Organisms exhibit various color patterns, which evolve through the processes of de novo origination and modification. While both of these modes are essential for the evolution of patterns, less attention has been given to understanding how the latter process takes place. The diverse wing pigmentation patterns in Drosophila species, ranging from a single dark spot to a complex mosaic pattern, is a good model system to investigate the genetic basis underlying de novo as well as modification processes. In particular, the male wings of the species of the suzukii subgroup, Drosophila suzukii, D. biarmipes, and D. subpulchrella, have a single dark spot at the distal tip that have undergone slight shifts regarding the position and size. The “shift” represents a modification process of the newly formed spot in the lineage leading to these species. In this study, we investigate how this modification process took place by analyzing the association with vein positions and the locations of Quantitative Trait Loci (QTL) using these species.
The wing pigmentation pattern in males was quantified as the relative wing spot position and size along the first and second longitudinal veins (L1 and L2). The measurement of multiple strains of these species indicated that the spot position of D. subpulchrella is located closer to the proximal direction of the wing and the spot size is significantly larger compared to those of the other two species. These comparisons inferred the wing pigmentation of D. subpulchrella is a derived phenotype that has evolved after the split from D. biarmipes. The intersection of the L1 and L2 vein, close to the area where the spot forms, has also shifted towards the proximal direction in D. subpulchrella.
To identify the genomic regions that are responsible for the shifts in wing spot pigmentation traits between D. suzukii and D. subpulchrella, we performed a QTL analysis using backcrossed individuals. The result revealed that the genomic regions that are responsible for wing spot size are on chromosome X and the right arm of chromosome 3. These regions consist of large structural rearrangements between the two species. Moreover, the semi-introgression lines using the introduced visible markers on those two genomic regions are being generated to examine the effects of these unique genomic regions at a sub-chromosomal scale.