- Genotype-specific interactions and the trade-off between host and parasite fitness


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Figure 1 Reaction norms of parasite transmission across the different host lines. Parasite transmission (± SE) measured as the total number of successful transmissions. (a) Parasite strains averaged by origin: Laboratory (black dots), Fribourg (white dots) and Orsay (grey dots). (b) The three parasite strains of laboratory origin: Emco (black dots), Emwa (white dots) and Noco (grey dots). (c) The two parasite strains from Orsay: Ors 3 (black dots) and Ors 5 (white dots). Raw data are presented in the Figures though analyses were performed on square-root transformed data. 

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Figure 2 Genetic correlations between host and parasite fitness. Genetic correlations between host seed production (total mass of seeds in milligrams) and parasite transmission (number of successful transmissions). Each symbol represents a unique combination of host line and parasite strain. Symbols are distinguished according to the host line involved in the combination: Gb (red dots), Fin (black dots), Pyr (green dots), Sue (yellow dots), Tch (pink dots), Tsu (blue dots). Solid line: significant regression across the seven strains and control in combination with the host line Gb (y = -0.28x + 97.58; Spearman'rho = 0.81, p-value = 0.015). Dashed lines: non significant regressions for the host lines Fin (y = -0.66x + 61.32; ρ = -0.26, p-value = 0.53), Pyr (y = 0.43x + 17.10; ρ = 0.55, p-value = 0.16), Sue (y = -0.03x + 62.97; ρ = -0.29, p-value = 0.49), Tch (y = -0.07x + 64.15; ρ = -0.24, p-value = 0.57), and Tsu (y = -0.06x + 57.04; ρ = 0.17, p-value = 0.69). 

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Figure 3 Graphical model representing how resources allocated to reproduction between a host and its parasite may vary. The surface of the circle represents the absolute amount of available resources accumulated by the host. The white portion is the amount of resources used by the host to produce its descendants, whereas the gray portion is diverted by the parasite to produce its own descendants. (a) non infected host: all the resources accumulated by the host are used for its reproduction. (b) host infected with a fully compatible parasite: an arbitrary proportion (a third in this case) of the resources accumulated by the host is diverted toward parasite reproduction. (c) host infected with a parasite with partially incompatible interaction: a part of the resources accumulated by the host are lost (dotted part of the circle), for instance due to the activation of host defenses, or to the inefficiency of resources use by the parasite. (c) illustrates the particular case where the relative diversion of resources by the parasite is the same as in case (b), but the absolute reproduction of both protagonists is lower. 

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Figure 4 Schematic representation of possible relationships between host and parasite fitness with the type of interaction described in figure 3. In fully compatible combinations of host and parasite genotypes there is no loss of resources and the absolute amount of available resources is always maximal. In such a situation, as the proportion diverted by the parasite increases, the portion left to the host, and thus its reproduction, decreases (relationship (a)). However, if all combinations are not fully compatible, host and parasite reproduction will also depend on the level of compatibility, i.e. the amount of lost resources. In this latter case, an increase in compatibility will increase both host and parasite reproduction and lead to a positive relationship between host and parasite fitness (relationship (b)). 


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