Department of Biology, University of Virginia, Charlottesville, Virginia, 22904, USA.
Department of Integrative Biology, University of South Florida, Tampa, Florida, 33620, USA.
Ecology. 2017 Apr;98(4):1036-1048. doi: 10.1002/ecy.1731.
The latitudinal herbivory defense hypothesis (LHDH) postulates that the prevalence of species interactions, including herbivory, is greater at lower latitudes, leading to selection for increased levels of plant defense. While latitudinal defense clines may be caused by spatial variation in herbivore pressure, optimal defense theory predicts that clines could also be caused by ecogeographic variation in the cost of defense. For instance, allocation of resources to defense may not increase plant fitness when growing seasons are short and plants must reproduce quickly. Here we use a common garden experiment to survey genetic variation for constitutive and induced phenylpropanoid glycoside (PPG) concentrations across 35 Mimulus guttatus populations over a ~13° latitudinal transect. Our sampling regime is unique among studies of the LHDH in that it allows us to disentangle the effects of growing season length from those of latitude, temperature, and elevation. For five of the seven PPGs surveyed, we find associations between latitude and plant defense that are robust to population structure. However, contrary to the LHDH, only two PPGs were found at higher levels in low latitude populations, and total PPG concentrations were higher at higher latitudes. PPG levels are strongly correlated with growing season length, with higher levels of PPGs in plants from areas with longer growing seasons. Further, flowering time is positively correlated with the concentration of nearly all PPGs, suggesting that there may be a strong trade-off between development time and defense production. Our results reveal that ecogeographic patterns in plant defense may reflect variation in the cost of producing defense compounds in addition to variation in herbivore pressure. Thus, the biogeographic pattern predicted by the LHDH may not be accurate because the underlying factors driving variation in defense, in this case, growing season length, are not always associated with latitude in the same manner. Given these results, we conclude that LHDH cannot be interpreted without considering life history, and we recommend that future work on the LHDH move beyond solely testing the core LHDH prediction and place greater emphasis on isolating agents of selection that generate spatial variation in defense and herbivore pressure.
纬度食草防御假说（LHDH）认为，包括食草在内的物种相互作用的流行程度在较低纬度地区更高，从而导致植物防御水平的提高。虽然纬度防御梯度可能是由食草动物压力的空间变化引起的，但最优防御理论预测，梯度也可能是由防御成本的生态地理变化引起的。例如，当生长季节较短且植物必须快速繁殖时，将资源分配到防御上可能不会增加植物的适应性。在这里，我们使用一个常见的花园实验，在一个约 13°的纬度范围内，对 35 个金光菊属（Mimulus guttatus）种群的组成性和诱导性苯丙素糖苷（PPG）浓度的遗传变异进行了调查。我们的采样方案在 LHDH 的研究中是独特的，因为它允许我们将生长季节长度的影响与纬度、温度和海拔的影响分开。在所调查的 7 种 PPG 中有 5 种，我们发现了与纬度有关的植物防御之间的关联，这种关联不受种群结构的影响。然而，与 LHDH 相反，只有两种 PPG 在低纬度种群中水平较高，而总 PPG 浓度在高纬度较高。PPG 水平与生长季节长度密切相关，生长季节较长的地区植物的 PPG 水平较高。此外，开花时间与几乎所有 PPG 的浓度呈正相关，这表明在发育时间和防御产物之间可能存在强烈的权衡。我们的研究结果表明，植物防御的生态地理模式可能反映了除食草动物压力变化外，防御化合物生产成本变化的差异。因此，LHDH 的生物地理模式可能不准确，因为在这种情况下，驱动防御变化的潜在因素（在这种情况下，生长季节长度）并不总是以相同的方式与纬度相关。考虑到这些结果，我们得出结论，在不考虑生活史的情况下，不能解释 LHDH，我们建议未来对 LHDH 的研究不仅仅是测试核心 LHDH 预测，而是更加重视分离产生防御和食草动物压力空间变化的选择代理。
