Introduction

Human activities and climate change have allowed many plant and animal species to recently expand their geographic ranges, a phenomenon likely to continue despite increasing quarantine efforts1. Impacts of biological invasions manifest at scales ranging from individuals to ecosystems and landscapes, potentially leading to evolutionary changes2,3. In the course of species’ expansion, the population dynamics and evolutionary processes of local flora and fauna may be affected4,5,6. Many factors, including genetic structure and variation in local populations, determine the capacity of native species to interact and adapt to the invader5. Higher trophic level species, such as parasitoids, in a given location may switch among taxonomically disparate but ecologically similar types of hosts by ecological sorting7, resulting in new associations or assemblages of related species8.

The diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), is a brassica-specialist herbivore of global significance9,10,11,12. It has achieved wide distribution across the world (i.e. East Asia and Oceania) in recent centuries, most likely due to human activities, such as globalization of trade and brassica crop cultivation10,13,14. In recent studies of P. xylostella, genetic homogeneity has been found in many populations across Asia-Pacific regions15,16,17. Insect movement due to air currents and transportation of agricultural products have been proposed as the main reasons for high levels of gene flow for P. xylostella15,16,18,19,20.

A broad range of natural enemies, including parasitoids, arthropod predators, pathogenic fungi and bacteria, attack P. xylostella9,10,11. Among these, Cotesia vestalis (=plutellae) Haliday (Hymenoptera: Braconidae) is one of its most important biocontrol agents10,11,21 occurring in 38 countries10. Whilst C. vestalis has been introduced to Australia, North America, and the Caribbean in over 20 classical biological control programs10,22, there are no records of it being introduced to Japan, Vietnam, Malaysia (Cameron Highlands) and China22,23,24,25. Yet, C. vestalis is reported to be among the most predominant parasitoids of P. xylostella across parts of East Asia25,26,27, most likely associated to its tolerance to high temperatures28 and insecticides25,26, and suggesting it may be native to this region.

The evolutionary success and diversification of insects has been aided, to some extent, by their complex associations with microorganisms29,30. Over the recent decades, Wolbachia symbionts have attracted intensive research effort due to their diverse behavioral effects in a broad range of insect hosts, ability to manipulate the host reproductive system, and potential role in biological control of pests31. Wolbachia are generally assumed to be maternally inheritable, with vertical transfer from egg cytoplasm to offspring, though recent studies have demonstrated horizontal transfer from infected to uninfected species32.

Invasive species may change distribution, genetic configuration and trophic interactions of other native species and trigger rapid adaptation33,34, which is increasingly becoming a major focus of research in evolutionary biology. In the present study, using samples of P. xylostella and C. vestalis collected from five Asian countries of China, Nepal, Thailand, Malaysia and Vietnam, we used a set of mitochondrial genes to analyze the phylogeographic relationships to (1) reveal the genetic diversity and demographic history of the two species; (2) identify the geographic origin of C. vestalis; and (3) address the adaptation of C. vestalis to the invasive P. xylostella in this region. Considering the Wolbachia-arthropod associations and the potential role in intra-specific interactions, we also investigated the impacts of horizontal transfer of Wolbachia on the genetic configuration and co-evolution of local assemblages.

Results

Genetic diversity

A total of 1,621 bp DNA was obtained from concatenation of three P. xylostella mitochondrial genes (hereafter referred as p3m). From 323 P. xylostella individuals coming from 29 sampling locations (Table 1), we found 187 polymorphic sites and 212 haplotypes, representing a high haplotype diversity with an average of 0.931. We identified 174 haplotypes represented by single individuals, with the remainders represented by multiple P. xylostella. Nucleotide diversity was overall low with an average of 0.329%, except for the samples from three locations of Quanzhou in China (FJQZ, 0.606%), Cameron Highlands in Malaysia (MLCH, 0.906%) and Katmandu in Nepal (NPKT, 0.682%) (Table 1).

Table 1 Parameters of Genetic Diversity and Demographic History of the P. xylostella and C. vestalis Populations Based on Three Mitochondrial Genes (CoxI, Cytb and NadhI).
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For C. vestalis, based on 1,232 bp DNA from three mitochondrial genes (hereafter referred as c3m), we found a relatively low haplotype and nucleotide diversity with an average of 0.415 and 0.172%, respectively. From 324 individuals, 43 polymorphic loci and 29 haplotypes were identified, with 19 haplotypes coming separately from single individuals and the rest from multiple individuals. We also observed higher haplotype and nucleotide diversity from two sampling locations of Guiyang (GZGY: Hd = 0.667 and θ = 0.431%) and Yuxi (YNYX: Hd = 0.769 and θ = 0.795%) in Southwest China (Table 1).

Phylogeny and haplotype network

The p3m-based phylogenetic tree revealed overall low genetic differentiation among individuals in terms of the branch length, and no isolated clusters containing individuals from specific geographic regions or populations, regardless of high genetic differentiation identified between Wolbachia PlutWB1-infected and -unfected individuals (Fig. 1a). The Cytb-based network exhibited a star-like shape with many unique haplotypes present at the terminals (Fig. 1b), indicating recent population expansion by P. xylostella. In terms of this Cytb-based network, haplotype 2 (H2) was dominant and present in most sampled populations. The frequency of identified haplotypes was randomly distributed in each of the P. xylostella populations. The c3m-based phylogenetic tree (Fig. 2a) and haplotype network (Fig. 2b) showed four separate lineages with multiple basal clusters/lineages containing the samples from Southwest China. Lineage 1 comprised the samples from part of China, Malaysia and Vietnam, Lineage 2 consisted of individuals from China, Thailand and Nepal, while lineage 3 was represented by individuals from China only. Using the CoxI gene sequences of our C. vestalis samples and additional sequences from India, Kenya, Benin, Hungary, Malaysia, New Zealand and Russia, we constructed a CoxI-based phylogeny (Fig. 2c) that does not change the overall 3cm-based topology of the basal position and paraphyly of samples from Southwest of China (Fig. 2a,b).

Figure 1
figure 1

Phylogenetic Tree and Haplotype Network of Plutella xylostella. (a) Phylogeny of P. xylostella based on the concatenated CoxI, Cytb and NadhI genes using maximum likelihood algorithm with 1000 bootstraps, with P. australiana as an outgroup. (b) Haplotype network based on Cytb for P. xylostella. Haplotypes with frequency ≤4 are illustrated in blue and labeled with sampling location acronyms and numbers; small empty circles represent unsampled haplotypes. Numbers upon branches are bootstrap values > 0.5.  PlutWB1: a specific Wolbachia strain previously identified in P. xylostella.

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Figure 2
figure 2

Phylogenetic Tree and Haplotype Network of Cotesia vestalis. (a) Phylogeny of C. vestalis based on the concatenated CoxI, Cytb and NadhI genes using maximum likelihood algorithm with 1000 bootstraps, with C. flavipes as an outgroup. (b) Haplotype network based on three concatenated genes, CoxI, Cytb and NadhI for C. vestalis. The number of mutations >1 is presented beside the corresponding branches; haplotypes with frequency ≤4 are illustrated in blue and labeled with sampling location acronyms and numbers; small empty circles represent unsampled haplotypes; haplotypes labeled YNYX are from Southwest China. (c) Phylogeny of global C. vestalis samples based on the CoxI gene (545 bp) using maximum likelihood algorithm with 1000 bootstraps, with C. flavipes as an outgroup. Individuals in green indicate recruited individuals from Europe, Africa, Oceania and Asia. Numbers upon branches are bootstrap values > 0.5.

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Four primary lineages were present on the Wsp-based Phylogeny of Wolbachia (Fig. 3). Hosts involved in lineage 1 and 2 are C. vestalis (Lineage 1) and P. xylostella (Lineage 2), respectively, while lineage 4 was identified in both of these two species (Fig. 3).

Figure 3
figure 3

The Wsp-based Phylogenetic Tree of Wolbachia. The gene sequences are coloured for different hosts (black: herbivore; red: parasitoid; blue: predator). PX = P. xylostella; CV = C. vestalis. Numbers upon branches are bootstrap values > 0.5.

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Demographic history

Neutrality tests for P. xylostella were conducted using Tajima’s D and Fu’s Fs statistics (Table 1). The p3m-based Tajima’s D and Fu’s Fs statistics were significantly negative (Tajima’s D = −2.501, P < 0.001; Fu’s Fs = −5.532, P < 0.05) when all sampled populations were considered as one group. A significantly negative Tajima’ D value was associated with a significantly negative Fu’s Fs value, suggePhysting a recent expansion of P. xylostella populations in this region. The p3m-based mismatch distribution was unimodal when all sampled individuals of P. xylostella were considered as one group for analysis (Fig. 4a1), further supporting the concept of a recent expansion of the P. xylostella populations in East Asia.

Figure 4
figure 4

Demographic Inference. (a1–a5) Mismatch distributions of P. xylostella and C. vestalis based on three concatenated genes, CoxI, Cytb and NadhI (a1) P. xylostella with all sampled individuals; (a2) C. vestalis with all sampled individuals; (a3) Lineage 1 of C. vestalis; (a4) Lineage 2 of C. vestalis; and (a5) Lineage 3 of C. vestalis). (b) Estimation of the CoxI-based TMRCA for P. xylostella and C. vestalis. PX: P. xylostella; CV: C. vestalis; OC: Oceania; OW: Old World.

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Based on the neutrality tests for C. vestalis, c3m-based Tajima’s D and Fu’s Fs statistics also had significantly negative values (Tajima’s D = −1.747, P < 0.05; Fu’s Fs = −10.475, P < 0.001) when all sampled individuals taken as one group. In the defined clusters, only Lineage 1 showed significantly negative values of Tajima’s D (D = −2.323, P < 0.01) and Fu’s Fs (Fs = −23.426, P < 0.001), suggesting a recent expansion event of the C. vestalis populations. No population expansion events could be inferred in Lineage 2 (Tajima’s D = −1.421, P > 0.05; and Fu’s Fs = −3.066, (0.01 < P < 0.05)) and Lineage 3 (Tajima’s D = −1.133, P > 0.05; and Fu’s Fs = −1.362, P > 0.05). The c3m-based mismatch distribution was multimodal when all sampled individuals were considered as one group, but the three defined lineages exhibited unimodal distributions (Fig. 4a2–a5).

Discussion

The paraphyly nature of samples from Southwest China, located at the early branching nodes of both c3m-based (Fig. 2a) and CoxI-based (Fig. 2c) phylogeny, suggests that Southwest China is the geographical origin of C. vestalis. Such an inference was also supported by the high nucleotide polymorphism of C. vestalis populations (YNYX and GZGY) in Southwest China (Table 1). We also found that all our sampled C. vestalis individuals derived from Southwest China based on the c3m-based phylogenetic tree (Fig. 2a) and haplotype network (Fig. 2b). According to the CoxI-based global phylogeny of C. vestalis (Fig. 2c, and supported by c3m tree, Fig. 2b), C. vestalis individuals from African, Oceanian and European countries were evolutionarily closely related to those from Thailand (TLPH) and Nepal (NPKT).

C. vestalis is one of the most common parasitic wasps of P. xylostella10 and previously considered to be native in Malaysia24, Japan23, and China25. It has also been suggested to originate from Europe based on the species description using Ukrainian specimens21. However, our Cox I-based phylogenetic analysis demonstrated that C. vestalis populations in Europe, Africa and Oceania derived from East Asia (Fig. 2b). Although no samples from the New World were included in this study, we speculate that the haplotypes from the New World may derive from the haplotypes of the Old World as C. vestalis was reported to be recently introduced into North America22,35 and South America21,35 as a biological control agent.

P. xylostella was recorded to colonize many regions (including East Asia) of the world in recent centuries14,15. A significantly negative Tajima’ D value was associated with a significantly negative Fu’s Fs value, suggesting a recent expansion of P. xylostella populations in East Asia, which is further supported by the unimodal p3m-based mismatch distribution of all sampled individuals. Our metadata analysis revealed a regional expansion of C. vestalis populations associated with the invasion and colonization of P. xylostella in East Asia. A comparable observation has also been reported for two Diadegma parasitoids of P. xylostella in Europe36. The results of BEAST analysis also showed that the most recent common ancestor (TMRCA) of C. vestalis was between the TMRCAs of P. xylostella in Old World (OW) and Oceania (OC), suggesting that the population expansion of C. vestalis could be related to the regional invasion of P. xylostella into Oceania through East Asia. Based on our results, we propose that the regional distribution of C. vestalis is a case of ecological sorting7 that add P. xylostella, which had become the dominant herbivore in brassica crops of East Asia11,37, to the hosts for C. vestalis38. Such a new trophic association may facilitate the rapid adaptation of C. vestalis, by the previously-documented means of altered the life history traits and physiological manipulation39,40, to P. xylostella.

The co-occurrence of plutWB141 in P. xylostella and C. vestalis (Lineage 4 in Fig. 3) suggested horizontal transfer of Wolbachia between herbivore and parasitoid. Our observation of Wolbachia-infected P. xylostella pupae and adults implied that the direction of this horizontal transfer was from P. xylostella to C. vestalis, given that C. vestalis would kill P. xylostella before pupation35. In addition, in concordance with findings of Delgado & Cook41, a distinct clade of five individuals (from different sampling locations) infected by plutWB1 were identified in the phylogenetic tree of P. xylostella (Fig. 1a), suggesting a long history of co-evolution between P. xylostella and this Wolbachia strain and a horizontal transfer scenario from the herbivore to its parasitoid. Another distinct clade (Lineage 3) in the wsp-based phylogenetic tree showed that the host of Wolbachia involved parasitoids, herbivores and predators, suggesting that the horizontal transfer of Wolbachia can occur across multiple trophic levels.

Li et al.42 reported that Bemisia tabaci-associated Wolbachia can be horizontally transferred between infected and uninfected individuals via plants. This can be further supported by a recent study of flowers and wild megachilid bees, in which the plants can act as hubs for bacterial transmission between multiple organisms43. In the present study, using our wsp sequences and the NCBI-based wsp sequences, we found that the individuals infected by Wolbachia were involved with several species of herbivores (Lineage 3 in Fig. 3). We assumed that the presence of such a Wolbachia lineage (Lineage 3) in various herbivores might have come from food intake or potentially from horizontal transfer between the species.

In this study, using an interactive system involving an invasive herbivore, P. xylostella, and its parasitoid, C. vestalis, we demonstrated how the invasion of an alien host herbivore (P. xylostella) could significantly affect the genetic variation of a higher trophic species (C. vestalis). Through ecological sorting, this parasitoid could have switched from an original local host to P. xylostella and, as an important agent of classical biological control, it underwent significant, human-aided, population expansion. In addition, during this expansion, the endosymbiont Wolbachia (plutWB1) in P. xylosetlla was also introduced into local species. Our work provides a comprehensive picture (Fig. 5) of how invasion by an alien species can trigger significant evolutionary changes in a newly associated parasitoid and the transmission of exotic bacteria into local species, leading to the formation of new biological interactions and the genetic configuration of local species.

Figure 5
figure 5

A Schematic Map Illustrating the Rapid Adaptation of C. vestalis to the Invaded P. xylostella (as a new host) through the Ecological Sorting Process.

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Methods

Sample collection and species identification

We collected P. xylostella and C. vestalis samples from the same or nearby cabbage and broccoli fields in China (25), Nepal (1), Thailand (1), Vietnam (1) and Malaysia (2) from 2012 to 2014 (Fig. S1; Table S1). Twenty-eight samples of P. xylostella and C. vestalis were collected from common locations, and one sample of P. xylostella was collected in Chongqing, China (CQ) and its counterpart C. vestalis sample from nearby, Luzhou in Sichuan Province, China (SCLZ) (Fig. S1; Table S1). The second and third instar larvae of P. xylostella were maintained on vegetable leaves for emergence of parasitoids. P. xylostella pupae and adults, and C. vestalis cocoons were morphologically identified and preserved in 95% ethanol. Specimens were stored at −80 °C prior to DNA extraction. A total of 323 P. xylostella and 324 C. vestalis individuals were used in this study. We used a 600 bp mitochondrial gene sequence (CoxI) (Table S2) and DNA barcoding criteria to individually confirm the species identity of P. xylostella and C. vestalis based on BOLD44.

DNA Extraction and sequencing

Total genomic DNA was extracted from individual insects using DNeasy Blood and Tissue Kit (Qiagen, Germany). Three mitochondrial genes, CoxI, cytochrome b (Ctyb), and NADH dehydrogenase subunit I (NadhI) were sequenced for both P. xylostella and C. vestalis (Table S2). Primers were developed for Cytb and NadhI of P. xylostella as well as Cytb and NadhI of C. vestalis using Primer Premier version 5 (Premier Biosoft International, Palo Alto, CA, USA) based on the reference mitochondrial genomes. Primers of other gene segments were referred to published references (Table S2).

PCR was conducted using the Mastercycler pro system (Eppendorf, Germany) under the following conditions: an initial denaturation for 2 min at 94 °C, followed by 35 cycles of 10 s at 96 °C, 15 s at specific annealing temperature of each genes (Tables S2), and 1 min at 72 °C, and a subsequent final extension for 10 min at 72 °C. Amplified products were purified and bidirectionally sequenced using the ABI 3730xl DNA Analyzer by Sanboyuanzhi Biotechnology Co., Ltd. (Beijing, China). All the sequences were deposited in Genebank database (accession number from KX604356 to KX606864).

Infection of P. xylostella and C. vestalis by Wolbachia was determined using ∼600 bp products of the wsp gene amplified with specific primers (Table S2). A positive control of PCR reaction (with DNA of Wolbachia infected samples as templates) was used to test infection of Wolbachia in samples.

Genetic analysis

Sequences for each of the gene fragments were aligned using MEGA5.245. All mitochondrial sequences for each of the individuals of both insect species were aligned independently using MAFFT-7.03746. Conservative regions selected by Gblock-0.91b47 were used for gene concatenation, which was performed by Sequence-Matrix-1.7.8 with default parameters48. Parameters of genetic diversity, haplotype diversity (Hd) and nucleotide diversity (θ), were calculated using the DnaSPv549. Populations with <5 individuals (3 populations of P. xylostella and 12 of C. vestalis) were not included in the calculation of the parameters related to the genetic diversity (Table 1).

Phylogenetic and network analysis

Phylogeographic analysis can be used to explore the evolutionary history of a species50,51,52 or to examine the temporal and spatial effects on co-evolutionary relationships of closely related species53,54. This type of study can help reveal the impacts of biological expansions on local communities over wide spatial scales. Using the sequences of three concatenated mitochondrial genes, phylogenetic relationships were constructed for P. xylostella (here after p3m) and C. vestalis (here after c3m). We selected Cotesia flapvis, which is from the Cotesia genus, as the outgroup for C. vestalis phylogenetic tree construction. The CoxI sequences of C. vestalis and C. flapvis (outgroup) from NCBI (http://www.ncbi.nlm.nih. gov/) were also downloaded for construction of a global phylogenetic tree of C. vestalis. The phylogeny of wsp gene was developed using the NCBI-based wsp sequences with the best hit (with <3 gaps and > = 99% identity) when BLAST conducted using the wsp sequences from this study, plus additional sequences of the wsp gene of Wolbachia parasitizing P. xylostella and C. vestalis.

Phylogenetic inferences were performed using the neighbor-joining (NJ) and maximum likelihood (ML) methods by PAUP*4.0b1055. The software MrModeltest version 2.356 was used to select the best-fit nucleotide substitution model. The General Time Reversible model was used with invariable sites and a gamma-shaped distribution of rates across sites (GTR + I + G) based on the Akaike Information Criterion (AIC).

The network analysis was conducted for mitochondrial genes of P. xylostella and C. vestalis using median-joining algorithm implemented in the software Network, version 4.6.1.357. We constructed the haplotype networks of both species for individual genes as well as the concatenated mitochondrial gene sequences. Haplotype type and frequency for each population were also recorded.

Demographic analysis

We calculated the Tajima’s D and Fu’s Fs for each of the populations (≥5 individuals) of the two species based on the concatenated mitochondrial genes using Dnasp V549. Analyses of mismatch distributions were also performed for both species. For C. vestalis, mismatch distribution was analyzed for not only a collection of all samples, but also three major clusters based on the phylogenetic tree of three concatenated mitochondrial genes.

BEAST58 was also used to calculate  the coalescent time of lineages in P. xylostella and C. vestalis, based on CoxI sequences, which has a reported mutation rate53. For P. xylostella, more samples from the Old World and Oceania36 were included to increase precision in coalescent time inference. As the mutation rates varied among insect lineages, we used lognormal relaxed clock while estimating the evolutionary timescales. The chain of Markov Chain Monte Carlo (MCMC) was set to 50 million with Log parameters in every 5000.