Abstract：
Fem is a W-linked gene that encodes a piRNA precursor, and its product, Fem piRNA, is a master factor of female determination in Bombyx mori. Fem has low similarity to any known sequences, and the origin of Fem remains unclear. So far, two hypotheses have been proposed for the origin of Fem: The first hypothesis is that Fem is an allele of Masc, which assumes that the W chromosome was originally a homologous chromosome of the Z chromosome. The second hypothesis is that Fem arose by the transposition of Masc to the W chromosome. To explore the origin of Fem, we determined the W chromosome sequences of B. mori and, as a comparison, a closely relative bombycid species of Trilocha varians with a Fem-independent sex determination system. To our surprise, although the sequences of W and Z chromosomes show no homology to each other, a few pairs of homologues are shared by W and Z chromosomes, indicating the W chromosome of both species originated from Z chromosome. In addition, the W chromosome of T. varians lacks Fem, while the W chromosome of B. mori has over 100 copies of Fem. The high-quality assembly of the W chromosome of B. mori arose the third hypothesis about the origin of Fem: Fem is a chimeric sequence of multiple transposons. More than half of one transcriptional unit of Fem shows a significant homology to RTE-BovB. Moreover, the Fem piRNA-producing region could correspond to the boundary of the two transposons, gypsy and satellite DNA.
摘要：
Fem是一个W连锁基因，编码piRNA前体，及其产品，FempiRNA,是家蚕女性决心的主要因素。Fem与任何已知序列的相似性都很低，Fem的起源尚不清楚。到目前为止,对于Fem的起源提出了两个假设：第一个假设是Fem是Masc的等位基因，假定W染色体最初是Z染色体的同源染色体。第二个假设是Fem是由Masc转座到W染色体引起的。为了探索Fem的起源，我们确定了B.mori的W染色体序列，作为比较,Trilochavarians的一种紧密相关的bombycid物种，具有与Fem无关的性别决定系统。令我们惊讶的是，尽管W和Z染色体的序列显示彼此没有同源性，W和Z染色体共有几对同源物，表明两个物种的W染色体均起源于Z染色体。此外,T.varians的W染色体缺乏Fem，而B.mori的W染色体有超过100个拷贝的Fem。B.mori的W染色体的高质量组装产生了关于Fem起源的第三个假设：Fem是多个转座子的嵌合序列。Fem的一个转录单位的一半以上显示与RTE-BovB的显著同源性。此外，产生FempiRNA的区域可以对应于两个转座子的边界，吉普赛人和卫星DNA.
