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If parasites transmit more readily between closely related hosts, then parasite burdens should decrease with increased genetic diversity of host populations. This important hypothesis is often accepted at face value—notorious epidemics of crop monocultures testify to the vulnerability of host populations that have been purged of diversity. Yet the relationship between genetic diversity and parasitism likely varies across contexts, differing between crop and noncrop hosts and between experimental and natural host populations. Here, we used a meta‐analytic approach to ask if host diversity confers protection against parasites over the range of contexts in which it has been tested. We synthesized the results of 102 studies, comprising 2004 effect sizes representing a diversity of approaches and host‐parasite systems. Our results validate a protective effect of genetic diversity, while revealing significant variation in its strength across biological and empirical contexts. In experimental host populations, genetic diversity reduces parasitism by ∼20% for noncrop hosts and by ∼50% for crop hosts. In contrast, observational studies of natural host populations show no consistent relationship between genetic diversity and parasitism, with both strong negative and positive correlations reported. This result supports the idea that, if parasites preferentially attack close relatives, the correlation of genetic diversity with parasitism could be positive or negative depending upon the potential for host populations to evolve in response to parasite selection. Taken together, these results reinforce genetic diversity as a priority for both conservation and agriculture and emphasize the challenges inherent to drawing comparisons between controlled experimental populations and dynamic natural populations. [ABSTRACT FROM AUTHOR]

In biological control, populations of both the biological control agent and the pest have the potential to evolve and even to coevolve. This feature marks the most powerful and unpredictable aspect of biological control strategies. In particular, evolutionary change in host specificity of the biological control agent could increase or decrease its efficacy. Here, we tested for change in host specificity in a field population of the biological control organism Pasteuria penetrans. Pasteuria penetrans is an obligate parasite of the plant parasitic nematodes Meloidogyne spp., which are major agricultural pests. From 2013 through 2016, we collected yearly samples of P. penetrans from eight plots in a field infested with M. arenaria. Plots were planted either with peanut (Arachis hypogaea) or with a rotation of peanut and soybean (Glycine max). To detect temporal change in host specificity, we tested P. penetrans samples annually for their ability to attach to (and thereby infect) four clonal lines of M. arenaria. After controlling for temporal variation in parasite abundance, we found that P. penetrans from each of the eight plots showed temporal variation in their attachment specificity to the clonal host lines. The trajectories of change in host specificity were largely unique to each plot. This result suggests that local forces, at the level of individual plots, drive change in specificity. We hypothesize that coevolution with local M. arenaria hosts may be one such force. Lastly, we observed an overall reduction in attachment rate with samples from rotation plots relative to samples from peanut plots. This result may reflect lower abundance of P. penetrans under crop rotation, potentially due to suppressed density of host nematodes. As a whole, the results show local change in specificity on a yearly basis, consistent with evolution of a biological control organism in its ability to infect and suppress its target pest. [ABSTRACT FROM AUTHOR]

Why do infectious diseases erupt in some host populations and not others? This question has spawned independent fields of research in evolution, ecology, public health, agriculture, and conservation. In the search for environmental and genetic factors that predict variation in parasitism, one hypothesis stands out for its generality and longevity: genetically homogeneous host populations are more likely to experience severe parasitism than genetically diverse populations. In this perspective piece, I draw on overlapping ideas from evolutionary biology, agriculture, and conservation to capture the far-reaching implications of the link between genetic diversity and disease. I first summarize the development of this hypothesis and the results of experimental tests. Given the convincing support for the protective effect of genetic diversity, I then address the following questions: (1) Where has this idea been put to use, in a basic and applied sense, and how can we better use genetic diversity to limit disease spread? (2) What new hypotheses does the established disease-diversity relationship compel us to test? I conclude that monitoring, preserving, and augmenting genetic diversity is one of our most promising evolutionarily informed strategies for buffering wild, domesticated, and human populations against future outbreaks., (© 2021 The Authors. Evolution © 2021 The Society for the Study of Evolution.)