Ecology Center, Utah State University, Logan, Utah 84322 USA;
Am J Bot. 2001 Feb;88(2):213-9.
The Ipomopsis aggregata species complex (Polemoniaceae) includes species pairs that hybridize readily in nature as well as pairs that meet along contact zones with no apparent hybridization. Artificial hybrids can be made between I. aggregata and I. arizonica, yet morphological intermediates between these two species have not been observed in natural populations. This apparent lack of hybridization is perplexing given that plants of the two species often grow within a few metres of each other and both species have red flowers visited by the same species of hummingbirds. We used trained hummingbirds to examine pollen transfer within and between species. We also hand-pollinated flowers to examine paternal success of heterospecific and conspecific pollen, testing paternity with electrophoretic examination of seeds. Hummingbirds were not simply better at transferring pollen within than between species. Instead, I. arizonica was a better pollen donor so that considerable pollen transfer was observed from I. arizonica to I. aggregata, but very little in the opposite direction. Conversely, once pollen arrived at stigmas, I. arizonica pollen performed very poorly on I. aggregata pistils. However, pollen from I. aggregata could, in some cases, sire seeds on I. arizonica. We hypothesize that hybrids are scarce in nature, in part, because of asymmetric barriers to reproduction: little pollen transfer in one direction and poor pollen performance in the other.
松果菊属（菊科）的聚合松果菊种复合体包括在自然界中容易杂交的种对，以及在接触区相遇但没有明显杂交的种对。可以在聚合松果菊和阿利桑那松果菊之间人工制作杂交种，但在自然种群中尚未观察到这两个物种之间的形态中间体。考虑到这两个物种的植物通常在彼此几公尺范围内生长，并且两种植物的红色花朵都被同一种蜂鸟访问，这种明显缺乏杂交的情况令人费解。我们使用受过训练的蜂鸟来检查种内和种间的花粉转移。我们还进行了人工授粉，以检查异交和同交花粉的父本成功，并通过电泳检查种子来检验亲子关系。蜂鸟并不是简单地更擅长在种内传递花粉，而是阿利桑那松果菊是更好的花粉供体，因此可以从阿利桑那松果菊向聚合松果菊观察到相当多的花粉转移，但相反的方向很少。相反，一旦花粉到达柱头，阿利桑那松果菊花粉在聚合松果菊雌蕊上的表现非常差。然而，聚合松果菊的花粉在某些情况下可以在阿利桑那松果菊上育出种子。我们假设，杂种在自然界中稀少，部分原因是繁殖的不对称障碍：一个方向的花粉转移很少，另一个方向的花粉表现很差。
