MED Q2 was first detected in 2013 at a site in Kanto District, Japan [28]. In 2015, MED Q2 was detected at two additional sites in the same area [29]. In the present study, we detected MED Q2 at ten sites in the same district (Supplementary Table S1). In a previous study, we also investigated two sites, Isesaki (locality no. 11) and Maebashi (locality no. 14) but did not detect MED Q2 (0/12 and 0/6 individuals, respectively) [28]. This suggests that MED Q2 has expanded its distribution area in Kanto District since 2013. MED Q2 was detected at Sites 11 and 14 over multiple years (Supplementary Table S1 and Fig. 1), suggesting that the MED Q2 population was established here and in the wider area of the Kanto District.
MED Q2 appeared to have expanded its distribution to many countries. In South Korea, MED Q2 was first identified in 2018 [30]. In southern Italy, the distribution of MED Q2 has expanded since it was first discovered, and the dominant type was replaced MED Q1 by MED Q2 in only a few years (2010–2013) [26]. In the 2017–2019 surveys conducted in central and southern Italy and Sicily, MED Q2 accounted for 87.6% of the total samples collected from greenhouse crops [58]. The high temperature tolerance and insecticide resistance of MED Q2 are thought to have contributed to its spread in Italy [26]. Therefore, global warming and insecticide use may be responsible for the expanding distribution of the MED Q2 in Japan. To prevent the spread of MED Q2, effective insecticides and other methods must be identified.
The infection status of MED Q2 has been previously reported in Israel [21, 53], Turkey [27, 54], and Italy [26]. The MED Q2 in these countries commonly indicated a high infection rate with Arsenophonus and Rickettsia, but a low infection rate with Wolbachia. The infection status of the Japanese MED Q2 group differed substantially from those of the other groups. In addition to the primary symbiont, Portiera, two types of S-symbionts, Rickettsia and Wolbachia, were detected in the Japanese MED Q2 (Fig. 1). The prevalence of Rickettsia infection was 100% throughout all survey periods; Wolbachia was only detected in some of the samples. Interestingly, Arsenophonus, which is highly prevalent in MED Q2 in other countries, has never been detected in the Japanese MED Q2 population. The possible reasons for this difference are as follows: (1) only MED Q2, which is uninfected with Arsenophonus, has entered Japan, and (2) Arsenophonus-uninfected MED Q2 has increased their population due to its better adaptability under Japanese field conditions. However, these hypotheses are not mutually exclusive. In future, a detailed analysis is necessary to clarify this issue.
Rickettsia was nearly fixed in MED Q2 populations in all regions, suggesting possible important contributions to survival, reproduction, and so on. The importance of Rickettsia in MED Q2 is also suggested by its population dynamics in host insects; Rickettsia titres increased during nymphal development and then rapidly decreased in males from 1 d after eclosion (Fig. 2). These bacterial population dynamics have been observed in many obligate symbionts of various insect hosts, including Buchnera in the pea aphid (Acyrthosiphon pisum), Sodalis in cereal weevils (Sitophilus oryzae), and Portiera and Hamiltonella in B. tabaci MEAM1 [59–62]. This phenomenon has been interpreted as the regulation of nutrient-compensating symbionts to meet the dietary needs of host insects. The Rickettsia titre in females remained at higher levels for a longer duration than that in males (Fig. 2). This suggests that symbiotic populations are regulated differently depending on the sex of the host. It is conceivable that higher Rickettsia levels in females evolved to ensure vertical transmission, as proposed for the obligate symbiont Hamiltonella in the MEAM1 of B. tabaci [62].
Previous genomic studies on B. tabaci MEAM1 and MED Q1 have shown that Portiera alone is unable to synthesise essential amino acids that are deficient in the host diet [63–65]; and the enzymes responsible for producing essential amino acids are found across Portiera, a coexisting symbiont (Hamiltonella or Arsenophonus), and B. tabaci. Hence, metabolic intermediates must be transported between the symbionts and the host multiple times for production. These symbionts coexist within the same bacteriocyte [43], which seems to be adaptive for the efficient production of essential nutrients via intertwined metabolic pathways [62]. Incomplete metabolic pathways were found in Portiera for both MEAM1 and MED Q1 [66, 67], suggesting that the loss of the enzymes in Portiera occurred before the divergence of these lineages. Therefore, it is likely that Portiera has incomplete metabolic pathways in the derived lineage MED Q2. In the present study, Rickettsia was not detected in the bacteriocytes of MED Q2 (Figs. 3, 4; Supplementary S2 and S3), in contrast to the previously reported "confined type" in MEAM1 and MED Q2 [43]. Instead, Rickettsia aggregated in the regions proximal to Portiera-containing bacteriocytes at all growth stages (Fig. 3; Supplementary Fig. S3). The physical proximity of Rickettsia and Portiera in MED Q2 suggests a close interaction via nutrient metabolism. To identify their metabolic pathways and test this possibility, genomic analyses of Rickettsia and Portiera in MED Q2 are required.
In MED Q1 and MEAM1, the co-obligate symbionts Portiera and Hamiltonella are passed on to the next generation by transferring a whole bacteriocyte bearing the symbionts to the developing egg [32]. Rickettsia in MED Q2 were detected in pre-vitellogenic oocytes (stage 1 egg as defined in [68]), which is the stage before the bacteriocyte enters the egg. A previous study reported that Rickettsia in MEAM1 attached to a bacteriocyte and transferred with the bacteriocyte to the developing egg [68]. However, in MED Q2, we did not observe Rickettsia on the bacteriocytes in eggs (Fig. 4). This result suggests that there is a mechanism in MED Q2 for the transmission of Rickettsia into eggs without the involvement of bacteriocytes. In the future, the invasion process of Rickettsia into early developing eggs should be clarified in detail using confocal and electron microscopy.
The most serious damage to crops caused by B. tabaci is due to the transmission of TYLCV. Therefore, the ability to transmit TYLCV is a critical factor in determining the risk of whiteflies becoming an agricultural pest. According to previous studies, Israeli MED Q2 has low ability to transmit TYLCV because it lacks infection with Hamiltonella, whose GroEL facilitates TYLCV transmission [69–72]. This study showed that Japanese MED Q2, which is free of Hamiltonella infection, is an effective vector for both TYLCV-IL and TYLCV-Mld, similar to Hamiltonella-infected MED Q1. MED Q2 tended to have higher TYLCV-IL retention (Fig. 5) and transmission rates to plants (Table 1) than MED Q1. TYLCV-Mld retention was significantly lower in MED Q2 than in MED Q1 (Fig. 5). However, the transmission rates in MED Q2 were as high as those in MED Q1 (Table 1). These results indicate that the Japanese MED Q2 is a high-risk agricultural pest, comparable to MED Q1, and possesses efficient TYLCV transmission machinery independent of Hamiltonella. Rickettsia may play a critical role in the transmission. In a previous study on MEAM1, Rickettsia was suggested to accelerates the uptake of TYLCV into the haemolymph, which may enhance TYLCV transmission to tomato plants [56]. Certain proteins, such as GroEL, produced by Rickettsia, may be involved in the transmission efficiency of TYLCV in MED Q2, similar to Hamiltonella GroEL in MED Q1 [71]. To investigate the potential role of Rickettsia in the high efficiency of TYLCV transmission in the Japanese MED Q2, future experiments will require the elimination of Rickettsia through antibiotic treatment and analysis of its GroEL interaction with TYLCV. Elucidation of the mechanisms underlying TYLCV transmission may lead to the development of more effective control technologies.