SEQ_FILES
The Genetic Basis of Rapidly Evolving Male Genital Morphology in Drosophila
The external genitalia are some of the most rapidly evolving morphological structures in insects.  The posterior lobe of the male genital arch shows striking differences in both size and shape among closely related species of the Drosophila melanogaster species subgroup.  Because this structure is involved in copulation, divergence in species-specific morphology is thought to have been driven by sexual selection.  Here, we dissect the genetic basis of posterior lobe morphology between D. mauritiana and D. sechellia, two island endemic species that last shared a common ancestor roughly 300,000 years ago.  We test a large collection of genome-wide homozygous D. mauritiana genetic introgressions, which collectively cover approximately 50% of the genome, for their morphological effects when placed in a D. sechellia genetic background.  We find several introgressions that have large effects on posterior lobe morphology, and that posterior lobe size and posterior lobe shape can be separated genetically for some of the loci that specify morphology.  Using next generation sequencing technology, we perform whole transcriptome gene expression analyses of the larval genital imaginal disc of D. mauritiana, D. sechellia, and two D. mauritiana-D. sechellia hybrid introgression genotypes that each have large effects on either posterior lobe size or posterior lobe shape, and find that both introgression genotypes possess genes in the insulin receptor signaling pathway that are expressed at D. mauritiana expression levels.  Moreover, both introgression hybrid genotypes show D. mauritiana expression levels in unique sets of genes, but they also show D. mauritiana expression levels in identical genes, which suggests that the insulin signaling pathway might integrate size and shape genetic inputs to establish differences in overall posterior lobe morphology between D. mauritiana and D. sechellia.  We propose a model that describes how the insulin and other growth related signaling pathways might integrate separate genetic inputs for size and shape to specify species-specific posterior lobe morphology.