Genetic analysis of differences in eye and face morphology between Drosophila simulans and Drosophila mauritiana. Alistair P. McGregor¹, Saad Arif¹, Maarten Hilbrant¹, Corinna Hopfen^2,3, Isabel Almudi¹, Maria D. S. Nunes¹, Nico Posnien^1,4, Linta Kuncheria¹, Kentaro Tanaka⁵, Philipp Mitteroecker⁶, Christian Schötterer². 1) Oxford Brookes University, Oxford, United Kingdom; 2) Institute for Population Genetics, Vetmeduni Vienna, Vienna, Austria; 3) Max Planck Institute for Biology of Ageing, Cologne, Germany; 4) Department of Developmental Biology, Georg August University, Göttingen; 5) Department of Population Genetics, National Institute of Genetics, Mishima, Shizuoka, Japan; 6) Department of Theoretical Biology, University of Vienna, Vienna, Austria.

   D. melanogaster subgroup species exhibit considerable variation in head and eye morphology. While all of these species exhibit a negative correlation between eye and face size, D. mauritiana generally has bigger eyes composed of larger ommatidia and conversely a narrower face than its sibling species. To better understand the evolution of these differences, we carried out QTL mapping of eye size and face width differences between D. mauritiana and D. simulans. We found that the major loci responsible for the differences in eye and face size between these species map to the chromosome X and 3L respectively, and to distinct regions of chromosome 2. We confirmed this finding by independently introgressing regions of chromosome X and 3L from D. mauritiana into D. simulans, which resulted in flies with larger eyes but no significant difference in face width for the X chromosome region and vice versa for the region on chromosome 3L. Fine mapping of these regions identified a number of candidate genes for these differences in eye size and face width. We also found that the difference in face width is detectable earlier in the development than the difference in the size of the retinal field. Our results suggest that different loci that act at different developmental stages underlie changes in eye and face width. Therefore, while there is a negative correlation between these traits in Drosophila, we show genetically that they also have the potential to evolve independently.