McKone Mark J, Ostertag Rebecca, Rauscher Jason T, Heiser David A, Russell F Leland
Department of Biology, Carleton College, 65057, Northfield, MN, USA.
Oecologia. 1995 Jan;101(1):68-74. doi: 10.1007/BF00328902.
Darwin pointed out that plants with vertical inflorescences are likely to be outcrossed if the inflorescence is acropetalous (flowers from the bottom up), the flowers are protandrous (pollen is dispersed before stigmas are receptive), and pollinators move upward on the inflorescence. This syndrome is common in species pollinated by bees and flies, and very few exceptions are known. We investigated flowering phenology and pollinator behavior in Besseya bullii (Scrophulariaceae) and found that it did not fit Darwin's syndrome. The vertical inflorescence was acropetalous but the flowers were distinctly protogynous, so flowers with newly receptive stigmas appeared on the inflorescence above those with dehiscing anthers. A number of small insects visited B. bullii; bees in the family Halictidae (Augochlorella striata and Dialictus spp.) were most common. When insects moved between gender phases within inflorescences, they moved up more often than down (61% versus 39% of observations, respectively) but this difference was only marginally significant. Most visits were to male-phase flowers only, and this preference was more pronounced for pollen-foraging insects than for nectar-foraging insects. B. bullii was self-compatible, so its flowering characteristics potentially could result in considerable self-pollination. However, an average of 38% of the lowermost flowers opened before any pollen was available on the same inflorescence; these "solo females" had a high probability of outcrossing (though fruit set was relatively low in the bottom portion of the inflorescence). Upper flowers may also be outcrossed because downward insect movement was not uncommon. Therefore protogyny in B. bullii may not necessarily lead to more selfing than would protandry.
达尔文指出，如果花序是向顶式的（花朵从下往上开）、花是雄蕊先熟的（花粉在柱头可接受之前就已散播）且传粉者在花序上向上移动，那么具有垂直花序的植物很可能进行异花授粉。这种综合征在由蜜蜂和苍蝇授粉的物种中很常见，已知的例外情况很少。我们研究了布氏贝斯亚草（玄参科）的开花物候和传粉者行为，发现它不符合达尔文综合征。其垂直花序是向顶式的，但花朵明显是雌蕊先熟的，所以柱头刚开始可接受的花朵出现在花序上那些花药开裂的花朵之上。有许多小昆虫拜访布氏贝斯亚草；隧蜂科的蜜蜂（细纹奥戈隧蜂和Dialictus属）最为常见。当昆虫在花序内的性别阶段之间移动时，它们向上移动的频率比向下移动的频率更高（分别为观察次数的61%和39%），但这种差异仅略微显著。大多数拜访仅针对雄花阶段的花朵，并且这种偏好对于采集花粉的昆虫比对采集花蜜的昆虫更为明显。布氏贝斯亚草是自交亲和的，所以其开花特征可能会导致相当数量的自花授粉。然而，平均有38%的最下部花朵在同一花序上有任何花粉之前就已开放；这些“单性雌花”有很高的异花授粉概率（尽管花序底部的坐果率相对较低）。上部的花朵也可能进行异花授粉，因为昆虫向下移动的情况也并不罕见。因此，布氏贝斯亚草的雌蕊先熟不一定会比雄蕊先熟导致更多的自交。
