Stowe Kirk A
Department of Biology, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, Missouri, 63121.
Evolution. 1998 Jun;52(3):703-712. doi: 10.1111/j.1558-5646.1998.tb03695.x.
The evolutionary response of plant populations to selection for increased defense may be constrained by costs of defense. The purpose of this study was to investigate such constraints on the evolution of defense due to a cost of defense manifested as a trade-off between defense and tolerance. Variation in the response to artificial damage (tolerance) among lines of Brassica rapa that had been artificially selected for foliar glucosinolate content (defense) was examined. Leaf area was removed from replicates of three selection lines (high glucosinolates, control, and low glucosinolates) at three damage levels (0%, 20%, and 60% damage). An external cost of defense would result in a statistically significant selection line by damage treatment interaction, with those selected for high defense expressing less tolerance than those selected for low defense. Damage treatment had a significant overall effect on estimated total fitness, with fitness declining with increasing damage level. Further, selection line also had a significant overall effect on estimated total fitness, with low-defense selection lines having higher fitness compared to both control and high-defense selection lines. More importantly, a cost of defense in terms of tolerance was demonstrated by a significant selection line-by-damage treatment interaction. This interaction was in the direction to demonstrate a genetic trade-off between defense and tolerance, with low-defense selection lines decreasing estimated total fitness in response to damage less than both control and high-defense selection lines. Variation in tolerance among selection lines was due to the greater ability of low-defense lines to maintain fruit and seed production despite the presence of damage. In terms of tolerance, this cost of glucosinolate production in B. rapa could constrain the evolution of increased defense and, in so doing, maintain individuals within the population that are poorly defended yet tolerant.
植物种群对增强防御的选择的进化反应可能受到防御成本的限制。本研究的目的是调查由于防御成本表现为防御与耐受性之间的权衡而对防御进化产生的此类限制。研究了在人工选择叶芥子油苷含量（防御）的甘蓝型油菜品系中，对人工损伤（耐受性）反应的差异。在三个损伤水平（0%、20%和60%损伤）下，从三个选择品系（高芥子油苷、对照和低芥子油苷）的重复样本中去除叶片面积。防御的外部成本将导致通过损伤处理相互作用产生具有统计学意义的选择品系，选择高防御的品系比选择低防御的品系表现出更低的耐受性。损伤处理对估计的总适合度有显著的总体影响，适合度随着损伤水平的增加而下降。此外，选择品系对估计的总适合度也有显著的总体影响，与对照和高防御选择品系相比，低防御选择品系具有更高的适合度。更重要的是，通过显著的选择品系×损伤处理相互作用证明了耐受性方面的防御成本。这种相互作用表明了防御与耐受性之间的遗传权衡，低防御选择品系对损伤反应导致的估计总适合度下降幅度小于对照和高防御选择品系。选择品系之间耐受性的差异是由于低防御品系尽管存在损伤仍具有更强的维持果实和种子产量的能力。就耐受性而言，甘蓝型油菜中芥子油苷产生的这种成本可能会限制增强防御的进化，并在此过程中维持种群中防御能力差但耐受性强的个体。
