The effects of climate change on parasites: toward an integrative and predictive approach – INCLIMPAR

In a first part, we will use a specific host-parasite interaction to identify climatic and non-climatic factors that may affect parasite colonization. Combining empirical observations, experimental works and molecular tools, we will focus on potential barriers to T. polycolpus colonization, and we will evaluate the strength of each of the following barriers to colonization:
(i) The dispersal ability of T. polycolpus (Is T. polycolpus able to freely disperse from one site to another?)
(ii) The abiotic requirement of T. polycolpus (What are the environmental factors that may restrain T. polycolpus expansion range?)
(iii) The ability of T. polycolpus to shift on new hosts species (If T. polycolpus arrives in a new site in which L. leuciscus is absent, will it be able to shift on another host species?)
(iv) The ability of T. polycolpus to adapt to foreign L. leuciscus genotypes (If T. polycolpus arrives in a new site in which L. leuciscus is naturally present, does local adaptation may restrain T. polycolpus to colonize this foreign genotype of L. Leuciscus?)
In a second part we will develop a predictive statistical framework that would consider conjointly the effects of climatic and non-climatic factors on parasite spread under several climatic scenarios. This integrative framework will aim at improving the forecasting of the effect of climate change on the future geographic range of T. polycolpus.
Finally, in a third part we will generalise our findings by using meta-analyses of the existing literature to quantify numerically changes in parasite geographic range in the last decades (over taxon and ecosystem types). In additioin, this meta-analysis as well as simulations will be used to assess the relative effects of various biological and environmental variables (climatic and human-related) on those changes in parasite geographic range.

During this first year, we focus on patterns and processes of alternative host use in Tracheliastes polycolpus. The ecological and evolutionary determinants underpinning alternative host use remain indeed elusive.
We investigated, in the wild, patterns of alternative host use in a Tracheliastes polycolpus, and we tested several a priori hypotheses regarding determinants underlying these patterns. We specifically answer two related questions: (i) why do parasites use alternative host species? and (ii) why do parasites preferentially use one particular alternative host species rather than another?
We first showed that T. polycolpus was able to use five alternative host species in addition to its principal host species, the dace (Leuciscus burdigalensis). Using causal analyses, we demonstrated that, overall, the rate of alternative host use was higher when parasite burden on the principal host was higher, providing support for the “parasite density” hypothesis. Then, phylogenetic regressions revealed that the specific use of these alternative host species does not occur randomly, but according to the “ecological similarity” hypothesis: parasites preferentially use host species that are ecologically close to the principal host species, irrespective of the phylogenetic distance and of the alternative host density. The fitness of parasites on alternative host species was similar to that of parasites on the principal host species, except for the smallest body-sized alternative host species for which parasite fitness was lower. Finally, using microsatellite markers, we demonstrated that this differential host use did not lead to genetically isolated parasite populations.
Our study suggests that encounter rate may be a key factor in predicting patterns of alternative host use, and unravels intriguing questions about the contribution of phenotypic plasticity to the use of a large host spectrum by a generalist parasite.

This project will allow setting a new statistical framework allowing to predict the spatial distribution of numerous pathogens while accounting for their biological particularities. This tool will be of great interest in a context of climate change, both for the human society (for predicting emergent diseases) and for the conservation of biodiversity. From a fundamental viewpoint, this project will be among the first at testing biological factors limiting or favoring the colonization of pathogens.

Our main result (see above) is being accepted for publication in Functional Ecology IF : 4,56.

Article title :
Patterns and processes of alternative host use in a generalist parasite: insights from a natural host-parasite interaction. Amélie Lootvoet, Simon Blanchet, Muriel Gevrey, Laetitia Buisson, Loïc Tudesque et Géraldine

A common wisdom holds that parasites will spread extensively, and will expand their geographic range northward as climate will go warmer. Although expansion range of parasites might greatly affect wildlife, predictions concerning animals and plants parasites are mostly conceptual. Moreover, the association between climate and parasite spread has recently been questioned. It has been argued (i) that parasites would undergo range shifts (and even range restriction) rather than range expansion and (ii) that many non-climatic factors may affect, and even overshadow, the effects of the climate. Such controversy has important fundamental and societal implications. Accordingly, the next generation of model-based predictions concerning climate change and parasites obviously need to consider all the processes that link parasites to their environment. Including non-climatic factors in predictive models is an efficient way to obtain a balanced and objective view of the risk posed by climate change on parasites spread. INCLIMPAR will contribute to fill the gap between the controversy around the association climate-parasite spread and societal needs.
In a first part (Tasks 2-5), we will use a specific host-parasite interaction (the ectoparasite Tracheliastes polycolpus and its fish host Leuciscus leuciscus) to identify climatic and non-climatic factors that may affect parasite colonization. Combining empirical observations, experimental works and molecular tools, we will evaluate the strength of the following potential barriers to T. polycolpus colonization:
(i) The dispersal ability of T. polycolpus (Is T. polycolpus able to freely disperse from one site to another?)
(ii) The abiotic requirement of T. polycolpus (What are the environmental factors that may restrain T. polycolpus expansion range?)
(iii) The ability of T. polycolpus to shift on new hosts species (If T. polycolpus arrives in a new site in which L. leuciscus is absent, will it be able to shift on another host species?)
(iv) The ability of T. polycolpus to adapt to foreign L. leuciscus genotypes (If T. polycolpus arrives in a new site in which L. leuciscus is naturally present, does local adaptation may restrain T. polycolpus to colonize this foreign genotype of L. Leuciscus?)
In a second part (Task 6) we will develop a predictive statistical framework that would consider conjointly the effects of climatic and non-climatic factors on parasite spread under several climatic scenarios. This integrative framework will aim at improving the forecasting of the effect of climate change on the future geographic range of T. polycolpus.
Finally, in a third part (Task 7) we will generalise our findings by performing a meta-analysis of the existing literature to quantify numerically changes in parasite geographic range in the last decades (over taxon and ecosystem types). This meta-analysis as well as simulations will be used to also assess the relative effects of various biological and environmental variables (climatic and human-related) on those changes in parasite geographic range.