Concentration and distribution of diaphorin, and expression of diaphorin synthesis genes during Asian citrus psyllid...

The Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Sternorrhyncha: Psylloidea: Liviidae) is an important pest of citrus species worldwide because it transmits Candidatus Liberibacter spp. (Alphaproteobacteria); the causative agents of an incurable citrus disease known as huanglongbing or greening disease. D. citri possesses a vertically transmitted intracellular symbiont, Candidatus Profftella armatura (Betaproteobacteria), which produces diaphorin; a polyketide that is toxic to various eukaryotic organisms. Our previous study demonstrated that the total amount and average concentration of diaphorin in adult D. citri, when homogeneous distribution is assumed within the insect, are sufficient to exert inhibitory effects on fungi and insects, including the Asian lady beetle Harmonia axyridis (Coleoptera: Coccinellidae); one of the major predators of D. citri. However, diaphorin may be localized to a limited body area within D. citri, and its concentration may change during development. In the present study, to better understand the physiological and ecological function of diaphorin, we assessed the distribution of diaphorin within the D. citri body and analyzed concentrations of diaphorin in various developmental stages. Expression of genes involved in diaphorin synthesis was also analyzed. The results demonstrated that diaphorin is distributed widely in the D. citri body, which appears to be a prerequisite for effective deterrence of natural enemies. The concentration of diaphorin was shown to change significantly during the development of D. citri. It was highest in mature adults, followed by embryos and teneral adults, and lowest in nymphs. The lowest concentrations of diaphorin observed in nymphs are still presumed to be effective in deterring invasive natural enemies, including parasites, parasitoids, and entomopathogenic fungi. Quantitative RT-PCR indicated that amounts of transcripts for diaphorin synthesis genes dipP and dipT were at a minimum in embryos, increased during the nymphal period, and reached a maximum level just after adult eclosion. The alteration pattern of the amounts of transcripts for diaphorin synthesis genes appeared to partially disagree with that of the concentration of diaphorin. The present study provides new insights into the function of diaphorin, which is essential for further investigations that aim to improve the efficacy of D. citri biological control.