Silence of the killers: discovery of male-killing suppression in a rearing strain of the small brown planthopper, Laodelphax striatellus

According to evolutionary theory, sex ratio distortions caused by reproductive parasites such asWolbachia andSpiroplasma are predicted to be rapidly normalized by the emergence of host nuclear suppressors. However, such processes in the evolutionary arms race are difficult to observe because sex ratio biases will be promptly hidden and become superficially unrecognizable. The evolution of genetic suppressors has been reported in just two insect species so far. In the small brown planthopper,Laodelphax striatellus , female-biases caused bySpiroplasma , which is a ‘late’ male-killer, have been found in some populations. During the continuous rearing ofL. striatellus , we noted that a rearing strain had a 1 : 1 sex ratio even though it harbouredSpiroplasma . Through introgression crossing experiments with a strain lacking suppressors, we revealed that theL. striatellus strain had the zygotic male-killing suppressor acting as a dominant trait. The male-killing phenotype was hidden by the suppressor even thoughSpiroplasma retained its male-killing ability. This is the first study to demonstrate the existence of a late male-killing suppressor and its mode of inheritance. Our results, together with those of previous studies, suggest that the inheritance modes of male-killing suppressors are similar regardless of insect order or early or late male killing.