Abstract
Generalist insect can utilize two different modes for regulating their detoxification genes, the constitutive mode and the induced mode. Here, we used the Bemisia tabaci sibling species MEAM1 and MED, as a model system for studying constitutive and induced detoxification resistance and their associated tradeoffs. B. tabaci adults were allowed to feed through membranes for 24 h on diet containing only sucrose or sucrose with various phytotoxins. Quantitative real-time PCR analyses of 18 detoxification genes, indicated that relatively few transcripts were changed in both the MEAM1 and MED species, in response to the addition of phytotoxins to the diet. Induced transcription of detoxification genes only in the MED species, in response to the presence of indole-3-carbinol in the insect’s diet, was correlated with maintenance of reproductive performance in comparison to significant reduction in performance of the MEAM1 species. Three genes, COE2, CYP6-like 5 and BtGST2, responded to more than one compound and were highly transcribed in the insect gut. Furthermore, functional assays showed that the BtGST2 gene encodes a protein capable of interacting with both flavonoids and glucosinolates. In conclusion, several detoxification genes were identified that could potentially be involved in the adaptation of B. tabaci to its host plants.
Highlights
Generalist insect can utilize two different modes for regulating their detoxification genes, the constitutive mode and the induced mode
We focused on four main topics: (I) The extent to which detoxification genes of B. tabaci (MEAM1 and MED) are induced in response to the presence of various phytotoxins in the insect diet? (II) How specific is the induced response? In other words, which mechanism rules: “specific gene/s for specific phytotoxin” or “few detoxifiers for many phytotoxins”? (III) How much do MEAM1 and MED differ in their constitutive and induced transcription of detoxification genes? Can these differences be related to ecological traits such as reproductive performance? (IV) Whether the induced transcription of detoxification genes might be related to a possible detoxification role of the encoded enzymes?
When the constitutive transcription levels of the detoxification genes were compared between MEAM1 and MED adults feeding on sucrose diet lacking phytotoxins (‘sucrose only’), it was found that eight of the 18 genes were transcribed significantly higher in MED while only two genes were transcribed significantly higher in MEAM1 (Fig. 1b)
Summary
Generalist insect can utilize two different modes for regulating their detoxification genes, the constitutive mode and the induced mode. Insect herbivores may utilize two different modes (strategies) for regulating genes coding for enzymes involved in detoxification resistance to plant defensive compounds: the constitutive mode, in which the detoxification genes are transcribed independent of encountering a defended plant, and the induced mode, in which the detoxification genes are activated only after contact with the plant toxic chemistry[16]. Despite the fact that transcriptional regulation of detoxification enzymes in response to dietary constituents should be challenging for broadly polyphagous species, which can encounter dozens of biosynthetically distinct toxins across their host range[13], induced detoxification resistance against phytotoxins was speculated to be the more widespread phenomenon in generalist herbivores, whereas constitutive resistance was considered to be rare and restricted to specialists[4]
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