The sex comb an array of specialized bristles on the foreleg, is a highly variable male trait of Drosophila that provides an ideal system for integrative studies of morphological evolution. Here, studies of the genetic and developmental architecture of sex comb bristle number variation in Drosophila melanogaster are described. Analysis of the response to twenty-four generations of divergent artificial selection indicated high genetic variance underlying this trait, and demonstrated a weak relationship with other, developmentally related non-sex bristle systems. I also present evidence showing bristle number is associated with mating success. Manipulation of diet in full-sib families confirmed that this trait is condition dependent, and that there is a genetic basis for condition dependence. Further partitioning of variance components using a half-sib mating design revealed a strong maternal, dominance and/or X chromosome effect on sex comb bristle number variation. Finally, sex comb bristle number was not correlated with comb orientation in wild type, High and Low artificial selection lines, or the mutant strain bric a brac PR72. Analysis of patterns of variation in comb orientation over ontogeny in these lines showed that this aspect of the sex comb phenotype is highly canalized. This body of work provides important insight into D. melanogaster sex comb evolvability, and represents a timely approach to bridging the gap between population genetics and development in studies of phenotypic evolution. / Thesis / Doctor of Philosophy (PhD)

This study is primarily concerned with assessing the influence of the sex determination genes, transformer (tra), doublesex (dsx) and fruitless (fru) on three sexually dimorphic traits within Drosophila; pheromone blend, courtship song and sex comb tooth number. The sex determination loci have all been implicated as possible candidate genes affecting these important traits that contribute to sexual isolation, which is a major cause of speciation. Quantitative Trait Loci (QTL) analysis is used to assess the effects of these known candidate genes on the naturally occurring variation of mean interpulse interval (IPI) of courtship song and the differing pheromone blend profiles between Drosophila simulans and D. sechellia. The QTL analysis for both song and pheromone blend variation incorporated Multiple Interval Mapping (MIM), which enables the detection for epistasis. The desaturase loci desat1, desat2 and desatF were also included in the assessment on pheromone blends (cuticular hydrocarbon compounds), since they facilitate ecological adaptation and are also candidate genes, which are likely to exert a large affect on this particular trait. The sex determination genes were not significantly influential on the interspecific variation of the cuticular hydrocarbon compounds between these two sibling species. However significant effects were detected from two of the desaturase loci. desat1 was associated with a strong effect on the interspecific variation of a saturated hydrocarbon chain compound (unbranched-23). Additionally the candidate gene desatF potentially exerts an influence on the variation of 7,11-heptacosadiene, through a large epistatic effect with unidentified loci, situated between the markers pros and Mtn. The candidate gene eloF is situated in this region, and is known to affect the elongation of unsaturated hydrocarbon chains. The QTL associated with the marker desatF influenced the variation of both diene compounds (7,11-heptacosadiene and 7,11-pentacosadiene). Intriguingly epistasis was only detected for the variation of these two diene compounds. The MIM analysis assessing the affects of the sex determination genes on interspecific variation of mean IPI detected the candidate gene fru as the closest marker associated with a significant QTL on the third chromosome. The MIM also found a significant QTL associated with the marker Dgα situated on the second chromosome. Moreover significant epistatic interactions were detected between a further QTL situated nearest the marker forked on the X-chromosome with both of the other significant QTL situated on the third and second chromosomes. The analysis of a number of Recombinant Inbred (RI) lines was also carried out to test for the affects of the sex determination genes on both mean IPI and sex comb tooth number. The fru locus was associated with a significant increase in mean IPI, whereas the opposite was true for the dsx locus. In the analysis of sex comb tooth variation, it appears that all RI lines homozygous for D. sechellia alleles at the sex determination loci had significantly higher numbers of sex comb teeth. The final data chapter involves the sequence analysis of the fruitless locus, including all 13 fru proteins between ten recently sequenced Drosophilid genomes. The PAML program was used to detect the possible influence of natural selection on sequence divergence. There was no significant positive selection detected at the BTB functional domain and the sequences encoding for this domain were extremely conserved. Positive selection was found to be acting on the exon encoding for the Zinc-finger C domain. This domain is present in two protein isoforms including the male sex-specific isoform FRUMC, and the common non-sex-specific isoform FRUComC. Interestingly positive selection was also found at the non sex-specific Zinc-finger D domain.

One of the best-known structures in Drosophila is the sex comb, an arrangement of modified bristles on the tarsal forelegs of males. This complex, sexually-dimorphic trait shows striking variation among closely related species, although most other aspects of the tarsal bristle pattern have been conserved. I studied the development of the sex comb in the model organism Drosophila melanogaster and six related species. I confirmed that the D. melanogaster sex comb, although longitudinal in the adult, originates in a transverse orientation and rotates during development, and showed that this process occurs through male-specific convergent extension. However, in the species that I examined that have longitudinally-oriented sex combs that extend the full length of the tarsus, including D. ficusphila and two species of the montium subgroup, the sex comb does not rotate, and instead forms from two longitudinal rows that converge during development. Another species of the montium subgroup, D. nikananu, has a sex comb that is convergently similar to D. melanogaster, but forms in a manner typical of its subgroup, showing that very similar combs can be formed through different processes. In all species, there is a strong correlation between the position of the sex comb and the transverse bristle row on the foreleg tarsus just proximal to it. To test whether it is possible to violate this apparent constraint on development, I perturbed the expression of the leg patterning gene dachshund to generate ectopic sex combs in D. melanogaster. I found that while most patterns showed the same correlation, a few circumvent the constraint. I also demonstrated that the ectopic combs were formed non-autonomously and that overexpression of dachshund can transform certain aspects of the sex comb phenotype to resemble the transverse bristles to which they are homologous.

One of the best-known structures in Drosophila is the sex comb, an arrangement of modified bristles on the tarsal forelegs of males. This complex, sexually-dimorphic trait shows striking variation among closely related species, although most other aspects of the tarsal bristle pattern have been conserved. I studied the development of the sex comb in the model organism Drosophila melanogaster and six related species. I confirmed that the D. melanogaster sex comb, although longitudinal in the adult, originates in a transverse orientation and rotates during development, and showed that this process occurs through male-specific convergent extension. However, in the species that I examined that have longitudinally-oriented sex combs that extend the full length of the tarsus, including D. ficusphila and two species of the montium subgroup, the sex comb does not rotate, and instead forms from two longitudinal rows that converge during development. Another species of the montium subgroup, D. nikananu, has a sex comb that is convergently similar to D. melanogaster, but forms in a manner typical of its subgroup, showing that very similar combs can be formed through different processes. In all species, there is a strong correlation between the position of the sex comb and the transverse bristle row on the foreleg tarsus just proximal to it. To test whether it is possible to violate this apparent constraint on development, I perturbed the expression of the leg patterning gene dachshund to generate ectopic sex combs in D. melanogaster. I found that while most patterns showed the same correlation, a few circumvent the constraint. I also demonstrated that the ectopic combs were formed non-autonomously and that overexpression of dachshund can transform certain aspects of the sex comb phenotype to resemble the transverse bristles to which they are homologous.