Meeting Abstract
Sepsid flies (Diptera: Sepsidae) have a novel abdominal appendage used for courtship and mating. This appendage is sexually dimorphic occurring only in males and is a jointed, mobile brush-like structure mounted on a highly modified sternite. The appendage develops from the 4th abdominal histoblast nest rather than an imaginal disc. Our objective is to identify the genes in the 4th histoblast nest that pattern the appendage and to compare histoblast nest size and structure between species to identify differences in the evolutionary history of the appendage. We developed a transcriptome for the sepsid fly Themira biloba and performed Illumina sequencing on the 3rd male, 4th male, and 4th female larval segments and identified transcripts which are differentially expressed in the 4th male segment which produces the appendage. We then collected larval epidermal tissue from 16 species across Sepsidae and one outgroup to test whether the complex evolutionary history of gain, secondary loss, and recovery of the appendage are the product of different mechanisms. We characterized the histoblast nests in all segments and both sexes, determining the nest size as well as the number and size of cells. The appendage-producing histoblast nest is sexually dimorphic in species after primary gain. Loss of the appendage shows a return to ancestral state while regain shows an increase in nest size in both sexes.
