![]() ![]() Four days after oviposition, well-developed but dead embryos (e.g., with black mouth parts and a differentiated cuticle) were abundant in all-female matrilines (SP12F and tr.SP11-20), but not in the normal line (SP11-20) (Supplementary Fig. We confirmed that egg hatch rates of the all-female matrilines, SP12F (the originally discovered all-female matriline) and tr.SP11-20 (the matriline generated by injecting SP12F homogenate into SP11-20) 5, were nearly half of those of SP11-20, which corresponds to a typical feature of early male-killing (Fig. Here we show that the cause of male-killing is a virus belonging to the family Partitiviridae, reveal the genome structure of the virus and identify a gene responsible for male-killing. However, the above-mentioned observations led us to suspect the incidence of virus-induced male-killing. This phenomenon, also known as male-killing, is one of the reproductive manipulations commonly exerted by endosymbiotic bacteria, such as Wolbachia 4. Males of the all-female matriline were considered to be killed during embryogenesis because the egg hatch rate was nearly half of that of a normal sex ratio line (SP11-20) despite there being no significant differences in the total number of eggs laid between the two lines 5. These features strongly suggested that a non-bacterial agent-possibly a virus-was responsible for the absence of males 5. We observed a lack of polymerase chain reaction (PCR) amplicons after the use of bacterial universal primers, the lack of an effect of antibiotic treatment on the sex ratio, transmissibility of the all-female trait after injecting fly homogenate sterilized using a 0.22-µm filter, and matrilineal inheritance of the all-female trait. Previously, we reported an all-female matriline (SP12F) in Drosophila biauraria 5. Recent studies investigating the metagenomics of various arthropod taxa have unearthed a wide array of viruses 7, some of which are likely to be vertically transmitted endosymbionts, and their effects on host ecology as well as evolution are gaining attention 8, 9, 10, 11. Such viruses are classically biparentally inherited and thus are not expected to manipulate host reproduction. Likewise, heritable viruses in insects are known but are generally thought to be uncommon (e.g., sigma virus 6). In arthropods, strongly female-biased sex ratios have been repeatedly observed in various arthropods, which resulted from the male-specific lethality (male-killing) induced by maternally inherited symbiotic bacteria such as Spiroplasma and Wolbachia 3, 4, 5. A female-biased sex ratio is advantageous for cytoplasmic genomes while a 1:1 sex ratio is adaptive for nuclear (autosomal) genomes thus, there exists a conflict over the sex ratio. In most eukaryotes, biparentally inherited nuclear genomes and maternally inherited cytoplasmic genomes have different evolutionary interests 1, 2. We anticipate that host-manipulating genes possessed by symbiotic viruses can be utilized for controlling arthropods. Our results imply that male-killing genes can be easily acquired by symbiotic viruses through reassortment and that symbiotic viruses are hidden players in arthropod evolution. ![]() We also show that a protein (designated PVMKp1 330 amino acids in size), encoded by a gene on the dsRNA4 segment, is necessary and sufficient for inducing male-killing. 1000 or more genes possessed by Spiroplasma or Wolbachia. DbMKPV1 localizes in the cytoplasm and possesses only four genes, i.e., one gene in each of the four genomic segments (dsRNA1−dsRNA4), in contrast to ca. Here we show that a maternally inherited double-stranded RNA (dsRNA) virus belonging to the family Partitiviridae (designated DbMKPV1) induces male-killing in Drosophila. ![]() However, despite some plausible case reports wherein viruses are raised as male-killers, it is not well understood how viruses, having much smaller genomes than bacteria, are capable of inducing male-killing. Strongly female-biased sex ratios that are repeatedly observed in various arthropods often result from the male-specific lethality (male-killing) induced by maternally inherited symbiotic bacteria such as Spiroplasma and Wolbachia. In most eukaryotes, biparentally inherited nuclear genomes and maternally inherited cytoplasmic genomes have different evolutionary interests. ![]()
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