This transformer is not like the others: Distinct features of a sex determination gene in a blow fly with a distinct sex determination mechanism

The blow fly Chrysomya rufifacies (Diptera: Calliphoridae) exhibits a distinct sex determination mechanism, where females produce either all male or all female offspring.  This mechansim appears to be a feature of the maternal genotype and segregates in a mendelian fashion.  Currently, the genes that drive this process are not understood.  We have conducted a series of genomic and transcriptomic sequencing efforts of arrhenogenic and thelygenic females, as well as male and female embryos.  This has enabled us to evaluate major sex determination factors known to affect sex determination in other fly species.  One interesting feature of these genes in this species is the transformer (tra) gene, which affects sex determination in other flies.  Traditionally, this splicing factor is alternatively spliced, with males producing a nonfunctional protein and females producing a functional splicing factor that can splice other genes that will ultimately lead to male or female differentiation.  The C. rufifacies gene assembled in our genomic assembly exhibits evidence of synteny with respect to Drosophila tra region gene order.  In addition, our RNA sequencing results indicate high similarity of sequenced RNAs to the genomic contig containing our putative tra sequence.  However, the C. rufifacies gene does not exhibit a cannonical blow fly splicing pattern.  Here we report evidence of male and female biased exons that are not reported in other blow flies.   In addition, we report an exon found only in female embryos.  These results are suggestive of tra as a major regulator of this distinct monogenic sex determination mechanism.