Singular and stacked corn (Zea mays): a study of tritrophic interactions

Abstract

Plants respond to insect injury and egg laying by synthesizing a complex mixture of volatile compounds, which are important chemical cues used by herbivores and natural enemies. With advances in molecular biology, it has been possible to develop genetically modified plants that have herbicide tolerance/insect resistance (stacked events), and are herbicide tolerant or pest resistant (singular events). Therefore, studies are needed to evaluate new transgenic technologies in the context of tritrophic interactions (plant-herbivore-natural enemy). The present study was divided into three articles. In the first article, the objective was to evaluate if events of maize, singular and stacked interfere in feeding preference and oviposition of Spodoptera frugiperda. The hybrids with the protein insertion of Bacillus thuringiensis (Bt), DKB390 YieldGard VTPROTM, DKB390 VTPRO2TM, DKB390 VTPRO3®, DKB390 (isogenic) and Ag 3700 RR2 were evaluated (the same hybrids were used in articles 2 and 3). In the tests with larvae, the food preference between transgenic and non-transgenic hybrid was evaluated, besides biological parameters. In the experiments with moths, oviposition preference was evaluated by greenhouse and field tests. Adult females were strongly attracted to transgenic hybrids, specifically Bt hybrids. However, the larvae selected the non-Bt hybrid in the choice tests. The results show that there is no relationship between larvae and moth preference. There were no significant effects on the behavior of choice regarding singular and stacked events. In the second article, the olfactory behavioral responses of the egg parasitoid Trichogramma pretiosum (Hymenoptera: Trichogrammatidae) to the volatiles emitted by single and stacked maize (Zea mays) events induced by S. frugiperda during the day and night were investigated. Behavioral assays with parasitoid wasps were conducted in a "Y" type olfactor. Possible effects of genetic modification on changes in gene expression and production of volatile compounds were evaluated. Using the technique of real time PCR (qRTPCR) was evaluated the gene expression of plants subjected to herbivory induction, seeking to understand if the insertion of one or more Bt proteins and the induction time interfere in the expression of constitutive genes of the plant. GC-MS results showed that herbivory-induced compounds were released during the day and night. However, some chemical compounds were emitted exclusively at night and were highly attractive to the parasitoid. Volatiles emitted by non-Bt plants were more attractive to parasitoids than those released by Bt plants. However, the glyphosate tolerant transgenic event was more preferred than the isogenic form. The genes studied showed higher expression during nocturnal induction. In addition, the pattern of gene expression varied among the tested hybrids. There were no significant differences in attractiveness between singular and stacked events, as well as volatile compound emissions between Bt hybrids. In the third article we investigated the behavioral olfactory responses of T. pretiosum to volatile mixtures emitted by singular maize and stacked events after S. frugiperda oviposition. Possible variations in gene expression and volatile compounds induced by oviposition were examined. The results show that T.pretiosum is attracted by the volatiles of transgenic hybrids induced after oviposition. Additionally, some chemical compounds were emitted exclusively by transgenic hybrids, when induced by oviposition. The relative expression of TPS10, LOX and STC genes was higher in transgenic plants. It was concluded that there were differences between Bt and non Bt hybrids, however no significant differences were observed between singular and staked events in the tritrophic interactions.