Transfection of Wolbachia in Lepidoptera: the feminizer of the adzuki bean borer Ostrinia scapulalis causes male killing in the Mediterranean flour moth Ephestia kuehniella.

Two species of Lepidoptera, Ostrinia scapulalis and Ephestia kuehniella, harbour Wolbachia, which are maternally transmitted intracellular bacteria that often cause reproductive abnormalities in arthropods. While the infection in O. scapulalis causes conversion of genetic males into functional females (feminization), that in E. kuehniella induces cytoplasmic incompatibility. In the present study, we investigated the relative importance of host and Wolbachia factors in the differential expression of reproductive alterations in these insects. We transferred the Wolbachia harboured by O. scapulalis to E. kuehniella in which the original infection had been cured by tetracycline treatment. The transfected strain of E. kuehniella expressed a maternally inherited, female-biased sex ratio. Unexpectedly, two lines of evidence suggested that the sex ratio distortion was due to male killing. First, higher mortality of young larvae was observed. Second, the removal of the transferred Wolbachia resulted in the recovery of a 1:1 sex ratio, whereas the removal of a feminizer should result in a male-biased sex ratio among offspring. To the authors' knowledge, this is the first report that a single Wolbachia strain can cause two distinct sexual abnormalities in different hosts. Our observations highlighted the importance of host-Wolbachia interactions in determining the phenotype of reproductive alterations.

Two mitochondrial lineages occur in the Asian corn borer, Ostrinia furnacalis (Lepidoptera: Crambidae), in Japan.

The genealogy and diversity of the mitochondrial cytochrome oxidase subunit II (COII) gene were investigated for Ostrinia furnacalis in Japan. A preliminary examination of mitochondrial lineages in China and the Philippines was also made. Two lineages (A and B) were found in the COII gene. Lineage A was frequent throughout the Japanese main islands (Hokkaido, Honshu, Shikoku and Kyushu), while the frequency of lineage B varied among these islands. No clear patterns of geographical population structure were found. Population genetic features suggested that the O. furnacalis population harboring the lineage A mitochondria expanded in the recent past, while lineage B showed weak signals of a population expansion. It is not clear whether the two lineages of mtDNA evolved in separate or identical geographical regions. We discuss two hypotheses regarding the two lineages of mtDNA: a cryptic race/species hypothesis and a selective sweep hypothesis.

Variation in mitochondrial COII gene sequences among two species of Japanese knotweed-boring moths, Ostrinia latipennis and O. ovalipennis (Lepidoptera: Crambidae).

The Ostrinia latipennis group contains two species, O. latipennis (Warren) and O. ovalipennis Ohno. These two species commonly utilize perennial knotweeds (Fallopia spp.) as their host plants, which are serious invasive weeds in Europe and North America. Ostrinia latipennis is widely distributed across north-east Asia including Japan whereas O. ovalipennis is restricted to north Japan (Hokkaido Is.) and highland areas of central Japan (Nagano Prefecture in Honshu Is.). To estimate the phylogenetic relatedness and geographical differentiation of the two species, mitochondrial COII gene sequences were determined for specimens covering their distribution ranges in Japan. The uncorrected sequence divergence between O. latipennis and O. ovalipennis was 0.6-0.7%, supporting a close relationship. According to the standard molecular clock proposed for arthropod mtDNA, the two species are speculated to have diverged about 0.3 Myr ago. A single COII gene haplotype was found in O. latipennis irrespective of collection locality. In contrast, two haplotypes were found in O. ovalipennis, and their frequencies were significantly different between the Hokkaido and Honshu populations. The patterns of geographical variation in the COII gene within the two species were in agreement with previously reported patterns of geographical differentiation in morphology of the two species in Japan. The present results support the hypothesis that gene flow among local populations of O. ovalipennis has been limited by geographical isolation.

Allozyme polymorphism and geographic variation in the small brown planthopper, Laodelphax striatellus (Homoptera: Delphacidae).

The small brown planthopper, Laodelphax striatellus, immigrates annually into Japan over the East China Sea from the Asian mainland. It is not known whether this long-distance dispersal has any effect on the genetic structure of Japanese L. striatellus populations. The dispersal of L. striatellus is suspected to be relevant to the population dynamics of infection with the parasitic bacterium Wolbachia, which causes cytoplasmic incompatibility in L. striatellus. Wolbachia infection has spread within and among Japanese L. striatellus populations due to this cytoplasmic incompatibility. In the present study, the geographic differences among II L. striatellus populations from Japan and Taiwan was investigated using allozyme polymorphism. FST values on three enzyme loci (GPI, PGM, and AK) indicated a geographically differentiated population structure. Significant differentiation was found even among populations located along the course of the long-distance dispersal. The results indicated that long-range dispersal of L. striatellus does not occur regularly over the main islands of Japan and that it does not have a large effect on the population structure of L. striatellus. This conclusion is in agreement with the geographically variable life history of L. striatellus adapted to local climates. The short-term rice stripe epidemic, which is vectored by L. striatellus, in northwestern Kyushu, Japan, during 1985 and 1986 corresponds to these results. Based on the present findings, short-distance dispersal was considered to drive the spatial spread of Wolbachia infection among L. striatellus populations.

PCR-based detection of Wolbachia, cytoplasmic incompatibility microorganisms, infected in natural populations of Laodelphax striatellus (Homoptera: Delphacidae) in central Japan: has the distribution of Wolbachia spread recently?

Cytoplasmic incompatibility is caused in various insects by intracellular infection with rickettsia-like microorganisms of the genus Wolbachia. In Japan Laodelphax striatellus shows unidirectional cytoplasmic incompatibility between northeastern and southwestern populations. In this study, nine natural populations of L. striatellus collected from central Japan, including the geographic boundary between the two cytotype populations, were analysed for Wolbachia infection by PCR using primers specific to Wolbachia 16S rDNA. The geographic pattern of the infection rates of the southwestern (high) and the northeastern (low or zero) populations broadly resembled that of a previous study of incompatibility. In populations which originated from the boundary regions between the southwestern and northeastern populations, the infected and uninfected cytotypes coexisted. It is suggested that in some populations of L. striatellus, which formerly had been uninfected with Wolbachia, the infection property has changed to the infected. Based on our results, we conclude that the distribution of Wolbachia-infected L. striatellus populations have spread northeasterly during the last 12 years.

Feminizing Wolbachia in an insect, Ostrinia furnacalis (Lepidoptera: Crambidae).

Wolbachia, which forms a group of maternally inherited bacteria in arthropods, often cause reproduction alterations in their hosts, such as cytoplasmic incompatibility, parthenogenesis, male-killing, hybrid breakdown and feminization. To date, Wolbachia-induced feminization has been reported only in isopods. Here we report that a Wolbachia strain feminizes an insect host, Ostrinia furnacalis. Among 79 wild females of O. furnacalis examined, Wolbachia infection was detected in 13 females. Twelve of the 13 infected females produced all-female progenies, and this trait was maternally inherited. Tetracycline treatment of thelygenic matrilines resulted in the production of all-male progenies. The present findings indicate that the Wolbachia infection induces feminization of genetic males in O. furnacalis. Differences in the Wolbachia-induced feminization in O. furnacalis and that in isopods are discussed along with the differences in sex determination mechanisms between insects and isopods. Phylogenetic analysis of the wsp gene sequence of Wolbachia suggests independent evolutionary origins for the Wolbachia-induced feminizations in O. furnacalis and in isopods. Our findings over 5 years suggest that the infection has been maintained at a low prevalence in the O. furnacalis population.

Wolbachia infection shared among planthoppers (Homoptera: Delphacidae) and their endoparasite (Strepsiptera: Elenchidae): a probable case of interspecies transmission.

Wolbachia, a group of parasitic bacteria of arthropods, are believed to be horizontally transmitted among arthropod taxa. We present a new probable example of interspecies horizontal transmission of Wolbachia by way of an endoparasite based on the conformity of Wolbachia gene sequences. Field samples of two rice planthoppers, Laodelphax striatellus and Sogatella furcifera possessed identical Wolbachia. Among three major endoparasites of planthoppers, a strepsipteran, Elenchus japonicus, harboured the identical Wolbachia strain, suggesting strepsipteran transmission of Wolbachia from one planthopper to the other. No Wolbachia was detected in a mermithid nematode Agamermis unka, and dryinid wasps possessed different types of Wolbachia.