Abstract
Insect compound eyes exhibit striking variation in size caused by changes in the number and/or diameter of ommatidia. These changes can affect the contrast sensitivity and visual acuity of the vision of these animals thereby facilitating adaptations to different lifestyles and habitats. However, the genetic basis of changes in insect eye size is poorly understood. Here, we describe extensive eye size variation within and between species of the Drosophila melanogaster species subgroup, which offers an opportunity to address both the genetic and developmental bases of size variation. We mapped quantitative trait loci (QTL) in D. melanogaster and D. simulans underlying intra-specific variation in eye size, mainly caused by differences in ommatidia number, and in inter-ocular distance. We found that in both cases QTL appear to be largely overlapping for both of these head traits, suggesting that the same loci may expand eye size at the expense of face cuticle and vice versa. Furthermore, although these traits have low heritability and are polygenic, we were able to identify some QTL, whose location estimates overlap between these two species. This may indicate some common genetic basis for eye size variation between these two species. However, we observed differences in eye fate commitment between strains of D. melanogaster and D. simulans suggesting different developmental mechanisms contribute to eye size variation in these species. Considering the results of previous studies, our findings suggest that the gene regulatory network that specifies eye size has evolved at multiple nodes to give rise to natural variation in this trait within and among species.