Gene expression responses of Bactrocera tryoni larvae feeding on different ripening stages of tomato fruit

The Queensland fruit fly, Bactrocera tryoni, is an economically important pest insect of horticultural crops in Australia, damaging a wide variety of fruits and vegetables. To study the B. tryoni–fruit interaction, we conducted a transcriptomic investigation of B. tryoni larvae feeding on three ripening stages of tomato fruit, which had been previously shown to influence larval performance. Fruit fly larval survival in immature tomato fruit is very poor, which we hypothesized to be due to the effect of tomato fruit toxins which are mostly expressed in green fruit. We found 2709 transcripts differentially expressed in larvae feeding on the three ripening stages. The transcript expression pattern was most divergent between larvae feeding in immature fruit compared with fully ripe fruit, with intermediate expression in larvae feeding in colour-break fruit. Gene Set Enrichment Analysis showed that larvae fed in immature fruit had a higher number of depleted GO terms in comparison with larvae fed on fully ripe fruit, particularly GO terms associated with larval development, metabolism and basic cellular functions. Enriched GO terms in larvae fed on fully ripe fruit showed the opposite pattern, with a greater number of enriched terms related to larval growth and detoxification processes. We concluded that poor larval survival in immature-green fruit is likely because the substrate offers a very poor diet for larvae, which limits normal developmental functions. By contrast, in colour-break and fully ripe fruit the substrate allows normal development, including the expression of metabolically costly detoxification mechanisms.