Identification and evolution of a novel instructor gene of sex determination in the haplodiploid wasp Nasonia

Although sex determination is a fundamental biological process, the underlying mechanisms are remarkably diverse in insects. There is a plethora of instructor signals that start the sex determination pathway. In many species, this instruction is related to sex chromosomes. However, all insects in the order Hymenoptera, that have haplodiploid sex determination, do not have sex chromosomes; males are haploid and develop from unfertilized eggs, whereas females are diploid and develop from fertilized eggs. The only molecularly characterized instructor in Hymenoptera is that of the honeybee which has complementary sex determination (CSD). However, most parasitoid wasps, including Nasonia, do not have CSD. In this thesis, a novel hymenopteran instructor gene, wasp overruler of masculinization (wom), of the haplodiploid wasp Nasonia, is identified. It is maternally silenced in haploid unfertilized eggs leading to male development, but is transcribed from the paternal allele in diploid fertilized eggs to initiate female development. It is the first characterized instructor gene with a parent-of-origin effect in sex determination. Phylogenetic analysis of wom suggests that it evolved recently by gene duplication and genomic rearrangements. These findings contribute to the understanding of the origin of instructor genes and the evolution of sex determination mechanisms in insects.