Abstract: 【Aim】 The aim of our study is to determine the diversity, abundance dynamics and infection site of the symbiotic bacterium Delftia (Burkholderiales: Comamonadaceae) in the cotton mealybug, Phenacoccus solenopsis. 【Methods】 By cloning and sequencing the bacterial 16S rRNA gene fragment, Delftia strains in the gut of the 3rd instar nymphs of five geographical populations ［Lanxi of Zhejiang (host: Hibiscus mutabilis), Dongyang of Zhejiang (host: Capsicum annuum), Xihu District, Hangzhou of Zhejiang (host: Portulaca grandiflora), Linping District, Hangzhou of Zhejiang (host: P. grandiflora) and Fangchenggang of Guangxi (host: Hibiscus rosa-sinensis)］ of P. solenopsis were identified and phylogenetically analyzed. The copy number of Delftia 16S rRNA gene of Delftia in the whole body of P. solenopsis at different developmental stages (1st-3rd instar nymphs, newly emerged adult, and 2-, 4- and 6-day-old adults, and adults at 2 and 4 d after beginning to produce nymphs) fed on cotton and tomato, and in the gut of adults at the above developmental stages and in the ovary of the 2-, 4- and 6-day-old adults fed on tomato were quantified using qPCR. Fluorescence in situ hybridization (FISH) was taken to detect the infection dynamics of Delftia in the gut and Malpighian tubules of the newly emerged adult of P. solenopsis. The possibility that P. solenopsis obtained Delftia from host plants was analyzed. 【Results】 The bacteria of Delftia from the 3rd instar nymphs of various geographical populations of P. solenopsis had quite similar 16S rRNA gene sequences (similarity>99%), being different strains of the same Delftia species, and they had close phylogenetic relationships with those found in some of other insects, plants and rhizosphere. During the nymphal stage, the abundance of Delftia in P. solenopsis fed on cotton and tomato increased gradually with instars. During the adult stage, however, the abundance of Delftia varied with the types of host plants. The abundance of Delftia in the 2-day-old adult of P. solenopsis fed on cotton was high, that in the 6-day-old adult fed on cotton decreased, and that in adult fed on cotton at 4 d after beginning to produce nymphs also decreased. By contrast, the abundance of Delftia in the 2-day-old adults of P. solenopsis fed on tomato showed a significant decline, that in the 6-day-old adult fed on tomato obviously increased, and that in adult fed on tomato at 4 d after beginning to produce nymphs also increased. The abundance of Delftia in the gut and ovary of the newly emerged adults was closely related to the developmental stage of P. solenopsis, decreasing slowly in the gut along with adult development while increasing gradually in the ovary from adult emergence to the 6-day-old adult. Delftia was present in both the midgut and Malpighian tubules of adults, with the latter being infected more abundantly. There existed extremely similar Delftia strains in P. solenopsis and their host plants, tomato and cotton. 【Conclusion】 The diversity of Delftia bacteria in P. solenopsis is very low, which are distributed in the gut, ovary and Malpighian tubules of hosts. The abundance of Delftia can be affected by developmental stages of P. solenopsis and their host plant types. Host plants might serve as a source of Delftia in P. solenopsis.

Key words: Phenacoccus solenopsis, symbiotic bacteria; Delftia, bacterial diversity, bacterial abundance, infection dynamics