Retrotransposable elements on the W chromosome of the silkworm, <i>Bombyx mori</i>

Abstract

The sex chromosomes of the silkworm, Bombyxmori, are designated ZW(XY) for females and ZZ(XX) for males. The W chromosome of B. mori does not recombine with the Z chromosome and autosomes and no genes for morphological characters have been mapped to the W chromosome as yet. Furthermore, femaleness is determined by the presence of a single W chromosome, regardless of the number of autosomes or Z chromosomes. To understand these interesting features of the W chromosome, it is necessary to analyze the W chromosome at the molecular biology level. Initially to isolate DNA sequences specific for the W chromosome as randomly amplified polymorphic DNA (RAPD) markers, we compared the genomic DNAs between males and females by PCR with arbitrary 10-mer primers. To the present, we have identified 12 W-specific RAPD markers, and with the exception of one RAPD marker, all of the deduced amino acid sequences of these W-specific RAPD markers show similarity to previously reported amino acid sequences of retrotransposable elements from various organisms. After constructing a genomic DNA lambda phage library of B. mori we obtained two lambda phage clones, one containing the W-Kabuki RAPD sequence and one containing the W-Samurai RAPD sequence and found that these DNA sequences comprised nested structures of many retrotransposable elements. To further analyze the W chromosome, we obtained 14 W-specific bacterial artificial chromosome (BAC) clones from three BAC libraries and subjected these clones to shotgun sequencing. The resulting assembly of sequences did not produce a single contiguous sequence due to the presence of many retrotransposable elements. Therefore, we coupled PCR with shotgun sequencing. Through these analyses, we found that many long terminal repeat (LTR) and non-LTR retrotransposons, retroposons, DNA transposons and their derivatives, have accumulated on the W chromosome as strata. These results strongly indicate that retrotransposable elements are the main structural component of the W chromosome.

Repression of tyrosine hydroxylase is responsible for the sex-linked chocolate mutation of the silkworm, Bombyx mori

“…Using primer sets near both ends of the insertion, we were not able to amplify a PCR band from the sch l mutant, even using long-range PCR. We therefore speculated that the insertion fragment was a long complex retrotransposon containing many other transposons and retrotransposons, like those present in W chromosomes (33,34). Using reverse PCR with primer sets near the insertion sites (Table S5), we identified and sequenced amplicons from both ends of the insertion fragment.…”

Linkage map of the peppered moth, Biston betularia (Lepidoptera, Geometridae): a model of industrial melanism

“…Not a single gene has been found on the W chromosome in Heliconius spp., B. anynana, or P. xylostella, in spite of comprehensive gene-based surveys (Pringle et al, 2007;Beldade et al, 2009;Baxter et al, 2011). An exceptionally small W chromosome may partly explain this paucity of genes in B. anynana ( Van't Hof et al, 2008), but there is no such association for the B. mori W chromosome that is almost entirely composed of repetitive sequences originating from retrotransposons (Abe et al, 2005). Some records of W-linked genes are disputed, such as the dark morph locus in Papilio glaucus (Andolfatto et al, 2003) and an egg size determining gene, Esd, in the silkworm (Kawamura, 1988; but see Fujii et al, 2010).…”

Recent Spread of a Retrotransposon in theSilene latifoliaGenome, Apart From the Y Chromosome

“…Accumulation of TE insertions and other repetitive DNA is also a typical feature of Y chromosomes as in degenerate Y chromosomes TE insertions are less likely to damage important genes and lack of recombination prevents ectopic exchange between the TE copies (Charlesworth et al 1994;Abe et al 2005;Steinemann and Steinemann 2005).…”