites is well established for the major histocompatibility (MHC) gene complex in jawed vertebrates and resistance (R) genes in plants (Jeffery and Bangham 2000;Hughes 2002;Radwan et al 2020), both of which have remarkably high allelic diversity (Sommer 2005;Baggs et al 2017). In particular, coevolution with parasites is linked to increased host diversity (Wang et al 2017;Duxbury et al 2019), and high diversity at resistance genes has been shown to be advantageous against parasites (Sommer 2005;Zhao et al 2016;Gösser et al 2019;Peters et al 2019;White et al 2020).…”

Changement climatique et biosphère

“…Ces baisses de rendement sont dues en partie à l'accélération et au raccourcissement du cycle annuel de développement évoqué précédemment. Dans ce cas précis des plantes cultivées (mais cela est aussi valable pour les animaux domestiqués), les pertes de diversité génétique dues à la réduction du nombre de variétés cultivées, pour ne garder que les plus productives et les plus intéressantes pour la commercialisation [Bélanger andPilling, 2019, Wolff, 2004], freinent maintenant la sélection de variétés mieux adaptées aux nouvelles conditions climatiques [Wolkovich et al, 2018] et altèrent les capacités des agroécosystèmes à résister aux aléas climatiques [Forest et al, 2015] et aux aléas naturels tels que les ravageurs et agents pathogènes [Ekroth et al, 2019, White et al, 2020. En effet, la diversité taxonomique et la diversité génétique permettent en général une meilleure résilience des communautés animales et végétales aux aléas [Oliver et al, 2015b].…”

“…Cette résilience est en général la résultante d'une redondance et d'une complémentarité entre les différentes espèces, variétés, et génotypes, dans l'exploitation des ressources (eau, lumière, nutriments), et dans les capacités de défense contre les ravageurs et pathogènes, et capacités de résistance aux aléas climatiques [Allan et al, 2011, Downing et al, 2014. La diversité taxonomique et la diversité génétique permettent aussi sur le long terme de ralentir l'adapta-tion génétique des ravageurs et pathogènes à leurs hôtes [White et al, 2020]. L'utilisation des interactions positives entre espèces, variétés/races et génotypes représente l'un des principes fondamentaux de l'agroécologie.…”