Abstract
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.
Highlights
Inter-population crosses and transplantations of plants among populations are used to restore genetically eroded populations [1,2] despite the fact that this can result in outbreeding depression, especially in fragmented, and genetically differentiated, plant populations (e.g., [2,3,4,5])
Due to strong directional selection, populations with low levels of genetic variation can be more strongly adapted to local conditions and more likely to suffer from outbreeding depression following crosses between distinct populations
We addressed the following questions: 1) Are plants from betweenpopulation crosses less resistant and tolerant against herbivory than plants from within-population crosses? 2) Do outbreeding effects on resistance and on tolerance vary among plant populations? 3) Does plant damage modify outbreeding effects on plant fitness?
Summary
Inter-population crosses and transplantations of plants among populations are used to restore genetically eroded populations [1,2] despite the fact that this can result in outbreeding depression, especially in fragmented, and genetically differentiated, plant populations (e.g., [2,3,4,5]). Outbreeding depression can arise because of breaking up co-adapted gene complexes [6], or due to the disruption of local adaptation, which is common at least in large plant populations [7]. Several mechanisms may contribute to the degree of expressed outbreeding depression depending on the effects of the genetic and environmental histories of populations, in particular on the interplay between selection, drift, gene flow and inbreeding [8]. Compared to large continuous populations, fragmented populations may experience lower outbreeding depression following inter-population crosses due to positive outbreeding effects on heterozygosity. Due to strong directional selection, populations with low levels of genetic variation can be more strongly adapted to local conditions and more likely to suffer from outbreeding depression following crosses between distinct populations
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