The differential expression of the selected immune- and stress-related genes used to determine the efficacy of our induction regime is in line with previous studies [6
]. Highest induction levels were recorded for the antimicrobial peptides upon injection of PGN, which is known for its high potency to elicit innate immune responses, whereas highest expression levels of the heat shock proteins occurred upon mild heat shock. The unexpected induction of defensin 3 and thaumatin in response to starvation is in agreement with the recently discovered expression of antimicrobial peptides in food-deprived Drosophila melanogaster.
The nuclear transcription factor FOXO, a key regulator of stress resistance, metabolism and ageing, has been attributed to mediate AMP expression in response to starvation in this model insect [12
]. The determined induction of CYP450 in female Tribolium
reflects solely a gender-specific response to injected PGN which can be explained by the need to protect developing eggs from toxins associated with pathogens. A similar female-specific investment in offspring explains the higher expression of HSP63 and HSP90 in female beetles upon mild heat shock (). At least HSP90 is known to function as a chaperon and to play a role in oogenesis [5
]. In sum, our real-time PCR-based approach provides evidence for differential- and gender-specific expression of immune- and stress-related genes upon exposure to environmental stressors. Since it has been established that miRNAs can target genes for translational inhibition or mRNA degradation [13
], we expected also diverse expression patterns for the selected miRNAs. To fill the growing gap between the increasing number of predicted miRNAs added to the global miRNA database and information about their expression and function [8
], we investigated whether particular miRNAs contribute to regulation of genes which are differentially expressed in response to biotic and abiotic stress. Indeed, 54 per cent out of a total of 455 microRNAs displayed stressor-dependent differential- and/or gender-specific expression.
Based on our findings, it is intriguing to speculate whether the observation that immediate stress (such as heat) result in a larger fraction of regulated miRNAs, while longer-term stress (e.g. starvation) lead to a lower number of differentially expressed miRNAs, could be advantageous for plastic responses of Tribolium
to environmental stress. This ultimately could also explain the remarkably higher number of miRNAs upregulated in females upon exposure to stress when compared with males (table 1). This discovered discrepancy in gender-specific miRNA expression could support the hypothesis called Batemańs principle in immunity [9
] because males gain fitness by increasing their mating success whereas females increase fitness through longevity since their reproductive effort is higher [10
]. Consequently, their ability to cope with environmental stressors should be higher than in males and this is reflected by the determined broader spectrum of miRNAs involved in regulation of immune and/or stress responses. While the precise function of individual miRNAs remains to be elucidated, our study provides for the first time information about miRNAs in T. castaneum
exhibiting differential- and gender-specific expression pattern upon exposure to different stressors. Whether these changes translate into environmentally induced heritable epigenetic changes [14
] remains to be elucidated.