Transposon fusion gave birth to Fem, Bombyx mori female determining gene

Fem is a W-linked piRNA precursor, and Fem piRNA is a master gene of female determination in Bombyx mori. Since Fem has significantly low similarity to any known sequences and the origin of Fem remains unclear, two hypotheses have been proposed for the origin of Fem. The first one is that Fem is an allele of Masc, while the second one is that Fem arose by transposition of Masc. To draw a solid conclusion, we determined the W chromosome sequences of B. mori and a closely relative bombycid species of Trilocha varians with a Fem-independent sex determination system. Comprehensive genome comparison led us to the "third" hypothesis: Fem is a chimeric sequence of transposons. Although we still cannot completely exclude the first hypothesis, at least a large portion of the parts other than the 44-bp Masc similarity region is derived from transposons, and even the 44-bp region could correspond to the boundary of the two transposons, gypsy and satellite DNA.