Pollinators, herbivores, and the evolution of floral traits | Science

![][1]Selection mediated by both pollinators and herbivores affects the evolution of floral traits.PHOTO: ARCO IMAGES GMBH/ALAMY STOCK PHOTOFloral traits, including flower shape, color, nectar, and scent, affect pollination success and mating systems by influencing attractiveness to pollinators and efficiency of cross- and self-pollination. Traditionally, the evolution and maintenance of differences in these traits have been related to selective pressures imposed by pollinators, and pollen and mate limitation. Recent years have seen increasing evidence that antagonists such as grazers and seed predators also mediate selection on floral traits ([ 1 ][2]–[ 4 ][3]). On page 193 of this issue, Ramos and Schiestl ([ 5 ][4]) show that both pollinators and herbivores influence the evolution of floral display and of self-pollination efficiency and that their effects are not necessarily additive. The study provides a proof-of-concept that should inspire investigation of the interactive effects of mutualists and antagonists on the evolution of plant reproductive traits in additional systems.There are several reasons why the evolution of floral display and efficiency of autonomous self-pollination might be affected by plant interactions with both pollinators and herbivores. For example, a prominent floral display may increase attractiveness to pollinators, but at the same time increase the risk of damage from herbivores. Optimal floral display will then be affected by the relative strength of these effects and their consequences for plant fitness ([ 6 ][5]). Moreover, because damage from herbivores can reduce attractiveness to pollinators, interactions with herbivores may influence the degree of pollen limitation and selection on traits affecting the ability to produce offspring through autonomous self-pollination ([ 2 ][6]). In addition, evolutionary response will depend not only on the strength and direction of selection, but also on the presence of genetic variation and genetically based correlations among traits.Experimental evolution is a powerful approach to examine how environmental conditions affect evolutionary trajectories. Ramos and Schiestl combined pollination and herbivory treatments to determine their independent and interactive effects on the evolution of floral traits. A rapid-cycling population of the annual plant Brassica rapa evolved under one of four treatment combinations: hand-pollination with or without herbivory, and bee pollination with or without herbivory. In two treatment combinations, pollination by hand ensured that flowers received a surplus of pollen, whereas in the other two, bumble bees determined the pollination success of individual plants. As predicted, selection by bumble bees resulted in the evolution of plant phenotypes that were more attractive to bumble bees. This effect was reduced when plants evolved in the presence of both bumble bees and leaf herbivores (caterpillars of the butterfly Pieris brassicae ). The observations are consistent with the idea that plant traits that influence attractiveness to pollinators may be subject to conflicting selection from herbivores. In addition, plants pollinated by bumble bees, which were presumably more strongly pollen-limited than were plants pollinated by hand, tended to evolve greater ability to produce offspring through autonomous self-pollination.More unexpectedly, the presence of herbivores was associated with an increased efficiency of autonomous self-pollination also among plants pollinated by hand. This cannot be explained by an effect of herbivores on pollen limitation. Instead, Ramos and Schiestl suggest that this is the result of genetic correlations between traits affecting resistance to herbivory and floral traits affecting efficiency of autonomous self-pollination. This hypothesis could be tested by artificial selection (selective breeding) on these traits.The experimental treatments were applied for six generations, which was sufficient for selection lines to diverge in several traits. One reason is that the population of B. rapa used in the study is genetically highly variable. The population was established through crosses between early-flowering genotypes and many generations of selection for early flowering, rapid seed maturation, absence of seed dormancy, small plant size, and high female fertility ([ 7 ][7]). Despite its history of long-term artificial selection, the population has maintained considerable genetic variation and has been used to examine the genetic basis of a wide range of traits and constraints to adaptive evolution [for example, ([ 8 ][8], [ 9 ][9])].Future studies should examine how frequently and how rapidly evolutionary trajectories of natural plant populations are affected by changes in pollination regime and herbivore abundance. This should depend on the strength of selection by pollinators and herbivores and the importance of indirect and nonadditive effects ([ 10 ][10], [ 11 ][1

Source: Pollinators, herbivores, and the evolution of floral traits | Science

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