Discovery of a symbiotic virus that prevents male development in the agricultural pest, Spodoptera litura — Male-lethal trait uniquely acquired during evolution

A research group led by Associate Professor Yoshinori Shintani and Professor Yoshiaki Kanno of the Department of Environmental and Horticultural Sciences at Minami Kyushu University, with Researcher Keisuke Nagamine and Senior Researcher Daisuke Kageyama of the Insect Control Technology Group at the Institute of Agrobiological Sciences of NARO (National Agriculture and Food Research Organization), in collaboration with Iwate University and Setsunan University, announced the discovery of a virus that symbiotically coexists with the moth Spodoptera litura, an agricultural pest, and stops egg production in males, turning the progeny into females only.

By sequencing the entire genome of this virus, S. litura male-killing virus (SIMKV) and comparing its phylogenetic relationship to symbiotic viruses known to possess male-lethal traits, it was found that this virus acquired the same traits independently. The virus is expected to be applied to pest control methods using viral reproductive manipulation. The results were published in the November 6 issue of the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

Various bacteria live symbiotically in insects, some of which cause reproductive manipulations such as developmental lethality and sex change of the host male, making the host female only. Insects contain not only symbiotic bacteria but also symbiotic viruses. Recently, symbiotic viruses with a male-lethal trait that stops the development of host males have been discovered in Homona magnanima Diakonoff (moths) and Drosophila biauraria (flies). Both species belong to the order Durnavirales.

The research began by virtue of Shintani's effort: he raised S. litura larvae captured in a greenhouse on the university's campus in September 2015 and noticed that all 45 were female. The larvae produced by mating these females with field-collected males were also all females. This indicates that this all-female trait is transmitted vertically from the female parent to the offspring. Furthermore, by continuously mating these adult females with males of the normal strain, it was possible to maintain a strain in which only females appeared (all-female strain). The mechanism of total feminization was studied.

Three possibilities were considered: only males die, males become females, or only females are born. The hatching rate of all female strains was observed to be about half that of the normal strains. Genetic and chromosomal studies revealed that males and females survived in the eggs in a 1:1 ratio, but only females hatched. Total feminization was due to male lethality.

To investigate the possibility of male lethality caused by symbiotic bacteria, individuals were observed after antibiotics were administered to eliminate bacteria. However, no particular changes were observed, leading the research team to suspect the possibility of a symbiotic virus.

Adults of the all-female strain were ground, and the resulting liquid was filtered through a filter with a size that is impermeable to bacteria and injected into the normal strain. The all-female trait was found to be transmitted to the next generation. Therefore, a virus, smaller than a bacterium, may be the cause.

Using RNA-seq analysis to compare the RNAs in adult females of the all-female strain and adult females of normal strains, five viral RNAs were found to be present only in the all-female strain. These viruses were named SIMKV.

Analysis of the SIMKV genome sequence revealed seven genes, including RNA-dependent RNA polymerase (RdRp). RdRp is a gene required for RNA replication, indicating that SIMKV is an RNA virus. SIMKV also had no known male-lethal genes. A comparison of phylogenetic relationships with the two previously discovered viruses (order Durnavirales) also showed that they are genetically distant from these two species.

SIMKV does not have genes similar to those responsible for male lethality in these two species and appears to have acquired them independently during evolution. It was also confirmed that infection of uninfected larvae with SIMKV causes male lethality at the cocoon stage. SIMKV is the third reported symbiotic virus with male lethal traits.

Although the male lethal gene of SIMKV has not yet been identified, its identification and elucidation of the mechanism are expected to lead to its use in the control of S. litura, a major agricultural pest that feeds on more than a hundred different crops.

Journal Information
Publication: National Academy of Sciences of the United States of America (PNAS)
Title: Male-killing virus in a noctuid moth Spodoptera litura
DOI: 10.1073/pnas.2312124120