The Interplay Between Developmental Stage and Environment Underlies the Adaptive Effect of a Natural Transposable Element Insertion.

Establishing causal links between adaptive mutations and ecologically relevant phenotypes is key to understanding the process of adaptation, which is a central goal in evolutionary biology with applications for conservation, medicine, and agriculture. Yet despite recent progress, the number of identified causal adaptive mutations remains limited. Linking genetic variation to fitness-related effects is complicated by gene-by-gene and gene-by-environment interactions, among other processes. Transposable elements, which are often ignored in the quest for the genetic basis of adaptive evolution, are a genome-wide source of regulatory elements across organisms that can potentially result in adaptive phenotypes. In this work, we combine gene expression, in vivo reporter assays, CRISPR/Cas9 genome editing, and survival experiments to characterize in detail the molecular and phenotypic consequences of a natural Drosophila melanogaster transposable element insertion: the roo solo-LTR FBti0019985. This transposable element provides an alternative promoter to the transcription factor Lime, involved in cold- and immune-stress responses. We found that the effect of FBti0019985 on Lime expression depends on the interplay between the developmental stage and environmental condition. We further establish a causal link between the presence of FBti0019985 and increased survival to cold- and immune-stress. Our results exemplify how several developmental stages and environmental conditions need to be considered to characterize the molecular and functional effects of a genetic variant, and add to the growing body of evidence that transposable elements can induce complex mutations with ecologically relevant effects.