Differences inorthodenticleexpression promote ommatidial size variation betweenDrosophilaspecies

AbstractThe compound eyes of insects exhibit extensive variation in ommatidia number and size, which affects how they see and underlies adaptations in their vision to different environments and lifestyles. However, very little is known about the genetic and developmental bases underlying differences in compound eye size. We previously showed that the larger eyes ofDrosophila mauritianacompared toD. simulansis caused by differences in ommatidia size rather than number. Furthermore, we identified an X-linked chromosomal region inD. mauritianathat results in larger eyes when introgressed intoD. simulans. Here, we used a combination of fine-scale mapping and gene expression analysis to further investigate positional candidate genes on the X chromosome. We found thatorthodenticleis expressed earlier inD. mauritianathan inD. simulansduring ommatidial maturation in third instar larvae, and we further show that this gene is required for the correct organisation and size of ommatidia inD. melanogaster. Using ATAC-seq, we have identified several candidate eye enhancers ofotdas well as potential direct targets of this transcription factor that are differentially expressed betweenD. mauritianaandD. simulans. Taken together, our results suggest that differential timing ofotdexpression contributes to natural variation in ommatidia size betweenD. mauritianaandD. simulans, which provides new insights into the mechanisms underlying the regulation and evolution of compound eye size in insects.