Herlihy Christopher R, Eckert Christopher G
Department of Biology, Queen's University, Kingston, Ontario K7L 3N6, Canada.
Evolution. 2007 Jul;61(7):1661-74. doi: 10.1111/j.1558-5646.2007.00137.x.
The mating system of flowering plant populations evolves through selection on genetically based phenotypic variation in floral traits. The physical separation of anthers and stigmas within flowers (herkogamy) is expected to be an important target of selection to limit self-fertilization. We investigated the pattern of phenotypic and genetic variation in herkogamy and its effect of self-fertilization in a broad sample of natural populations of Aquilegia canadensis, a species that is highly selfing despite strong inbreeding depression. Within natural populations, plants exhibit substantial phenotypic variation in herkogamy caused primarily by variation in pistil length rather than stamen length. Compared to other floral traits, herkogamy is much more variable and a greater proportion of variation is distributed among rather than within individuals. We tested for a genetic component of this marked phenotypic variation by growing naturally pollinated seed families from five populations in a common greenhouse environment. For three populations, we detected a significant variation in herkogamy among families, and a positive regression between parental herkogamy measured in the field and progeny herkogamy in the greenhouse, suggesting that there is often genetic variation in herkogamy within natural populations. We estimated levels of self-fertilization for groups of flowers that differed in herkogamy and show that, as expected, herkogamy was associated with reduced selfing in 13 of 19 populations. In six of these populations, we performed floral emasculations to show that this decrease in selfing is due to decreased autogamy (within-flower selfing), the mode of selfing that herkogamy should most directly influence. Taken together, these results suggest that increased herkogamy should be selected to reduce the production of low-quality selfed seed. The combination of high selfing and substantial genetic variation for herkogamy in A. canadensis is enigmatic, and reconciling this observation will require a more integrated analysis of how herkogamy influences not only self-fertilization, but also patterns of outcross pollen import and export.
开花植物种群的交配系统通过对花部性状基于遗传的表型变异进行选择而进化。花内花药和柱头的物理分离（雌雄异位）有望成为限制自花受精的重要选择目标。我们在加拿大耧斗菜（Aquilegia canadensis）的大量自然种群样本中，研究了雌雄异位的表型和遗传变异模式及其对自花受精的影响。加拿大耧斗菜尽管存在强烈的近亲繁殖衰退，但自交率很高。在自然种群中，植物的雌雄异位表现出显著的表型变异，主要是由雌蕊长度而非雄蕊长度的变异引起的。与其他花部性状相比，雌雄异位的变异更大，且更大比例的变异分布在个体间而非个体内。我们通过在一个普通温室环境中种植来自五个种群的自然授粉种子家系，来测试这种显著表型变异的遗传成分。对于三个种群，我们检测到家系间雌雄异位存在显著变异，并且在田间测量的亲本雌雄异位与温室中后代雌雄异位之间存在正相关，这表明自然种群中雌雄异位通常存在遗传变异。我们估计了雌雄异位不同的花组的自花受精水平，结果表明，正如预期的那样，在19个种群中的13个种群中，雌雄异位与自交减少相关。在其中六个种群中，我们进行了花部去雄操作，以表明自交减少是由于同株异花受精（花内自交）减少所致，而同株异花受精是雌雄异位最直接影响的自交方式。综合来看，这些结果表明应选择增加雌雄异位以减少低质量自交种子的产生。加拿大耧斗菜自交率高且雌雄异位存在大量遗传变异，这一现象令人费解，要解释这一观察结果，需要对雌雄异位如何不仅影响自花受精而且影响异交花粉的输入和输出模式进行更综合的分析。
