Takebayashi N, Wolf D E, Delph L F
Department of Biology, Indiana University, Bloomington, Indiana 47405-3700, USA.
Heredity (Edinb). 2006 Feb;96(2):159-65. doi: 10.1038/sj.hdy.6800780.
The diverse features of floral morphology are often thought to be well-designed mechanisms to manipulate plant mating systems. We evaluated the effectiveness of one such mechanism, anther-stigma separation (herkogamy), in controlling variation in the level of outcrossing among plants in a population of Gilia achilleifolia. Variation in outcrossing rates within populations has the potential to influence the coevolution between inbreeding depression and mating system. Using four polymorphic allozymes, we compared the outcrossing-rate estimates of two groups of individuals under natural conditions: one group with low herkogamy and another with high herkogamy. The high herkogamy group had a higher outcrossing rate (0.572) than the low herkogamy group (0.335). This suggests that the within-population variation in outcrossing rate could potentially cause the previously observed association between herkogamy and inbreeding depression (Takebayashi and Delph, 2000). A previous study of floral traits among G. achilleifolia populations failed to detect a relationship between herkogamy and outcrossing rate, demonstrating that the functionality of traits may be obscured in among-population studies as a consequence of uncontrolled environmental variation. Additionally, the effect of herkogamy on outcrossing rate in delayed selfers such as G. achilleifolia may be particularly prominent when pollinator availability is low. Our population-level estimate of outcrossing rate (0.444) was somewhat lower than an estimate from the same population, 15 years prior to our study (0.75), suggesting that pollinator availability may fluctuate among years. Both within-year and among-year variation in the outcrossing rate may have a strong influence on mating-system evolution.
花形态的多样特征通常被认为是操纵植物交配系统的精心设计的机制。我们评估了一种这样的机制，即花药 - 柱头分离（雌雄异位），在控制吉莉草种群中植物异交水平变化方面的有效性。种群内异交率的变化有可能影响近亲繁殖衰退与交配系统之间的协同进化。我们使用四种多态性等位酶，比较了两组个体在自然条件下的异交率估计值：一组雌雄异位程度低，另一组雌雄异位程度高。雌雄异位程度高的组比雌雄异位程度低的组有更高的异交率（0.572），而后者为（0.335）。这表明种群内异交率的变化可能会导致之前观察到的雌雄异位与近亲繁殖衰退之间的关联（武林和德尔夫，2000年）。先前对吉莉草种群间花性状的研究未能检测到雌雄异位与异交率之间的关系，这表明由于环境变化未得到控制，性状的功能在种群间研究中可能会被掩盖。此外，在传粉者可利用性较低时，像吉莉草这样的延迟自花授粉植物中雌雄异位对异交率的影响可能会特别显著。我们对种群水平异交率的估计值（0.444）略低于我们研究前15年对同一种群的估计值（0.75），这表明传粉者可利用性可能会随年份波动。异交率在年内和年间的变化都可能对交配系统的进化产生强烈影响。
