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, 5 (9), e12614

Between-population Outbreeding Affects Plant Defence


Between-population Outbreeding Affects Plant Defence

Roosa Leimu et al. PLoS One.


Between-population crosses may replenish genetic variation of populations, but may also result in outbreeding depression. Apart from direct effects on plant fitness, these outbreeding effects can also alter plant-herbivore interactions by influencing plant tolerance and resistance to herbivory. We investigated effects of experimental within- and between-population outbreeding on herbivore resistance, tolerance and plant fitness using plants from 13 to 19 Lychnis flos-cuculi populations. We found no evidence for outbreeding depression in resistance reflected by the amount of leaf area consumed. However, herbivore performance was greater when fed on plants from between-population compared to within-population crosses. This can reflect outbreeding depression in resistance and/or outbreeding effects on plant quality for the herbivores. The effects of type of cross on the relationship between herbivore damage and plant fitness varied among populations. This demonstrates how between-population outbreeding effects on tolerance range from outbreeding depression to outbreeding benefits among plant populations. Finally, herbivore damage strengthened the observed outbreeding effects on plant fitness in several populations. These results raise novel considerations on the impact of outbreeding on the joint evolution of resistance and tolerance, and on the evolution of multiple defence strategies.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.


Figure 1
Figure 1. Mean effects of outbreeding on resistance and tolerance.
Average resistance, measured as a) 1- proportion of damaged leaf area, b) final snail mass in the snail herbivory experiment with plants resulting from two generations of within- and between-population crosses of plants from 13 populations of Lychnis flos-cuculi. c) Effects of outbreeding on tolerance to clipping in the clipping experiment with plants resulting from two generations of within- and between-population crosses of plants from 19 populations of Lychnis flos-cuculi. Average effects of clipping on number of fruits produced after damage (1c). Covariate-adjusted least-squares means estimates and estimated standard errors are presented.
Figure 2
Figure 2. Among-population variation in outbreeding effects on tolerance and plant fitness.
Among-population variation in a) outbreeding effects on tolerance and in the effects of snail damage on the number of fruits produced for b) plants from within-population and c) between-population crosses in the snail herbivory experiment. Tolerance is assessed as slope of fitness (number of fruits) and leaf damage %. Thus, tolerance is given in units of change in the number of fruits per % leaf area damaged. The squares in a) denote slopes for plants from within- (grey symbols) and between-population (white symbols) crosses for each population. The lines illustrate differences in tolerance between plants within-and between population crosses for each population. Tolerance >0 indicates overcompensation, tolerance  = 0 full compensation, and tolerance <0 undercompensation. In figures b) and c) each line represents a population.

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