Effect of multilevel selection on the evolution of gynodioecy

点击翻译按钮获取中文摘要

Most traits are thought to evolve because they increase an individual's survival and reproduction. However, there are some traits that appear to decrease an individual's fitness, yet still evolve repeatedly. One such trait is gynodioecy, a type of sexual polymorphism in which hermaphroditic and female plants coexist within a population. Females have an inherent fitness disadvantage because they can reproduce only via seeds, whereas hermaphrodites can reproduce via both seeds and pollen. Despite this disadvantage, females, and thus gynodioecy, have evolved repeatedly in the flowering plants. Females could evolve in response to individual-level selection that acts only under a restricted set of conditions, or in response to selection at levels above and/or below the individual, including the group and the gamete. Measuring selection at multiple levels is important because traits evolve differently based on the level at which selection acts: even when the strength and direction of selection are similar across levels, both the causes of and responses to selection can differ.Our long-term goal is to determine how the evolution of gynodioecy has been affected by selection at multiple levels. Our recent work suggests that gynodioecy can evolve in response to frequency-dependent selection (where an individual's fitness depends in part on the mean phenotype of its group) and gametic selection (where the viability of a gamete depends on which allele it carries). However, we also found that the frequency of females varies widely among gynodioecious populations, suggesting that the strength of selection varies. To determine the why the strength of selection varies among gynodioecious populations, we will focus on two short-term goals. First, we will identify the mechanisms that cause frequency-dependent selection on female plants by testing why females have frequency-dependent fitness and whether the strength of frequency-dependent selection depends on conspecific density. Second, we will identify the mechanisms that cause gametic selection on sex-determining genes by testing whether variation in the strength of gametic selection is caused by ecological or genetic factors and whether these genetic factors are in the cytoplasmic or nuclear genome. All of this work will be done in the gynodioecious species Lobelia siphilitica. Although gynodioecy was first studied by Darwin, there is still uncertainty as to why it evolves in some populations but not in others. The proposed research, by focusing on the mechanisms that cause frequency-dependent selection and gametic selection, will allow us to predict the ecological and genetic contexts in which gynodioecy is more and less likely to evolve. More generally, the proposed research will demonstrate how measuring selection at multiple levels can lead to new explanations for the repeated evolution of traits that appear to decrease an individual's fitness.