Abstract
BackgroundWhile the transcription of innate immunity genes in response to bacterial infection has been well-characterised in the Drosophila model, we recently demonstrated the capacity for such transcription to evolve in flies selected for improved antibacterial defense. Here we use this experimental system to examine how insects invest in constitutive versus infection-induced transcription of immunity genes. These two strategies carry with them different consequences with respect to energetic and pleiotropic costs and may be more or less effective in improving defense depending on whether the genes contribute to humoral or cellular aspects of immunity.FindingsContrary to expectation we show that selection preferentially increased the infection-induced expression of both cellular and humoral immunity genes. Given their functional roles, infection induced increases in expression were expected for the humoral genes, while increases in constitutive expression were expected for the cellular genes. We also report a restricted ability to improve transcription of immunity genes that is on the order of 2-3 fold regardless of total transcription level of the gene.ConclusionsThe evolved increases in infection-induced expression of the cellular genes may result from specific cross talk with humoral pathways or from generalised strategies for enhancing immunity gene transcription. A failure to see improvements in constitutive expression of the cellular genes suggests either that increases might come at too great a cost or that patterns of expression in adults are decoupled from the larval phase where increases would be most effective. The similarity in fold change increase across all immunity genes may suggest a shared mechanism for the evolution of increased transcription in small, discrete units such as duplication of cis-regulatory elements.
Highlights
Using selection experiments, we examined the ability of Drosophila melanogaster to evolve in response to systemic infection by the opportunistic bacterial pathogen, Pseudomonas aeruginosa [1]
The evolved increases in infection-induced expression of the cellular genes may result from specific cross talk with humoral pathways or from generalised strategies for enhancing immunity gene transcription
A failure to see improvements in constitutive expression of the cellular genes suggests either that increases might come at too great a cost or that patterns of expression in adults are decoupled from the larval phase where increases would be most effective
Summary
Using selection experiments, we examined the ability of Drosophila melanogaster to evolve in response to systemic infection by the opportunistic bacterial pathogen, Pseudomonas aeruginosa [1]. While the original study demonstrated the involvement of both cellular and humoral aspects of insect immunity in the evolved defense, the design did not allow for the partitioning of the total transcriptional change into its constitutive versus infection-induced components These two avenues for transcriptional change should have different energetic or pleiotropic costs and [2] may be more or less effective in improving immunity depending on the functional role of the gene. While the transcription of innate immunity genes in response to bacterial infection has been wellcharacterised in the Drosophila model, we recently demonstrated the capacity for such transcription to evolve in flies selected for improved antibacterial defense We use this experimental system to examine how insects invest in constitutive versus infection-induced transcription of immunity genes. These two strategies carry with them different consequences with respect to energetic and pleiotropic costs and may be more or less effective in improving defense depending on whether the genes contribute to humoral or cellular aspects of immunity
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