Diversity and evolution of the endosymbionts of Bemisia tabaci in China

Introduction

Bemisia tabaci is a cryptic species complex comprising a minimum of 40 morphologically similar species (De Barro et al., 2011; Dinsdale et al., 2010; Hu et al., 2018; Wang, Li & Liu, 2017). Among members of the complex, the Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) groups (commonly known as the B and Q biotypes, respectively) have drawn much attention due to their global invasion and vectoring important plant pathogens (e.g., tomato yellow leaf curl virus) (Brown, 1994; Cohen & Nitzany, 1966). In China, B. tabaci was not considered as a serious pest until the arrival of the MEAM1 group in the mid-1990s (Qiu et al., 2007). In 2004, the MED group was detected, and rapidly became widely distributed in China, causing considerable damage to a wide range of vegetables, fibers, and ornamental crops (Chu et al., 2005). It is interesting that MED has been replacing the earlier invader MEAM1 as well as several indigenous species of whiteflies (e.g., Asia II and China 1) in many regions (Liu et al., 2007; Sun et al., 2013).

Associations between insects and endosymbionts are quite common in nature. It has been estimated that at least 15−20% of all insect species live in symbiotic relationships with bacteria (Douglas, 1998; Gosalbes et al., 2010). Endosymbionts associated with insects can be classified into primary endosymbionts (P-endosymbionts) and secondary endosymbionts (S-endosymbionts) (Baumann, 2005). The P-endosymbionts are obligate and usually have mutualist relationships with their hosts. Besides, P-endosymbionts are generally localized in bacteriocytes of bacteriome (Caspi-Fluger et al., 2011) and transmitted vertically from mother to progeny (Werren & O’Neill, 1997). In contrast, the S-endosymbionts are usually facultative symbionts, and they could reside in several host tissues such as gut, hemolymph, Malpighian tubules, salivary glands or ovarian cells (Cicero, Fisher & Brown, 2016; Cooper, Sengoda & Munyaneza, 2014; Dobson et al., 1999; Zchori-Fein, Roush & Rosen, 1998). Infection of secondary endosymbionts can be either maternally inherited or horizontally transmitted (Moran & Baumann, 2000). It has been discovered that the P-endosymbionts Portiera aleyrodidarum and S-endosymbionts such as Arsenophonus, Cardinium, Fritschea, Hamiltonella, Rickettsia, and Wolbachia were infected in whiteflies (Bing et al., 2013; Chiel et al., 2007; Chu et al., 2011; Everett et al., 2005; Karut et al., 2017; Thao & Baumann, 2004; Zchori-Fein, Lahav & Freilich, 2014). Previous studies have investigated the prevalence, diversity and evolution of endosymbionts in the B. tabaci species complex from different countries or regions (e.g., Turkey, China, Brazil, and Africa) (Ahmed et al., 2010; Bing et al., 2013, 2014; Ghosh, Bouvaine & Maruthi, 2015; Hashmi et al., 2018; Jahan et al., 2015; Karut et al., 2017; Marubayashi et al., 2014; Santos-Garcia et al., 2015; Sseruwagi et al., 2018; Thierry et al., 2011, 2015). However, most of these reports only focused on the two invasive cryptic species MEAM1 and MED; furthermore, for studies from China, the sample size and distribution range were limited. For example, regarding prevalence of S-endosymbionts in B. tabaci, only the laboratory samples from two provinces were investigated (Bing et al., 2013); only one S-endosymbiont (Wolbachia) was explored from a slightly larger geographical scale (Bing et al., 2014).

Therefore, although substantial datasets regarding endosymbionts prevalence of B. tabaci species complex are present, their current situation in whole China is still not very clear and further investigation is essential. In the present study, our first goal is to investigate the prevalence and diversity of the P-endosymbionts P. aleyrodidarum and two common S-endosymbionts Arsenophonus and Cardinium (as representatives of S-endosymbionts) within B. tabaci in a wider range of China. We also aimed to explore the evolutionary relationships between these three endosymbionts and their host based on phylogenetic analyses of 16S and 23S ribosomal DNA (rDNA) (from endosymbionts) as well as mitochondrial cytochrome oxidase 1 (mtCO1) gene (from B. tabaci). Our study will not only uncover the current status of endosymbionts infection within B. tabaci in China, but also greatly provide a supplement for studies of B. tabaci endosymbionts worldwide.

Materials and Methods

Sample collection

A total of 1,510 B. tabaci individuals were collected from 71 geographical locations, including 19 provinces and four municipalities in China (Fig. 1). At each location, B. tabaci samples were collected from different leaves of separate plants. The collection details, geographical sites and host plants were described in Table S1.

Results

Prevalence of endosymbionts among five species of B. tabaci

As expected, P. aleyrodidarum was detected in all whitefly individuals and species (Fig. 2A); 21.7% (327/1,510) individuals harbored S-endosymbiont (Arsenophonus or Cardinium); 5.6% (84/1,510) whiteflies were co-infected with both, and the remained majority (72.8%; 1,099/1,510) lacked an infection with either of the two S-endosymbionts. Infection frequencies with S-endosymbionts varied across B. tabaci species. In detail, Arsenophonus was abundant in the indigenous species (Asia II 1, Asia II 3, and China 1), with infection rates ranging from 50.7% to 55.7%; however, it was infrequent in MED (11.5%) and MEAM1 (0.0%) (Fig. 2B). Cardinium was moderately common in MED, Asia II 1 and Asia II 3 populations with frequencies of 20.3–40.7%, but rarely detected in MEAM1 (0.5%) and not found in China 1 (Fig. 2C).

Phylogenetic analysis of B. tabaci and endosymbionts

Phylogenetic trees were constructed for B. tabaci, P. aleyrodidarum, Arsenophonus, and Cardinium based on mtCO1 gene, 16s rDNA, 23S rDNA, and 16s rDNA sequences of haplotypes, respectively. We also pulled the related sequences from NCBI to explore the phylogenetic status of our haplotypes. The GenBank number of those related sequences could be found in phylogenetic trees. Based on the mtCO1 gene, we found five distinct genetic groups of B. tabaci, which was corresponding with the five cryptic species including the MEAM1, MED, Asia II 1, Asia II 3, and China 1 (Fig. 3). We can also find that all of our MED individuals belonged to Q1 subclade (Chu et al., 2011). For phylogenetic trees of endosymbionts, several distinct bacterial strains existed within individual bacterium. It is interesting that the phylogenetic tree of P. aleyrodidarum were similar to the mtCO1 tree and exhibited four groups corresponding to MEAM1+MED, Asia II 1, Asia II 3, and China 1 clades to some extent (Fig. 4). While there are still several differences. For example, the sequences of P. aleyrodidarum of China 1 were not well clustered and P. aleyrodidarum from B. tabaci species MED (KP201113) was also present in that China 1 clade. In addition, the MEAM1+MED clade (Fig. 4) contained sequences of P. aleyrodidarum from both MEAM1 and MED; however, it was separated into two distinct clades in the B. tabaci tree (Fig. 3). In contrast, the phylogenetic trees for Arsenophonus and Cardinium were totally incongruent with the mtCO1 tree, exhibiting four (A1–A4) and three groups (C1–C3), respectively (Figs. 5 and 6).

Discussion

In this study, we conducted an extensive screening of B. tabaci for the presence of one P-endosymbiont and two common S-endosymbionts, along with phylogenetic analyses of these symbionts to compare with host species from the cryptic B. tabaci complex. The reason we chose Arsenophonus and Cardinium as representatives of S-endosymbionts because they are the very common S-endosymbionts in whiteflies, and Wolbachia has been thoroughly investigated in the study of Bing et al. (2014). The results showed that P-endosymbiont P. aleyrodidarum was detected in all whitefly individuals while S-endosymbionts infection were varied among species. The variation in the prevalence of endosymbionts could be influenced by numerous factors such as host, environmental conditions, geographical location or even climate (Chu et al., 2011; Karut & Tok, 2014; Morag et al., 2012; Skaljac et al., 2010). In our study, Arsenophonus was abundant in Asia II 1, Asia II 3, and China 1 species but absent in the invasive species MEAM1, which is exactly consistent with previous studies (Bing et al., 2013; Karut et al., 2017); Cardinium was present in the MED, Asia II 1 and Asia II 3 species (20.3–40.7%) but was rarely detected in MEAM1 and not detected in China 1. Taken together, it seemed that these two S-endosymbionts had high prevalence in native species rather than invasive species, which is consistent with another S-endosymbiont Wolbachia but in contrast to Hamiltonella; Hamiltonella was found abundant in invasive species rather than native species (Bing et al., 2013).

Previous studies showed that B. tabaci could be co-infected with particular pairs of S-endosymbionts, including Rickettsia and Hamiltonella, Hamiltonella and Cardinium, or Rickettsia and Arsenophonous; others were less common, such as Cardinium and Rickettsia, Hamiltonella and Arsenophonous, Cardinium and Wolbachia, and even three or four endosymbionts together (Gueguen et al., 2010; Karut & Tok, 2014; Marubayashi et al., 2014; Pan et al., 2012; Skaljac et al., 2010). In our study, we found evidence for a low rate (5.6%) of co-infection with Arsenophonous and Cardinium in four B. tabaci species (MEAM1 was the exception since Arsenophonous was not detected in this species), which has also been reported before (Bing et al., 2013; Chu et al., 2011; Parrella et al., 2014; Zchori-Fein, Lahav & Freilich, 2014). However, the reason of so few rate of co-infection by Arsenophonous and Cardinium is that Arsenophonous and Cardinium are potential reproductive manipulators that compete for resources inside the bacteriocytes, thus compromising the fitness of host (Gottlieb et al., 2008). We have one plausible explanation for the co-infection status, that is Cardinium is not restricted to the bacteriocytes (Skaljac et al., 2010), and perhaps the non-overlapping niche makes co-infection of Arsenophonous and Cardinium possible. In addition, the co-infection symbiont system in whiteflies may indicate the roles of dual endosymbionts: work as important mutually dependents to provide full complement of nutrients to their host (Rao et al., 2015) or affect the fitness and biology of the B. tabaci (Ghosh et al., 2018).

Our phylogenetic analyses indicated that B. tabacia mtCO1 sequences could be assigned to five distinct clades, which conformed to existing MEAM1, MED, Asia II 1, Asia II 3, and China 1 clades. Similarly, the sequences of the P-endosymbionts P. aleyrodidarum were assigned to their own clade and the phylogeny was similar with that of B. tabaci genetic groups to some extent. This may potentially indicate an ancient infection followed by vertical transmission and subsequent co-evolutionary diversification (Baumann, 2005). Meanwhile, it is important to note that the correlation was not perfect: sequences of P. aleyrodidarum from MEAM1 and MED were assigned to the same clade instead of the two distinct clades presented in the mtCO1 tree. The reason could be the dissemination of the MEAM1 and MED species; furthermore, the spread of these two invasive species in China has been frequently associated with founder effects that fix specific mtDNA variation(s) along with particular endosymbionts (Chu et al., 2011; Gueguen et al., 2010). Taken together, although there was similarity between the two trees of P. aleyrodidarum and B. tabaci, the genetic variation of primary symbiont might not be an ideal reflecting the genetic variation of the cryptic B. tabaci.

The S-endosymbionts, Arsenophonus and Cardinium, both showed a lack of congruence with the B. tabaci mtCO1 phylogram. This is consistent with the finding from Ahmed et al. (2013), who provided evidence for horizontal transmission of S-endosymbionts in the B. tabaci cryptic species complex based on phylogenies studies. There are substantial phylogenetic evidences showing that S-endosymbionts such as Wolbachia and Arsenophonus, undergoing horizontal transfer among host arthropod species (Ahmed et al., 2013; Chrostek et al., 2017; Kolasa et al., 2017; Li et al., 2017; Russell et al., 2003; Vavre et al., 1999). In some cases, the mechanisms for horizontal transmission of S-endosymbionts are already known, including transferring through parasitoid wasps (Ahmed et al., 2015; Gehrer & Vorburger, 2012), plants (Caspi-Fluger et al., 2012; Li et al., 2017) or even sexual transmission (Moran & Dunbar, 2006). Therefore, the potential horizontal transfer of S-endosymbionts in our samples could be one or combination of the above ways.

In summary, this study reported the varied prevalence of three endosymbionts within five B. tabaci cryptic species. The P-endosymbiont P. aleyrodidarum was detected in all whitefly individuals; the S-endosymbionts Arsenophonus was abundant in native species while Cardinium was common in the invasive species. In addition, the phylogenetic relationships between endosymbionts and their hosts B. tabaci probably indicated the vertical transmission and co-evolution of P. aleyrodidarum and B. tabaci; meanwhile, horizontal transfer of Arsenophonus and Cardinium may happen in our collecting samples. Our study not only reported current infection status of endosymbionts within B. tabaci populations in China, but also demonstrated that S-endosymbionts genetic variation may not reflect host genetic variation and should not be used to infer taxonomic relationships within the host species complex. If funding allows, more endosymbionts should be investigated and future investigations could be the contribution of endosymbionts to invasiveness, population expansion, or even competitiveness of whitefly species.

Supplemental Information