Gange Alan C, Nice Helen E
School of Biological Sciences, Royal Holloway University of London, Egham Hill, Egham, Surrey TW20 OEX, UK.
New Phytol. 1997 Oct;137(2):335-343. doi: 10.1046/j.1469-8137.1997.00813.x.
Many hypotheses have been developed to explain the adaptive nature of insect galls. One of these, the nutrition hypothesis, states that gall formers have advantages over other insects because gall tissue provides a better (higher quality) food source than unmodified tissue. However, this has rarely been experimentally tested. In a test of this hypothesis, we grew plants of Cirsium arvense (L.) Scop. in a factorial design with two main treatments: the addition of nitrogen (to enhance foliar N levels) and of fungicide (to reduce colonization of roots by arbuscular mycorrhizal fungi). Mycorrhizal fungi have been shown previously to reduce the N concentration of host plants. Plants were exposed to adult gall flies, Urophora cardui L., and maintained through one season to allow maturation of galls. Reduction of the percentage mycorrhizal colonization by fungicide resulted in an elevation of total stem N comparable to that achieved by N addition, but gall N concentration remained unchanged in all treatments. Nitrogen application elevated stem N levels when mycorrhizal fungi were present, but application of both compounds together did not result in any increase over either single treatment. Fungicide application resulted in larger galls, which contained more larval chambers, with more live, and heavier, larvae. However, the main effects of N were not significant, as N addition only increased fly performance on plants where mycorrhizas were not reduced. It is suggested that U. cardui gall inhabitants can manipulate N at an optimal level and thus might conform to a modified version of the nutrition hypothesis. Mycorrhizal colonization might reduce gall fly performance by delaying the appearance, or impairing the quality, of secondary nutritive tissue in the gall. Future tests of the nutrition hypothesis should include a consideration of the plant's mycorrhizal status.
人们已经提出了许多假说来解释虫瘿的适应性本质。其中之一是营养假说，该假说认为形成虫瘿的昆虫比其他昆虫具有优势，因为虫瘿组织比未改变的组织提供了更好（更高质量）的食物来源。然而，这很少经过实验验证。在对这一假说的测试中，我们采用析因设计种植了田蓟（Cirsium arvense (L.) Scop.），有两个主要处理：添加氮（以提高叶片氮水平）和添加杀菌剂（以减少丛枝菌根真菌对根部的定殖）。先前已表明菌根真菌会降低宿主植物的氮浓度。将植物暴露于成年瘿蚊（Urophora cardui L.）中，并维持一个季节以使虫瘿成熟。杀菌剂降低菌根定殖百分比导致总茎氮含量升高，与添加氮所达到的水平相当，但在所有处理中虫瘿氮浓度保持不变。当存在菌根真菌时，施氮提高了茎氮水平，但同时施用这两种化合物并没有比单一处理产生更大的增加。施用杀菌剂导致虫瘿更大，其中包含更多的幼虫室，有更多存活且更重的幼虫。然而，氮的主要影响并不显著，因为仅在菌根未减少的植物上添加氮才提高了瘿蚊的表现。有人认为，U. cardui虫瘿内的昆虫可以将氮调节到最佳水平，因此可能符合营养假说的一个修正版本。菌根定殖可能通过延迟虫瘿中次生营养组织的出现或损害其质量来降低瘿蚊的表现。未来对营养假说的测试应考虑植物的菌根状态。
