Waser Nicholas M, Price Mary V
Department of Biology, University of California, Riverside, CA, 92521.
Rocky Mountain Biological Laboratory, Crested Butte, CO, 81224.
Evolution. 1989 Aug;43(5):1097-1109. doi: 10.1111/j.1558-5646.1989.tb02554.x.
Restricted gene flow and localized selection should establish a correlation between physical proximity and genetic similarity in many plant populations. Given this situation, fitness may decline in crosses between nearby plants (inbreeding depression), and in crosses between more widely separated plants ("outbreeding depression") mostly as a result of disruption of local adaptation. It follows that seed set and offspring fitness may be greatest in crosses over an intermediate "optimal outcrossing distance." This prediction was supported for Ipomopsis aggregata, a long-lived herbaceous plant pollinated by hummingbirds. In six replicate pollination experiments, mean seed set per flower was higher with an outcrossing distance of 1-10 m than with selfing or outcrossing over 100 m. A similar pattern appeared in the performance of offspring from experimental crosses grown under natural conditions and censused for a seven-year period. Offspring from 10-m crosses had higher survival, greater chance of flowering, and earlier flowering than those from 1-m or 100-m crosses. As a result, 1-m and 100-m offspring achieved only 47% and 68%, respectively, of the lifetime fitness of 10-m offspring. Offspring fitness also declined with planting distance from the seed parent over a range of 1-30 m, so that adaptation to the maternal environment is a plausible mechanism for outbreeding depression. Censuses in a representative I. aggregata population indicated that the herbaceous vegetation changes over a range of 2-150 m, suggesting that there is spatial variation in selection regimes on a scale commensurate with the observed effects of outbreeding depression and planting distance. We discuss the possibility that differences in seed set might in part reflect maternal mate discrimination and emphasize the desirability of measuring offspring fitness under natural conditions in assessing outcrossing effects.
在许多植物种群中，受限的基因流动和局部选择会使植株间的物理距离与遗传相似性之间建立起一种关联。在这种情况下，邻近植株之间的杂交（近亲繁殖衰退）以及相隔较远植株之间的杂交（远交衰退），其适合度可能都会下降，这主要是局部适应性遭到破坏的结果。由此可知，在中等“最佳异交距离”的杂交中，结实率和子代适合度可能最高。这一预测在聚合花葱（Ipomopsis aggregata）中得到了证实，聚合花葱是一种由蜂鸟授粉的多年生草本植物。在六次重复授粉实验中，异交距离为1至10米时，每朵花的平均结实率高于自交或异交距离超过100米时的结实率。在自然条件下种植并连续七年进行统计的实验杂交子代表现中，也出现了类似的模式。来自10米异交的子代比来自1米或100米异交的子代具有更高的存活率、更大的开花机会以及更早的开花时间。因此，来自1米和100米异交的子代，其终生适合度分别仅为来自10米异交子代的47%和68%。在1至30米的范围内，子代适合度也随着与种子亲本的种植距离增加而下降，所以对母本环境的适应性是远交衰退的一种合理机制。对聚合花葱一个代表性种群的统计表明，草本植被在2至150米的范围内会发生变化，这表明选择机制存在空间变异，其尺度与观察到的远交衰退和种植距离的影响相当。我们讨论了结实率差异可能部分反映母本配偶选择的可能性，并强调在评估异交效应时，在自然条件下测量子代适合度的必要性。
