Diversity and role of bacterial endosymbionts infecting the cassava whitefly, Bemisia tabaci (Gennadius) in Sub-Saharan Africa

The main aims of this study were to investigate the diversity of endosymbiotic bacteria infecting cassava whiteflies, understand their putative role on insect population dynamics and cassava virus disease epidemics in sub-Saharan Africa (SSA). The genetic diversity of field-collected cassava whiteflies, Bemisia tabaci, and their endosymbionts from Tanzania, Uganda, Malawi and Nigeria were described. Cassava in these countries is infested by five whitefly populations, which were infected with diverse endosymbiotic bacteria with varied prevalence. Wolbachia was the most predominant symbiont with infection levels varying from 21 to 97%. Arsenophonus varied from 17 to 64% and that of Rickettsia was 0 to 53%. Sequence analysis of the bacteria revealed high genetic diversity. Several strains of Rickettsia, Arsenophonus and Wolbachia were found infecting cassava whiteflies. Hamiltonella and Fritschea were absent in all the samples tested. Fluorescent in situ hybridisation was used to localise Arsenophonus, Rickettsia and Wolbachia. These were confined within specialised cells called bacteriocytes in both nymphs and adults. Isofemale lines of different whitefly populations with varied symbiont infections were developed for comparing the effect of bacterial infection on the whitefly fitness and biology. Isofemale colonies of sub-Saharan Africa 1-subgroup 3 (SSA1-SG3), infected with Arsenophonus and Rickettsia (AR+) and those free of AR (AR-) were compared for fecundity, nymph development, adult emergence and development time on healthy and East African cassava mosaic virus-Uganda (EACMV-UG)-infected cassava plants var. Ebwanateraka. The fecundity (number of eggs laid) and nymph development was not affected by the presence of endosymbionts or the infection of cassava by the virus. AR- whiteflies, however, produced significantly higher number of adults than AR+ on both healthy and virus-infected plants, indicating that bacterial infections negatively affect cassava whitefly development. The AR+ whiteflies took longer time to emerge on healthy and virus-infected plants than AR-. AR+ and AR- populations were further compared for EACMV-UG acquisition and retention after 48 hours each of acquisition access and inoculation access on diseased and healthy cassava var. Ebwanateraka, respectively. Higher proportion of AR- acquired and retained EACMV-UG than AR+. Similarly, AR- also retained higher (~9 folds) titres of virus than AR+. This indicated that bacteria-free SSA1-SG3 AR-whiteflies had higher adult emergence, quicker life cycle and better vectoring abilities than those infected with bacteria. However, innate immune response genes were upregulated in AR+ than AR- populations, possibly indicating a protective role of bacterial infections in AR+ whiteflies against pathogenesis. Endosymbiotic bacteria infecting cassava whiteflies during the Uganda cassava mosaic disease (CMD) pandemic in 1997 were characterized. No statistically significant association was found between symbiont infections of whiteflies and the CMD pandemic. However, SSA1-SG2 and SSA2, which were predominant in the 1990s and have become rare recently, had higher prevalence and titres of Wolbachia. Simulation experiments to investigate the effect of bacterial infections on different whiteflies showed that Wolbachia-infected SSA1-SG2 was quickly outcompeted and almost eliminated by the Wolbachia-free SSA1-SG3. These results indicated fitness costs of Wolbachia infections on cassava whitefly biology, which may have led to the decline of SSA1-SG2 and SSA2 in cassava fields. SSA2 whiteflies with different symbiont infections were also compared for EACMV-UG retention and expression of immune genes. SSA2 with Arsenophonus retained higher titres of virus than those infected with Wolbachia and free of symbionts. SSA2 with Wolbachia, however, had higher expression of antimicrobial peptides, which may have led to reduced virus retention. Put together, these results indicate the various effects of bacterial symbionts on whitefly and viruses they transmit. These should be studied further to better understand and control superabundant whitefly populations and cassava disease pandemics in eastern Africa