Department of Biological Science, Florida State University, Tallahassee, Florida 32306-4295 USA.
Am J Bot. 2009 Nov;96(11):2074-9. doi: 10.3732/ajb.0900048. Epub 2009 Oct 23.
The production of both potentially outcrossed (chasmogamous) and obligately self-fertilized (cleistogamous) flowers presents a clear exception to the prediction that the only evolutionarily stable mating systems are complete selfing and complete outcrossing. Although cleistogamy has evolved repeatedly, the reason for its stability is not known for any species. We tested the hypothesis that the production of cleistogamous and chasmogamous flowers by a perennial violet constitutes adaptive phenotypic plasticity. We manipulated the season of flowering for each flower type and determined fruit set and the germination percentage of seeds produced by cleistogamous and chasmogamous flowers to test the hypothesis that adaptive plastic response to seasonal environmental variation makes mixed mating stable. Cleistogamous flowers had greater fruit set in all seasons and produced seeds with germination percentages as great as or greater than those from chasmogamous flowers. The consistent advantage of cleistogamous flowers is clearly not consistent with a role of adaptive plastic response to seasonal variation. The biomass cost of seed production by chasmogamous flowers was nearly three times that for cleistogamous flowers. Explaining why chasmogamous flower have not been eliminated by natural selection requires that this difference be balanced by an advantage to chasmogamous flowers that has not yet been identified.
生产潜在的异交（开花）和强制自交（闭花）的花朵显然违反了这样的预测，即唯一进化稳定的交配系统是完全自交和完全异交。尽管闭花受精已经多次进化，但对于任何物种来说，其稳定性的原因都不得而知。我们测试了这样一个假设，即多年生紫罗兰产生闭花和开花花朵是适应性表型可塑性的结果。我们操纵了每一种花型的开花季节，并确定了闭花和开花花朵产生的果实结实率和种子发芽率，以检验这样一个假设，即对季节性环境变化的适应性塑性反应使混合交配稳定。在所有季节中，闭花的结实率更高，并产生的种子发芽率与开花的种子发芽率一样高或更高。闭花的明显优势显然不符合对季节性变化的适应性塑性反应的作用。开花花朵的种子产量的生物量成本几乎是闭花花朵的三倍。要解释为什么开花花朵没有被自然选择淘汰，就需要这种差异被尚未确定的开花花朵的优势所平衡。
