Abstract
Host specialization is a ubiquitous character of phytophagous insects. The polyphagous population is usually composed of some subpopulations that can use only a few closely related plants. Cotton-melon aphids, Aphis gossypii Glover exhibited strong host specialization, and the cotton- and cucurbits-specialized biotypes had been clearly identified. However, the experimental work that addressed the roles of plant species in determining diet breadth of phytophagous insects is rare. In the present study, we took the artificial host transfer method to assess the role of two special plants, zucchini Cucurbita zucchini L. and cowpea Vigna unguiculata (Linn.) Walp, in regulating diet breadth of cotton- and cucurbits-specialized A. gossypii collected from cotton and cucumber fields and reared separately on the native host plant for ten years. The results showed that the cotton-specialized aphids did not directly use cucumber whereas the cucurbits-specialized did not use cotton regardless of the coexistence or separation of cotton and cucumber plants. Neither of the cotton- and cucurbits-specialized aphids could use capsicum Capsicum annuum, eggplant Solanum melongenahttp://en.wikipedia.org/wiki/Carolus_Linnaeus, tomato Solanum lycopersicum, maize Zea mayshttp://en.wikipedia.org/wiki/Carl_Linnaeus, and radish Raphanus sativus, however, both of them could use zucchini and cowpea. Moreover, the feeding experience on zucchini led the cotton-specialized aphids to use cucumber well and finally to be transformed into the cucurbits-specialized biotype. The short-term feeding experience on cowpea resulted in the diet breadth expansion of the cucurbits-specialized aphids to use cotton. On the other hand, the diet breadth expansion of the cucurbits- and cotton-specialized aphids was only realized by different species of plant. It concluded that the special host plant did induce the conversion of feeding habits in the cotton- and cucurbits-specialized aphids, and consequently broke the host specialization. The plant species is an underlying factor to determine the diet breadth of phytophagous insects.
Highlights
Evolutionary direction of feeding habit is various in phytophagous insects
Our results showed that the cotton-specialized aphids could not directly use cucumber, and the cucurbits-specialized aphids could not use cotton too, but both of them could use zucchini and cowpea
The fact that the two host-specialized biotypes of A. gossypii still shared the same host plants illustrated that the specialized aphid was not wholly constrained on a species of host plant
Summary
Evolutionary direction of feeding habit is various in phytophagous insects. The specialization in habitat or resource use of polyphagous insects might be a common evolutionary trend [8,9]. Nosile studied the evolution direction of resource use from phylogenies of 15 groups of phytophagous insects including walking sticks, butterflies, beetles, treehoppers and aphids, and found that the forward transition rate from generalization to specialization was significantly higher than the reverse transition rate from specialization to generalization, but specialization did not always represent an evolution dead-end [10]. The hypothesis that specialization does not restrict the further evolution of feeding habits had been proposed, the directional experimental data to support the hypothesis are still rare
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