Males are from Mars, females are from Venus… and Earth? Adventures in sex determination and transcriptomics

Chrysomya rufifacies (Diptera: Calliphoridae) is a species of blow fly invasive to the New World. Unlike other flies which have heteromorphic sex chromosomes and a sex determination system based on embryonic genotype, C. rufifacies has homomorphic sex chromosomes and a monogenic system of sex determination in which the maternal genotype determines the sex of her offspring. Based on QTL mapping, tissue exchange, and proteomics studies, female-producing (thylegenic) females are heterozygote-dominant for a protein produced in the germ-line and incorporated into the soma of the eggs that predetermines the sex of the offspring. One possibility was that endosymbionts had a role in sex determination, however none were detected with metagenomics or specific PCR primers.   A de novo transcriptome assembly using Illumina HiSeq data was completed to compare patterns of gene expression between males, male-producing (arrhenogenic) females, and female-producing (thylegenic) females. Orthologs of most of the genes identified in studies of the sex-determination cascades of Drosophila melanogaster (Diptera: Drosophilidae) have been identified, including transformer, which is the master regulator of sex in other blow flies.  However, transformer was difficult to find and was only detected with blow fly data.  In addition, within the genes differentially expressed between thylegenic and arrhenogenic females, most (~99%) had no sequence homology with Drosophila  and their function is unknown. Discussion of pertinence of these findings to the evolution of sex determination and application to forensic entomology follows.