Evolution and genetics of functionally-related traits, male wing pigmentation and courtship behavior, in the oriental Drosophila melanogaster species group

The evolution of one morphological trait usually depends on or is facilitated by functionally related traits, such as physiological, behavioral, or other morphological traits. In the Oriental Drosophila melanogaster species group, some species possess male-specific pigmentation in the apical wing area, referred to as wing spots. Wing spots have been proposed to evolutionarily couple with frontal-wing-display in courtship. But the proposal that sexual selection shapes the evolution of wing spots is still untested. In theory, sexual selection on a wing spot display could be promoted by genetic correlations between the pigmentation and behavioral components of this complex trait. How often this scenario might occur nature is unknown. In this dissertation, the functionality, evolutionary history, and genetic architecture of wing spots and courtship behavior were explored using various approaches. First, wing spots were found to serve as a visual stimulus via frontal wing display in behavioral essays, which support the hypothesis that sexual selection drives the evolution of wing spots and wing display behavior. Second, the association between wing pigmentation and wing display behavior was confirmed to be robust phylogenetically. This part of the study also revealed that courtship songs, another male display component, evolve rapidly and exhibit novel features in this clade. Third, genetic analysis in a pair of crossable species, one of which having lost both wing spots and wing displays, elucidated the genetic correlation of these two traits. Interestingly, the major quantitative trait locus (QTL) for wing pigmentation and one QTL for wing display behavior are located on the same region of the X chromosome, indicating either close linkage or pleiotropy. This finding provides a possible genetic mechanism for the coordinate evolution of these two traits. A central focus of modern evolutionary biology has been on the genetic underpinnings of morphological evolution. This study takes this question one step further by providing insights on the evolutionary integration of a morphological trait and the behavior by which it is utilized.