Rideout, Elizabeth Jane
Investigating the neurobiological basis underlying the sex-specific production of courtship song in Drosophila: the roles of sex determination genes fruitless and doublesex.
PhD thesis, University of Glasgow.
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The elaborate courtship ritual performed by Drosophila melanogaster males to interest females in copulation is used as a model to investigate the genetic and neurobiological basis for the specification of complex behaviours. One component of this courtship ritual is the ‘lovesong’, which both promotes mating and carries vital species-specific information. Sex mosaic studies have shown that specific regions of the central nervous system (CNS) must be male in order to produce this courtship song. Indeed, two genes in the sex determination hierarchy, fruitless (fru) and doublesex (dsx), which are responsible for sexual differentiation in both the CNS and soma, are required for wild-type song production. However, the critical differences underlying the sexually dimorphic production of courtship song are unknown; moreover, the relative contributions of fru and dsx to the generation of these differences are not known.
The central aim of this thesis was to investigate the neurobiological basis for the sexually dimorphic production of courtship song; and in addition, to determine the individual and combined contributions of fru and dsx in creating these essential differences. The long-term goal of this study was to determine the mechanism by which genetic factors such as fru and dsx can create the potential for courtship song by specifying aspects of CNS development and function. As a first step towards this long-term goal, the work presented in this thesis has identified the individual and combined contributions of fru and dsx to the production of courtship song. Moreover, a requirement for both fru and dsx in the specification of a neural substrate underlying courtship song was shown.
Using females constitutively expressing the male-specific fru isoforms, the individual contribution of fru to the production of courtship song was determined; and the amount and quality of song generated by these females was found to be highly aberrant. Thus fru alone does not specify courtship song production. In fact, wild-type courtship song was only achieved when females expressed the male-specific isoforms of both fru and dsx, demonstrating that both genes are required for the specification of courtship song.
Next, the co-expression of fru and dsx was examined, where fru and dsx were found to be co-expressed in three distinct regions of the CNS. One of these regions, the mesothoracic ganglion (Msg), is strongly implicated in the production of courtship song, suggesting that fru and dsx may act in concert to specify courtship song production. A closer examination of this region revealed a male-specific population of fru-expressing neurons in the Msg. This population of neurons was not present in females constitutively expressing fru’s male-specific isoforms, demonstrating that fru alone does not specify the development of a sexually dimorphic Msg. Instead, a critical requirement for dsx, alongside fru, in the specification of this sexually dimorphic population of neurons was shown. Thus a critical requirement for both fru and dsx in the specification of courtship song, and in the creation of a sexually dimorphic population of neurons in the Msg has been revealed.
Finally, to determine how this sexually dimorphic population of neurons is linked to courtship song production, this thesis described the generation of genetic tools to allow the visualization and manipulation of these sex-specific neurons. Using the UAS/GAL4 system of targeted gene expression, another sexually dimorphic population of neurons was identified in the posterior brain, which is, like the Msg, a region where fru and dsx are co-expressed. Together, these results suggest that fru and dsx act to generate sexually dimorphic populations of neurons in regions of co-expression, where these neurons may form part of the sex-specific neural circuits underlying the performance of sexually dimorphic behaviours.
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