Transcriptomic analysis of inbreeding depression in cold-sensitive Drosophila melanogaster shows upregulation of the immune response

In sexually reproducing species, increased homozygosity often causes a decline in fitness, called inbreeding depression. Recently, researchers started describing the functional genomic changes that occur during inbreeding, both in benign conditions and under environmental stress. To further this aim, we have performed a genome-wide gene expression study of inbreeding depression, manifesting as cold sensitivity and conditional lethality. Our focus was to describe general patterns of gene expression during inbreeding depression and to identify specific processes affected in our line. There was a clear difference in gene expression between the stressful restrictive environment and the benign permissive environment in both the affected inbred line and the inbred control line. We noted a strong inbreeding-by-environment interaction, whereby virtually all transcriptional differences between lines were found in the restrictive environment. Functional annotation showed enrichment of transcripts coding for serine proteases and their inhibitors (serpins and BPTI/Kunitz family), which indicates activation of the innate immune response. These genes have previously been shown to respond transcriptionally to cold stress, suggesting the conditional lethal effect is associated with an exaggerated cold stress response. The set of differentially expressed genes significantly overlapped with those found in three other studies of inbreeding depression, demonstrating that it is possible to detect a common signature across different genetic backgrounds.