Identification of the female-determining region of the W chromosome in Bombyx mori

The W chromosome of the silkworm Bombyx mori is devoid of functional genes, except for the putative female-determining gene (Fem). To localize Fem, we investigated the presence of W-specific DNA markers on strains in which an autosomal fragment containing dominant marker genes was attached to the W chromosome. We produced new W-chromosomal fragments from the existing Zebra-W strain (T(W;3)Ze chromosome) by X-irradiation, and then carried out deletion mapping of these and sex-limited yellow cocoon strains (T(W;2)Y-Chu, -Abe and -Ban types) from different Japanese stock centers. Of 12 RAPD markers identified in the normal W chromosomes of most silkworm strains in Japan, the newly irradiated W(B-YL-YS)Ze chromosome contained three, the T(W;2)Y-Chu chromosome contained six, and the T(W;2)Y-Abe and -Ban chromosomes contained only one (W-Rikishi). To investigate the ability of the reduced W-chromosome translocation fragments to form heterochromatin bodies, which are found in nuclei of normal adult female sucking stomachs, we examined cells of the normal type p50 strain and the T(W;2)Y-Chu and -Abe strains. A single sex heterochromatin body was found in nuclei of p50 females, whereas we detected only small sex heterochromatin bodies in the T(W;2)Y-Chu strain and no sex heterochromatin body in the T(W;2)Y-Abe strain. Since adult females of all strains were normal and fertile, we conclude that only extremely limited region, containing the W-Rikishi RAPD sequence of the W chromosome, is required to determine femaleness. Based on a comparison of the normal W-chromosome and 7 translocation and W-deletion strains we present a map of Fem relative to the 12 W-specific RAPD markers.

The female-killing chromosome of the silkworm, Bombyx mori, was generated by translocation between the Z and W chromosomes

Bombyx mori is a female-heterogametic organism (female, ZW; male, ZZ) that appears to have a putative feminizing gene (Fem) on the W chromosome. The paternally transmitted mutant W chromosome, Df(p ( Sa ) + ( p )W + ( od ))Fem, derived from the translocation-carrying W chromosome (p ( Sa ) + ( p )W + ( od )), is inert as femaleness determinant. Moreover, this Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome has been thought to have a female-killing factor because no female larvae having the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome are produced. Initially, to investigate whether the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome contains any region of the W chromosome or not, we analyzed the presence or absence of 12 W-specific RAPD markers. The Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome contained 3 of 12 W-specific RAPD markers. These results strongly indicate that the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome contains the region of the W chromosome. Moreover, by using phenotypic and molecular markers, we confirmed that the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome is connected with a partially deleted Z chromosome and that this fused chromosome behaves as a Z chromosome during male meiosis. Furthermore, we demonstrated that the ZZW-type triploid female having the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome is viable. Therefore, we concluded that the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome does not have a female-killing factor but that partial deletion of the Z chromosome causes the death of the ZW-type diploid female having the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome. Additionally, our results of detailed genetic analyses strongly indicate that the female-killing chromosome composed of the Df(p ( Sa ) + ( p )W + ( od ))Fem chromosome and deleted Z chromosome was generated by translocation between the Z chromosome and the translocation-carrying W chromosome, p ( Sa ) + ( p )W + ( od ).

Partial deletions of the W chromosome due to reciprocal translocation in the silkworm Bombyx mori

In the silkworm, Bombyx mori (female, ZW; male, ZZ), femaleness is determined by the presence of a single W chromosome, irrespective of the number of autosomes or Z chromosomes. The W chromosome is devoid of functional genes, except the putative female-determining gene (Fem). However, there are strains in which chromosomal fragments containing autosomal markers have been translocated on to W. In this study, we analysed the W chromosomal regions of the Zebra-W strain (T(W;3)Ze chromosome) and the Black-egg-W strain (T(W;10)+(w-2) chromosome) at the molecular level. Initially, we undertook a project to identify W-specific RAPD markers, in addition to the three already established W-specific RAPD markers (W-Kabuki, W-Samurai and W-Kamikaze). Following the screening of 3648 arbitrary 10-mer primers, we obtained nine W-specific RAPD marker sequences (W-Bonsai, W-Mikan, W-Musashi, W-Rikishi, W-Sakura, W-Sasuke, W-Yukemuri-L, W-Yukemuri-S and BMC1-Kabuki), almost all of which contained the border regions of retrotransposons, namely portions of nested retrotransposons. We confirmed the presence of eleven out of twelve W-specific RAPD markers in the normal W chromosomes of twenty-five silkworm strains maintained in Japan. These results indicate that the W chromosomes of the strains in Japan are almost identical in type. The Zebra-W strain (T(W;3)Ze chromosome) lacked the W-Samurai and W-Mikan RAPD markers and the Black-egg-W strain (T(W;10)+(w-2) chromosome) lacked the W-Mikan RAPD marker. These results strongly indicate that the regions containing the W-Samurai and W-Mikan RAPD markers or the W-Mikan RAPD marker were deleted in the T(W;3)Ze and T(W;10)+(w-2) chromosomes, respectively, due to reciprocal translocation between the W chromosome and the autosome. This deletion apparently does not affect the expression of Fem; therefore, this deleted region of the W chromosome does not contain the putative Fem gene.

Retrotransposable elements on the W chromosome of the silkworm, Bombyx mori

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.

The effect of W chromosome origin on sex-chromosome pairing in ZZWW tetraploid females of the domesticated silkworm, Bombyx mori, and the congenic wild silkworm, Bombyx mandarina

To analyze the degree of pairing of the Z and W chromosomes in ZZWW tetraploid female silkworms that have the W chromosomes of the domesticated silkworm, Bombyx mori, and those of the wild silkworm, Bobyx mandarina, we induced two types of ZZWW tetraploid female silkworms (Cr4n, Wr4n) through cold treatment of the eggs. The Wr4n female is congenic to the Cr4n female for W chromosomes; namely, the W chromosomes of the Wr4n female are derived from those of B. mandarina. Each of the sex ratios (female/male) in filial triploids from the Cr4n females was shown to be in the range of 3.9-5.3 (4.6 as an average of six cases). On the other hand, each of the sex ratios (female/male) in filial triploids from the Wr4n females was shown to be in the range of 6.2-9.0 (6.9 as an average of nine cases). The results of a t-test indicated that the difference in sex ratios in the two groups is highly significant (at the 0.1% level). These results suggest that, in the meiosis of the ZZWW tetraploid female, the frequency of pairing of the W chromosome of B. mandarina and the Z chromosome of B. mori is lower than that of the pairing of the W and Z chromosomes of B. mori. Furthermore, the t-test results are evidence that the W chromosomes have undergone significant evolutional change.

Nested retrotransposons on the W chromosome of the wild silkworm Bombyx mandarina

The W chromosome of the silkworms Bombyx mori or B. mandarina is recombinationally isolated from the Z chromosome and the autosomes. We previously characterized a female-specific randomly amplified polymorphic DNA (RAPD), designated W-Yamato, derived from the W chromosome of the wild silkworm Bombyx mandarina. To further analyse the W chromosome of B. mandarina, we obtained a lambda phage clone that contains the W-Yamato RAPD sequence and sequenced the 16.7 kb DNA insert. We found that this DNA comprises a nested structure of at least seven elements: six retrotransposons and one transposable element-like sequence. The transposable element-like sequence is inserted into a micropia-like retrotransposon (Karate). The Karate and the non-long terminal repeat (non-LTR) retrotransposon BMC1 are inserted into a 412-like retrotransposon (Judo). Furthermore, this Judo, and two non-LTR retrotransposons (Kurosawa and Kendo) are inserted into a Pao-like retrotransposon (Yamato). These results indicate that the retrotransposons inserted into the W chromosome are not efficiently removed but accumulate gradually as strata without recombination.

Cytogenetic analysis shows that the unusually large chromosome in the sex-limited pB silkworm (Bombyx mori) strain consists of three chromosomes

We have discovered an inordinately large chromosome pair at the pachytene stage in the oocyte of the sex-limited pB (black larval marking) silkworm (Bombyx mori) strain (TWPB). We have analyzed the composition and arrangement of this large chromosome. A genetic linkage analysis shows that the large chromosome is made up of the W chromosome, the second chromosome fragment (pB fragment), and the fifth chromosome (linkage group) containing at least the region from map position 0.0 to 40.8. We also observed a sex heterochromatin body (SB) that we deduced to be made up of condensed W chromosomes. The number of SBs in each female nucleus among the sucking stomach cells of the TWPB strain was variable. Evidently, the W chromosome of the TWPB strain is attached to another chromosome. The composition of the W chromosome, the second chromosome fragment, and the fifth chromosome was studied through linkage analysis for these three chromosomes. We used two strains derived from the TWPB strain, the sex-limited pM (moricaud larval marking)-like (TWPML) and the autosomal pM-like (T5PML). The results show that the TWPML strain originates through a detachment of the fifth chromosome from the large chromosome of the TWPB strain, and the T5PML strain originates through a detachment of the W chromosome from that. Accordingly, the large chromosome of the TWPB strain is arranged in the order W chromosome--second chromosome fragment--fifth chromosome.

Two novel Pao-like retrotransposons (Kamikaze and Yamato) from the silkworm species Bombyx mori and B. mandarina: common structural features of Pao-like elements

To characterize the structural features common to Pao-like retrotransposons, we analyzed two lambda phage clones which contain the Pao-like elements from the silkworm species Bombyx mori and B. mandarinia, and copies of Pao itself and ninja of Drosophila simulans, amplified by PCR. We previously identified two randomly amplified polymorphic DNAs (RAPDs), W-Kamikaze and W-Yamato, from B. mori and B. mandarina, which are part of two novel Pao-like retrotransposons, Kamikaze and Yamato, respectively. Complete characterization of these and other elements of this group reported here shows that Pao-like elements have common features that distinguish them from the other groups of LTR-retrotransposons. While the elements of the Ty1-copia group encode only one cysteine and histidine (Cys) motif in their gag-like region, the Pao-like elements specify three Cys motifs. The highly conserved D(35)E motif in the integrase domain of the retrotransposon polyprotein seems to be conserved in Pao-like elements, but the number of amino acid residues between D and E varies and is greater than 35. A comparison of the deduced amino acid sequences of the reverse transcriptase domain revealed the Pao-like elements are members of neither the Ty1-copia nor the gypsy-Ty3 groups. Therefore, we confirmed that the long-terminal-repeat (LTR) retrotransposons should be divided into three major groups (or families), namely the Ty1-copia, gypsy-Ty3, and Pao-like groups.

Molecular structure of a novel gypsy-Ty3-like retrotransposon (Kabuki) and nested retrotransposable elements on the W chromosome of the silkworm Bombyx mori

We previously characterized a female-specific randomly amplified polymorphic DNA (RAPD), designated W-Kabuki, derived from the W chromosome of the silkworm, Bombyx mori. To further analyze the W chromosome of B. mori, we obtained a lambda phage clone which contains the W-Kabuki RAPD sequence and sequenced the 18.1-kb DNA insert. We found that this DNA comprises a nested structure of at least seven elements; three retrotransposons, two retroposons, one functionally unknown insertion, and one Bombyx repetitive sequence. The non-LTR retrotransposon BMC1, the retroposon Bm1, a functionally unknown inserted DNA (FUI), and a copia-like LTR retrotransposon (Yokozuma) are themselves inserted into a novel gypsy-Ty3-like LTR retrotransposon, named Kabuki. Furthermore, this Kabuki element is itself inserted into another copy of Bm1. The BMCI and Yokozuna elements inserted in the Kabuki sequence are intact. Moreover, the Kabuki element is largely intact. These results suggest that many retrotransposable elements have accumulated on the W chromosome, and these elements are expected to evolve more slowly than those on other chromosomes.

Identification and genetic mapping of RAPD markers linked to the densonucleosis refractoriness gene, nsd-2, in the silkworm, Bombyx mori

In the silkworm, Bombyx mori, nonsusceptibility to B. mori densonucleosis virus type-2 (BmDNV-2) is controlled by a recessive gene, nsd-2 (nonsusceptibility to DNV-2). We investigated the genetic linkage between two random-amplified polymorphic DNA (RAPD) markers and the +nsd-2 gene. Initially, we constructed the JSD-2 strain (nsd-2/+), which is congenic to strain J137 (nsd-2/nsd-2) with respect to the chromosome containing the +nsd-2 gene, starting with a female of strain J137 and a male of strain C137 (+nsd-2/+nsd-2). Genomic DNAs were compared between infected individuals of the JSD-2 strain and J137 by a polymerase chain reaction (PCR) with 700 arbitrary 10-mer primers. Two RAPD markers (OPH19R and OPP01R) linked to the +nsd-2 gene were found. For the crossing-over experiment, a female of J137 was crossed with a male (nsd-2/+) of JSD-2. Segregation analysis showed that the most closely linked RAPD marker (OPP01R) was mapped 4.7 cM distant from +nsd-2.

A complete full-length non-LTR retrotransposon, BMC1, on the W chromosome of the silkworm, Bombyx mori

In the silkworm, Bombyx mori, a non-long terminal repeat (non-LTR) retrotransposon, BMC1, is considered to be a LINE (long interspersed nuclear element)-like element. So far, a BMC1 containing two intact open reading frames (ORFs) has not been found. However, we discovered a complete full-length BMC1 on the W chromosome. This BMC1 is 5091 bp and contains a 5' untranslated region (5'-UTR), two intact ORFs, and 3'-UTR which terminates in a poly(A) tail. ORF1 encodes a putative nucleic acid-binding protein, while ORF2 encodes a protein containing an endonuclease domain and a reverse transcriptase domain.

Molecular structure of the copia-like retrotransposable element Yokozuna on the W chromosome of the silkworm, Bombyx mori

We discovered a novel retrotransposable element, designated Yokozuna, on the W chromosome of Bombyx mori. The size of this element is 4738 bp, including a 208-bp long terminal repeat (LTR) on one side and a 183-bp LTR on the other. This retrotransposable element is flanked by a 5-bp target site duplication, TAATT. Yokozuna contains a single long open reading frame (ORF) and the whole deduced amino acid sequence of ORF reveals strong homology with copia of Drosophila. Moreover, an alignment analysis of the reverse transcriptase (RT) sequences suggested that the Yokozuna element is the first Bombyx retrotransposable element belonging to the Ty1-copia group. The number of Yokozuna per haploid genome is approximately four copies. Although Yokozuna was discovered on the W chromosome, it is not specific for the W chromosome.

Identification of novel random amplified polymorphic DNAs (RAPDs) on the W chromosome of the domesticated silkworm, Bombyx mori, and the wild silkworm, B. mandarina, and their retrotransposable element-related nucleotide sequences

Genomic DNAs were compared between males and females of the domesticated silkworm, Bombyx mori, strains C108, C137, J137, p50, and WILD-W (constructed by crossing a wild silkworm, B. mandarina, female with a male of strain C108) by polymerase chain reaction (PCR) with 700 arbitrary 10-mer primers. Four female-specific RAPDs (W-Kabuki, W-Samurai, W-Kamikaze, and W-Yamato) were found. The sex chromosome formulas of B. mori and B. mandarina are ZW (XY) for the female and ZZ (XX) for the male. The four female-specific RAPDs are assumed to be derived from the W chromosome because the other chromosomes are shared by both sexes. A computer search for deduced amino acid sequences of these four RAPDs revealed that all of them showed homology to previously reported amino acid sequences encoded in known retrotransposable elements from various organisms.

Genetic mapping of RAPD markers linked to the densonucleosis refractoriness gene, nsd-1, in the silkworm, Bombyx mori

In the silkworm, Bombyx mori, nonsusceptibility to B. mori densonucleosis virus type-1 (BmDNV-1) is controlled by a recessive gene, nsd-1 (nonsusceptibility to DNV-1), located on the twenty-first chromosome. We investigated genetic linkage between five random amplified polymorphic DNA (RAPD) markers and the +nsd-1 gene. Initially, we constructed the CSD-1 strain (nsd-1/+) which is congenic to strain C137 (nsd-1/nsd-1) for the twenty-first chromosome, starting with a female of C137 and a male of strain J137 (+nsd-1/+nsd-1). For the crossing over experiment, a female of C137 was crossed with a male (nsd-1/+) of CSD-1. Segregation analysis showed that the most closely linked RAPD marker mapped 3.0 cM distant from +nsd-1. A more specific marker for +nsd-1 was made by converting this RAPD band into a sequence characterized amplified region (SCAR) using a series of newly designed primer pairs based on its DNA sequence.