Cardinium and Rickettsia abundance in Bemisia tabaci MED collected globally
The 2bRAD-M sequencing results showed that Cardinium infection rate varied significantly among B. tabaci MED collected from the different areas (P < 0.001, Kruskal–Wallis test, Fig. 1 left panel). Whiteflies collected in Israel (IL population) had no Cardinium (no sample’s relative abundance of Cardinium exceeded 0.01%). Whiteflies collected from South Korea (Kr) had a relative higher Cardinium abundance (21.54 ± 3.24%, Mean ± SEM) compared to whiteflies collected in other areas (Fig. 1, left panel). Rickettsia was abundant in B. tabaci MED populations with low Cardinium infection such as IL (68.07 ± 4.57%, Mean ± SEM), CY (69.84 ± 7.14%, Mean ± SEM), ZJ (48.21 ± 13.19%, Mean ± SEM) and FJ (43.76 ± 5.30%, Mean ± SEM) populations, but was lower in Cardinium-abundant populations, such as in Kr population (relative abundance of Rickettsia lower than 0.01%) (Fig. 1, right panel).
Another interesting finding was that the relative abundance of Cardinium was higher in high-latitude populations compared to those in relatively low-latitude populations. In East Asia, high-latitude populations such as JL, XJ and Kr populations (more than 7.38%) had a relatively higher Cardinium abundance than low-latitude populations including FJ, GD, HN populations (lower than 4.16%). In Europe and the Mediterranean region, the relatively higher latitude populations including such as CR population also had higher Cardinium abundance (above 14.16%) than BH, CY and IL populations (lower 1.00%) which were collected in relatively low-latitude areas (Fig. 1, left panel).
Microbial composition of different Bemisia tabaci MED populations
The microbial communities in each B. tabaci MED population, was identified to the species level. One primary symbiont Portiera aleyrodidarum and five secondary symbionts (Hamiltonella, Cardinium, Wolbachia, Rickettsia and Arsenophonus) were discovered in all 49 B. tabaci MED populations. Portiera aleyrodidarum was the most abundant bacteria in almost all geographical populations except for IL and CY populations (in which the abundance of Rickettsia was higher than Portiera), with a relative abundance being above 28.82% (lowest in CY population) in all whitefly populations.
Rickettsia was abundant in the CY population (69.84% of all bacteria) and IL population (68.07%). However, no sequences of Rickettsia were identified in the AH, JX, HuB, XJ and all South Korea populations. The Rickettsia species identified in the samples included Rickettsia sp. Strain MEAM1, Rickettsia hoogstraalii, Rickettsia felis and Rickettsia bellii. Of these, Rickettsia sp. Strain MEAM1 was the dominant species in all Rickettsia-infected samples. Rickettsia bellii showed a lower infection rate in ZZ, JN, HN and USA populations (relative abundance lower than 0.1‰). Likewise, Rickettsia hoogstraalii showed a lower infection rate in SG, HN, JL, CR, CY populations (relative abundance lower than 0.1‰). Rickettsia felis (relative abundance 1.14‰) was identified only in the BH population.
Interestingly, in all populations which had a high Rickettsia abundance, showed lower Cardinium abundance. On the other hand, Cardinium-abundant populations (such as the Korea populations) showed no or less abundant Rickettsia, which imply a potential antagonistic interaction between Cardinium and Rickettsia in B. tabaci MED.
Four Cardinium species were identified in this study (Cardinium cBtQ, Cardinium cEper1, Cardinium endosymbiont of Culicoides punctatus and Cardinium cSfur). Of these, Cardinium cBtQ was the dominant species in all Cardinium-infected samples. Cardinium cEper1 had a lower infection rate (relative abundance lower than 0.1‰), in the JS, Kr2, Kr5, ZZ, LC, AH and JL populations. Likewise, Cardinium cSfur had a lower infection rate in the GD, SG, SPA3 and LC populations, while Cardinium endosymbiont of Culicoides punctatus was only detected in the Kr1 population.
The results showed that all whitefly populations were infected with Hamiltonella defensa, with a relative abundance ranging from 0.02% (lowest in the JX population) to 18.37% (highest in the HuB population).
Seven Wolbachia species were identified from the whitefly samples (Wolbachia endosymbiont of Bemisia tabaci, Wolbachia endosymbiont of Cylisticus convexus, Wolbachia endosymbiont of Diaphorina citri, Wolbachia endosymbiont of Folsomia candida, Wolbachia endosymbiont of Laodelphax striatellus, Wolbachia endosymbiont of Leptopilina clavipes, Wolbachia endosymbiont of Nilaparvata lugens, Wolbachia pipientis wAlbB). Of these, Wolbachia endosymbiont of Bemisia tabaci was the dominant in all Wolbachia-infected samples. However, its abundance was relatively low (below 5.94%) in all samples, except for the JS2 population, which had a relative abundance of 21.12% of all bacterial communities. The relative abundance of the other Wolbachia species was much lower (relative abundance lower than 0.1‰) in all samples. In some East Asia populations including BJ, HN, Kr1, Kr2 and Kr3 populations, no Wolbachia was detected.
Arsenophonus was detected in several populations (including AH, JX, BH, CR, CY and IL populations) with a low-abundance rate below 2.8% (Fig. 2). Four Arsenophonus species was identified from all samples in this study (Arsenophonus endosymbiont of Bemisia tabaci Asia II3, Arsenophonus endosymbiont of Aleurodicus floccissimus, Arsenophonus endosymbiont str Hangzhou of Nilaparvata lugens, Arsenophonus nasoniae). Of these, Arsenophonus endosymbiont of Bemisia tabaci Asia II3 was the dominant in all Arsenophonus-infected samples.
Bacterial diversity in geographically different Bemisia tabaci MED populations
Three α-diversity indexes (including Chao1, Simpson and Shannon indexes) were applied to determine the diversities of the bacterial communities in each whitefly population. The diversity indexes were all analyzed with the Kruskal–Wallis test and Dunn’s test with Bonferroni correction for multiple comparisons (Fig. 3), as none followed a normal distribution. The Shannon and Simpson index analysis showed a similar result for the community diversity index for all whitefly populations. Results showed that the JL population had the highest Simpson index (0.55 ± 0.02, Mean ± SEM) and a relatively high Shannon index (0.97 ± 0.05, Mean ± SEM), while the CR population had the highest Shannon index (1.04 ± 0.12, Mean ± SEM) and a relatively high Simpson index (0.49 ± 0.06, Mean ± SEM). On the other hand, the JX population had the lowest Shannon index (0.21 ± 0.05, Mean ± SEM) and the lowest Simpson index (0.09 ± 0.03, Mean ± SEM) (Fig. 3A and 3B). Results of the Chao1 diversity index showed that the AH population had the highest bacterial community richness (60.40 ± 9.18, Mean ± SEM), while the BJ population had the lowest Chao1 index (17.00 ± 1.91, Mean ± SEM) (Fig. 3C). Interestingly, the α-diversity indexes of Cardinium abundant populations (such as JL, CR, USA and Kr populations) were higher than diversity indexes of Cardinium limited populations (such as JX, BJ, IL and HuB populations) (Fig. 3).
Relationship between symbionts and bacterial diversities in Bemisia tabaci MED
The correlations between the relative abundance of symbionts and the α-diversity indexes (including Shannon, Chao1 and Simpson indexes) of microbial communities in whitefly, were analyzed using Spearman correlation. The microbiota diversity indexes were computed after removing the compared symbiont in this experiment. The results revealed that Cardinium abundance significantly influenced the B. tabaci microbiota Simpson diversity index negatively (r = -0.449, P < 0.01) (Fig. 4B). Hamiltonella had a significant negative effect on both the Shannon diversity (r = -0.498, P < 0.01) and Simpson diversity (r = -0.498, P < 0.01) of host whitefly (Fig. 4G and 4H), while Portiera and Rickettsia did not significantly influence the diversity indexes of bacterial communities in B. tabaci (Fig. 4). Categorical analysis showed that both Wolbachia and Arsenophonus did not significantly affect the diversity of host whitefly (Fig. S1 and S2).
Relationships among various symbionts in Bemisia tabaci MED
Pearson correlation was applied to clarify the complex relationships among the different symbionts in B. tabaci MED. Results showed that the relative abundance of Hamiltonella significantly negatively correlated with the relative abundance of Rickettsia (r = -0.403, P < 0.01) and Wolbachia (r = -0.351, P < 0.05) (5ig. 4A and 5B). The relative abundance of Portiera had a significant positive correlation with the relative abundance of Hamiltonella (r = 0.296, P < 0.05), but a significant negative correlation with the relative abundance of Rickettsia (r = -0.886, P < 0.001) (Fig. 5C and 5D). Results from the generalized linear models revealed a significant correlation between Rickettsia and Hamiltonella (Table 2). Categorical analysis showed that Wolbachia and Arsenophonus significantly affected the abundance of each other positively (Fig. S1E and S2E).
Table 2
Generalized linear models of symbionts, bacterial diversities and environmental factors in Bemisia tabaci MED.
Dependent Variable | Covariates | B | Wald Chi-Square | P-value |
Rickettsia | Hamiltonella | 0.156 | 14.496 | 0.003 |
Portiera | X | 0.039 | 14.913 | < 0.001 |
Y | -0.365 | 7.255 | 0.007 |
Bio1 | -0.202 | 9.324 | 0.002 |
Bio12 | 0.003 | 10.085 | 0.001 |
X*bio1 | 0.001 | 5.536 | 0.019 |
X*bio5 | -0.002 | 17.685 | < 0.001 |
X*bio12 | -0.000014 | 9.398716 | 0.002 |
Y*bio5 | 0.011 | 4.996 | 0.025 |
Rickettsia | X | -0.44 | 17.247 | < 0.001 |
Y | -0.365 | 7.255 | < 0.001 |
Bio1 | -0.202 | 9.324 | 0.002 |
Bio12 | 0.003 | 10.085 | 0.001 |
X*bio1 | 0.001 | 5.536 | 0.019 |
X*bio5 | -0.002 | 17.685 | < 0.001 |
X*bio12 | -0.000014 | 9.398716 | 0.002 |
Y*bio5 | 0.011 | 4.996 | 0.025 |
Hamiltonella | X | 0.006 | 4.132 | 0.042 |
Shannon | Y | 0.312 | 4.396 | 0.036 |
X*bio1 | -0.001 | 6.702 | 0.010 |
X*bio5 | 0.001 | 7.969 | 0.005 |
X*bio12 | 0.000012 | 5.911 | 0.015 |
Simpson | X | -0.016 | 4.982 | 0.026 |
Y | 0.234 | 5.895 | 0.015 |
X*bio1 | -0.001 | 6.007 | 0.014 |
X*bio5 | 0.001 | 8.947 | 0.003 |
X*bio12 | 0.000009 | 7.838 | 0.005 |
Y*bio5 | -0.007 | 4.020 | 0.045 |
Chao1 | Y | 29.251 | 4.619 | 0.032 |
X*bio12 | 0.001 | 5.763 | 0.016 |
Y*bio1 | 0.361 | 4.817 | 0.028 |
Y*bio5 | -1.081 | 4.627 | 0.031 |
Note: “X” means “longitude”, “Y” means “latitude”. |
Influence of ecological factors on the microbial communities in global Bemisia tabaci MED populations
To understand whether and to what extent ecological factors shaped the microbial communities across the B. tabaci MED populations, Pearson analysis and structural equation model (SEM) were used to resolve the relationships between microbial community structure (including symbionts and α-diversity indexes) and 5 putative predictor variables including annual mean temperature, mean temperature of warmest quarter, annual precipitation, latitude, and longitude. The diversity and abundance indexes of symbionts as influenced by climate or geographical factors were described by a structural equations model (SEM). Results showed that the annual mean temperature significantly and positively influenced the relative abundance of Rickettsia (r = 0.326, P < 0.05) (Fig. 6A), while annual precipitation significantly and positively influenced the Arsenophonus abundance (r = 0.325, P < 0.05) and the Chao1 index (r = 0.405, P < 0.01) (Fig. 6B and 6C) in global whitefly populations. Generalized linear models also revealed a significantly influence of ecological factors on the abundance of symbionts Portiera, Rickettsia and Hamiltonella (Table 2).
The relationships between the microbial communities of B. tabaci MED and longitude were also explored. Results showed that longitude positively influenced the abundance of Portiera (r = 0.499, P < 0.001) but negatively influenced the abundance of Rickettsia (r = -0.411, P < 0.01) (Fig. 7). Also, longitude negatively influenced the Shannon (r = -0.402, P < 0.01) and Simpson indexes (r = -0.321, P < 0.05) (Fig. 8).
The SEM analysis was used to clarify the complex interactions among symbionts, bacterial communities and ecological factors. The SEM models were simplified based on the lowest value of Akaike Information Criterion (AIC). The simplified models were not rejected based on the P value of chi-square test > 0.05, comparative Index (CFI) > 0.90, standardized root mean square residual (SRMR) < 0.05. For the relationships among different symbionts and ecological factors (Fig. 9). Results showed that longitude positively influenced Portiera abundance (r = 0.40 ± 0.12, z = 2.04, P < 0.05), but negatively influenced Rickettsia abundance (r = -0.42 ± 0.12, z = 3.40, P < 0.001). Further, longitude positively influenced Hamiltonella abundance but negatively influenced Wolbachia and Arsenophonus abundances. Latitude positively influenced both the abundance of Cardinium and Hamiltonella, but negatively influenced Rickettsia abundance. Annual mean temperature negatively affected Portiera abundance and positively affected Hamiltonella abundance, but mean temperature did not significantly influence Rickettsia abundance. The mean temperature of warmest quarter negatively influenced Hamiltonella abundance and Wolbachia abundance, but positively influenced Portiera abundance. Annual precipitation positively influenced Arsenophonus abundance and positively influenced Wolbachia abundance, but negatively affected Hamiltonella abundance. Also, SEM analysis indicated a negative correlation between Portiera and Rickettsia, as well as a negative correlation between Hamiltonella and Wolbachia. Additionally, SEM analysis also showed a negative correlation between Rickettsia and Cardinium abundance (r = -0.27 ± 0.12, z = 2.36, P < 0.05) (Fig. 9).
The relationships between the diversities of microbial communities and ecological factors were also explored by SEM (Fig. 10). Results showed that longitude negatively influenced the Shannon index, while latitude positively influenced both the Shannon and Chao1 indexes. Annual precipitation also positively influenced the Chao1 index, which result was similar to the Pearson analysis (Fig. 6C), and also positively influenced the Shannon index. Annual temperature and mean temperature positively influenced both the Shannon and Simpson indexes, while the mean temperature of warmest quarter negatively influenced both the Shannon and Simpson indexes (Fig. 10).