Johnson Scott N, Douglas Angela E, Woodward Stephen, Hartley Susan E
Department of Biology, University of York, P.O. Box 373, York, YO10 5YW UK.
Oecologia. 2003 Feb;134(3):388-96. doi: 10.1007/s00442-002-1139-6. Epub 2003 Jan 9.
The role of indirect interactions in structuring communities is becoming increasingly recognised. Plant fungi can bring about changes in plant chemistry which may affect insect herbivores that share the same plant, and hence the two may interact indirectly. This study investigated the indirect effects of a fungal pathogen ( Marssonina betulae) of silver birch ( Betula pendula) on an aphid ( Euceraphis betulae), and the processes underpinning the interaction. There was a strong positive association between natural populations of the aphid and leaves bearing high fungal infection. In choice tests, significantly more aphids settled on leaves inoculated with the fungus than on asymptomatic leaves. Individual aphids reared on inoculated leaves were heavier, possessed longer hind tibiae and displayed enhanced embryo development compared with aphids reared on asymptomatic leaves; population growth rate was also positively correlated with fungal infection when groups of aphids were reared on inoculated branches. Changes in leaf chemistry were associated with fungal infection with inoculated leaves containing higher concentrations of free-amino acids. This may reflect a plant-initiated response to fungal attack in which free amino acids from the degradation of mesophyll cells are translocated out of infected leaves via the phloem. These changes in plant chemistry are similar to those occurring during leaf senescence, and are proposed as the mechanistic basis for the positive interaction between the fungus and aphid.
间接相互作用在构建群落中的作用正日益受到认可。植物真菌可引起植物化学变化，这可能会影响取食同一植物的植食性昆虫，因此二者可能存在间接相互作用。本研究调查了白桦（Betula pendula）的一种真菌病原体（桦叶栅锈菌Marssonina betulae）对一种蚜虫（桦大蚜Euceraphis betulae）的间接影响，以及支撑这种相互作用的过程。在自然种群中，该蚜虫与真菌感染严重的叶片之间存在很强的正相关关系。在选择试验中，与无症状叶片相比，显著更多的蚜虫落在接种了真菌的叶片上。与在无症状叶片上饲养的蚜虫相比，在接种叶片上饲养的单个蚜虫体重更重、后胫节更长，且胚胎发育增强；当蚜虫群体在接种枝条上饲养时，种群增长率也与真菌感染呈正相关。叶片化学变化与真菌感染有关，接种叶片含有更高浓度的游离氨基酸。这可能反映了植物对真菌攻击的一种初始反应，即叶肉细胞降解产生的游离氨基酸通过韧皮部转运出受感染的叶片。这些植物化学变化与叶片衰老过程中发生的变化相似，并被认为是真菌与蚜虫之间正相互作用的机制基础。
