Background

Fleas (Insecta: Siphonaptera) are small, bilaterally flattened, wingless and diverse blood-feeding ectoparasites of mammals and birds [1]. They belong to the order Siphonaptera that includes more than 2500 valid species in 16 families [2, 3]. Fleas are one of the most common ectoparasites that serve as vectors of disease-causing agents, such as Bartonella henselae (cat scratch disease), Francisella tularensis (tularemia), Rickettsia typhi (murine typhus) and Yersinia pestis (plague) [4]. The human flea Pulex irritans and the dog flea Ctenocephalides canis have a worldwide distribution and are of high medical/veterinary importance [2, 5].

Accurate differentiation and identification of flea species are essential when diagnosing disease and in fundamental and applied research on these important ectoparasites [5,6,7,8,9]. C. canis and the cat flea C. felis have often been misidentified based on morphology because chaetotaxic variation is common [6]. In addition, the phylogeny of the order Siphonaptera within holometabolous insects is controversial. For example, while the monophyly of the order Siphonaptera is strongly supported by morphological features [2, 10], Tihelka et al. recently suggested that fleas should be treated as an infraorder of the order Mecoptera rather than as a separate order [11]. A very recent preprint has shown that fleas and mecopterans are sister groups, but the data were insufficient to distinguish whether the order Siphonaptera is sister to the order Mecoptera because the order Mecoptera is paraphyletic [12]. Thus, to date, the phylogenetic relationships of fleas remain unclear. The mitochondrial (mt) genome has been often used in systematics and phylogenetic studies across various taxonomic levels of different ectoparasites due to its nature of maternal inheritance, lack recombination, simple structure and rapid evolutionary rate [7,8,9, 13]. However, information on the mt genomes of fleas is limited [14,15,16,17,18], a deficiency which has greatly hindered the study of flea biology, genetics and phylogenetics. Therefore, there is a need to obtain more mt genomic data from more flea species. Such data would help to better understand the phylogenetic relationships of the order Siphonaptera, which notably include P. irritans (the primary vector of plague agents) and C. canis (vector of dipylidiasis pathogens).

The objectives of this study were: (i) to characterize the mt genomes of P. irritans and C. canis; (ii) to compare the mt genome sequences of C. canis with that of C. felis China isolate; and (iii) to assess the phylogenetic position of the order Siphonaptera within holometabolous insects.

Methods

Sample collection and DNA extraction

Adults of P. irritans and C. canis were collected from dogs brought by their owners to pet hospitals in Henan province, China. All animals were handled in strict accordance with good animal practice as defined by the relevant national and/or local animal welfare bodies, and all animal work was approved by the appropriate committee (No. 43321503). All fleas were stored in 70% ethanol immediately after collection and stored at − 80 °C until use. Prior to DNA extraction, the stored fleas were washed twice in physiological saline and air dried at room temperature. Genomic DNA was extracted from individual fleas using a Tissue DNA Kit (Promega, Madison, WI, USA) according to the manufacturer's instructions. DNA quantities was monitored on the Qubit 2.0 Fluorometer (Thermo Fisher Scientific, Waltham, MA, USA). Species identification of individual fleas was molecularly determined by PCR-based sequencing of the nuclear elongation factor 1 α (EF-1α) and mt cox2 genes as previously described [7, 13]. The sequences of EF-1α and the cox2 genes of human fleas had 100% and 98% identity to those of P. irritans originated from the USA (GenBank accession nos. AF423871 and MF136072), respectively. The sequences of EF-1α and the cox2 genes of dog fleas had 99% and 100% similarity to those of dog fleas from the Czech Republic and Hungary (GenBank accession nos. MG586747 and MG637389), respectively. These data collectively confirm that these fleas are P. irritans and C. canis, respectively.

Sequencing, assembling and verification

For P. irritans, a genomic DNA library of approximately 350 bp was constructed and used for high-throughput sequencing on the NovaSeq 6000 platform (Agilent Technologies, Santa Clara, CA, USA) with 250-bp paired-end reads. The raw reads in the FASTQ format were exported and then cleaned by removing adaptor reads, highly repetitive reads and ‘N’-rich reads using the fastp program [19]. The resulting clean reads were de novo assembled using the Velvet algorithm in Geneious Prime 2021.2.2 [20] based on the obtained cox2 sequence. The criteria were 1% mismatch, a maximum gap of 5 bp and a minimum overlap of 150 bp. A complete mt genome of P. irritans was assembled and was further confirmed by PCR using three pairs of specific primers (Additional file 5: Table S1) for all gene-coding regions.

For C. canis, specific primers (Additional file 5: Table S2) were designed based on cat flea C. felis China isolate (Genbank accession number: MW420044) [18]. The seven overlapping PCR amplicons covered regions between the AT region and nad2(approx. 1.4 kb), between transfer RNA (tRNA)-Ile and cox1 (approx. 1.7 kb), between cox1 and cox2 (approx. 1.9 kb), between cox2 and cox3 (approx. 2.0 kb), between cox3 and nad5 (approx. 2.5 kb), between nad5 and cytb (approx. 4.0 kb) and between cytb and the AT region (approx. 3.9 kb). The PCR mix (reaction volume: 25 μl) included 10.5 μl ddH2O, 0.5 μl each of the sense and antisense (2 μM) primer, 12.5 μl Master mix (Takara Bio, Kusatsu, Shiga, Japan) and 1 μl genomic DNA. The thermal cycling program consisted of an initial denaturing at 94 °C for 1 min, followed by 35 cycles of 98 °C for 10 s, 45–65 °C for 40 s depending upon the primers used, 68 °C for 4 min, with a final elongation for 8 min at 72 °C. Purified PCR amplicons were sequenced in both directions (Beijing Genomics Institute, Shenzhen, China).

Genome annotation

The assembled mt genomes were annotated using MITOS webservers [21]. The boundaries of the protein-coding genes and ribosomal RNA (rRNA) genes were discerned by alignment with the homologs of C. felis China isolate using MAFFT 7.122 [22]. tRNA genes were annotated using ARWEN [23] and tRNAscan-SE [24]. Nucleotide composition, amino acid sequences of individual protein-coding genes and codon usage were analyzed using MEGA X [25].

Phylogenetic analysis

The representative mt genome sequences of holometabolous insects, along with Philaenus spumarius (GenBank accession number: NC005944) as an outgroup [26], were obtained from GenBank for phylogenetic analysis (Additional file 5: Table S3). Individual amino acid sequences of all 13 mt protein-coding genes were aligned using MAFFT 7.122. The aligned sequences were then concatenated to form a single dataset. Ambiguous positions were excluded using Gblocks 0.91b [27] with the option for a less stringent selection.

Phylogenetic trees were reconstructed using Bayesian inference (BI) in MrBayes 3.2.6 [28] and by maximum likelihood (ML) in IQ-TREE v.2.1.3 [29]. For BI analysis, the alignment was partitioned by gene, and the MtArt model of amino acid evolution was selected as the most suitable model of evolution by the ProtTest 3.4 [30] based on the Akaike information criterion (AIC). As the MtArt model is not implemented in the current version of MrBayes, an alternative model, MtREV, was used in the Bayesian analysis. Four independent Markov chains were run for 10 million generations. The trees were sampled every 1000 generations with the first 25% discarded as burn-in. For the ML analysis, the optimal partitioning scheme and the best evolutionary model for each partition was selected under the corrected AIC in IQ-TREE. The ML tree was selected with IQ-TREE by an ultrafast bootstrap approximation approach with 10,000 replicates. The phylogenetic trees were visualized using FigTree v.1.42.

Results

General features of the mt genomes

A total of 6 Gb of Illumina short-read sequence datasets was generated for the mt genome of P. irritans, resulting in 13,123,958 × 2 clean reads. The complete mt genome with 20,337 bp in size was submitted to GenBank with accession no. ON100828 (Fig. 1). It was further verified by three PCR amplicons covering the entire gene-coding region (Additional file 1: Figure S1). The nearly complete mt genome, with the exception of the partial non-coding region of C. canis (GenBank accession no. ON109770), was 15,609 bp (Fig. 1). Again, this structure was confirmed by seven overlapped PCR amplicons (Additional file 2: Figure S2). Both mt genomes contained 37 genes, including 13 protein-coding genes (cox1-3, nad1-6, nad4L, atp6, atp8 and cytb), two rRNA genes and 22 tRNA genes (Table 1; Fig. 1). Twenty-three genes were on the heavy strand, and the rest were on the light strand (Table 1). The genes in the mt genome of P. irritans overlapped in 10 locations, comprising 37 bp in total, with overlaps of 1–13 bp per location. There were 10 intergenic regions consisting of a total of 188 bp, with the longest intergenic region located between tRNA-Met and nad2 (Table 1). Similarly, the mt genome of C. canis overlapped at eight locations, comprising 36 bp in total, with overlaps of 1–13 bp per location, and had nine intergenic regions ranging from 1 to 38 bp (Table 1). The nucleotide composition of P. irritans was: A = 5658 bp (38.4%), T = 5974 bp (40.6%), G = 1207 bp (8.2%) and C = 1892 bp (12.8%); this was similar to the nucleotide composition of C. canis: A = 5783 bp (39.5%), T = 5922 bp (40.5%), G = 1173 bp (8.0%) and C = 1759 bp (12.0%).

Fig. 1
figure 1

The complete mt genome of human flea Pulex irritans, and the nearly complete mt genome (except for partial non-coding region) of dog flea Ctenocephalides canis. The names and transcription orientation of the genes are indicated in the coding region. Protein-coding and rRNA genes are indicated using standard nomenclature. tRNA genes are indicated with the one-letter code of their corresponding amino acids. There are two tRNA genes for leucine: L1 for codons CUN and L2 for UUR; and two tRNA genes for serine: S1 for codons AGN and S2 for UCN

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Table 1 Organization of the mitochondrial genomes of human flea Pulex irritans and dog flea Ctenocephalides canis
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Annotation

All protein-coding genes in the P. irritans mt genome used ATT, ATG, TTG or ATC as a start codon, and TAA, TAG, TA or T as a stop codon (Table 1). In the C. canis mt genome, ATT, ATG, TTG or TTT were used as start codons, and ATA, T or TA were used as stop codons (Table 1). The large subunit of rRNA gene (rrnL) was located between tRNA-LeuCUN (L1) and tRNA-Val(V), and the small subunit of rRNA gene (rrnS) was located between tRNA-Val (V) and non-coding region (Table 1; Fig. 1). The rrnL and rrnS genes of P. irritans were 1294 and 793 bp, respectively, and those of C. canis were 1300 and 798 bp, respectively (Table 1). A + T contents of rrnL and rrnS of P. irritans were 82.8% and 82.1%, respectively, and those of C. canis were 83.5% and 81.8%, respectively. The 22 tRNA genes of both P. irritans and C. canis ranged in length from 60 to 71 bp (Table 1). The predicated secondary structures of 22 tRNA genes (Additional file 3: Figure S3; Additional file 4: Figure S4) were similar to those of C. felis, as previously reported [18].

Comparative analyses of the mt genomes of C. canis and C. felis China isolate

The coding regions of the mt genome of C. canis were in total 1 bp shorter than those of the C. felis China isolate (14,638 bp). The coding regions of both mt genomes were arranged in the same way. There were 6.5% nucleotide sequence differences among all genes between C. canis and the C. felis China isolate. The nad6 gene showed the greatest variation in nucleotide composition (9.9%), whereas the rrnS gene showed the least (3.0%) (Table 2). We also compared the predicted amino acid sequences of individual mt genes of C. canis with those of the C. felis China isolate (Table 2). The differences ranged from 0.4% to 10.2%, with COX2 being the most conserved protein and NAD6 the least conserved (Table 2). The sequence variation of the 22 tRNA genes was 3.2% between C. canis and the C. felis China isolate. The rrnL and rrnS genes showed 4.2% and 3.0% sequence differences, respectively. Taken together, the mt genome datasets presented here confirm that C. canis and C. felis represent distinct flea species.

Table 2 Nucleotide and/or predicted amino acid sequence differences in mitochondrial genes between C. canis and C. felis upon pairwise comparison
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Phylogenetic relationships

Two phylogenetic analyses of the concatenated amino acid sequences of all 13 proteins encoded by the mt genome showed that eight flea species used to construct the phylogenetic trees in this study grouped together (Figs. 2, 3). Our phylogenomic analysis further showed that the order Siphonaptera was monophyletic, as strongly supported by the calculated Bayesian posterior probability (Bpp) value (Bpp = 1.0) in the BI analysis and UFBoot value (UFBoot = 1.0) in the ML analysis. The C. canis was more closely related to C. felis than to the other members of the family Pulicidae (Figs. 2, 3). In addition, Siphonaptera is a sister group of orders Diptera + Mecoptera + Megaloptera + Neuroptera, with a strong support in the BI analysis (Bpp = 1.0) and a moderate support in the ML analysis (UFBoot = 77) (Figs. 2, 3). In contrast, the order Mecoptera was not monophyletic (Figs. 2, 3).

Fig. 2
figure 2

Phylogenetic relationships among 52 species of Endopterygota insects inferred from Bayesian inference (BI) analysis of deduced amino acid sequences of 13 mt proteins. Philaenus spumarius (GenBank accession number: NC005944) was used as the outgroup. Bayesian posterior probability (Bpp) values are indicated at nodes. Details of mt genomes, including accession numbers, are included in Additional file 5: Table S3

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Fig. 3
figure 3

Phylogenetic relationships among 52 species of Endopterygota insects inferred from maximum likelihood (ML) analysis of deduced amino acid sequences of 13 mt proteins. Philaenus spumarius (GenBank accession number: NC005944) was used as the outgroup. Ultrafast bootstrap approximation (UFBoot) values are indicated at nodes. Details of mt genomes, including accession numbers, are included in Additional file 5: Table S3

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Discussion

Fleas are the most common ectoparasites infesting dogs and cats worldwide, and they can also severely affect human health. The accurate identification and differentiation of flea species has important implications for the diagnosis of flea-borne diseases and the prevention and control of fleas and these diseases. Flea species such as C. canis and C. felis are usually identified by morphology [31]. However, the identification and differentiation of closely related flea species are often technically challenging [6].

In the present study, characterization of the mt genomes of both P. irritans and C. canis provides a complementary tool to investigate the genetic composition of flea species. Previous studies have used genetic markers in the internal transcribed spacer 1 and 2 (ITS-1 and ITS-2, respectively) regions of nuclear rDNA [32] and mt cox1 and cox2 genes [13] in the molecular identification of P. irritans and C. canis. In addition, molecular and phylogenetic analyses have detected two cryptic P. irritans species [33]. However, mt genes cox1 and cox2 are better suited for such studies than the ITS-1 and ITS-2 regions owing to their high level of nucleotide diversity [5].

In the present study, characterization of the mt genome of C. canis provides a molecular marker for enriching comparative analyses in flea taxa. Comparison between the mt genomes of C. canis and C. felis revealed a sequence variation of 6.5% across the coding region of these genomes. This level of nucleotide sequence difference (6.5%) is high. Previous studies of other insects have detected a similar difference in their mt genomes. For example, the difference in the nucleotide sequences of the coding region between Neochauliodes sinensis (GenBank accession number: MW642295) and N. meridionalis was 6.1% (GenBank accession number: MW642293), and the difference between N. rotundatus (GenBank accession number: MW642294) and N. sparsus was 6.2% (GenBank accession number: MW642296) [34]. In the present study, a clean genetic distinctiveness was detected between C. canis and C. felis China isolate, but host affiliation is not strict [4, 6]. Cross-infection of C. canis has often been found in cats, and in many geographical regions C. felis has been more often found on dogs than C. canis on dogs [4, 6]. Despite the compelling evidence of genetic distinctiveness between C. canis and C. felis China isolate, further study is required to confirm the genetic and phylogenetic relationships among species or subspecies of Ctenocephalides using larger numbers of specimens from broader geographical locations. Simultaneously, detailed morphological redescriptions of these fleas are needed.

Our characterization of the mt genomes of P. irritans and C. canis in the present study also stimulates reassessing the phylogenetic position of the order Siphonaptera among the holometabolous insects using mt genomic datasets. Phylogenetic analyses using a small number of genes, including 18S and 28S rRNA, cox2 and EF-1α have demonstrated that the order Mecoptera is paraphyletic. The order Siphonaptera nests within the order Mecoptera as a sister group to the family Boreidae, and the obscure family Nannochoristidae is placed as a sister group to Boreidae + Siphonaptera [10, 35,36,37]. Recently, the results of an analysis similar to the one presented here using the largest molecular dataset to date indicated fleas as a nested group within the order Scorpionflies as a sister group to the enigmatic Southern Hemisphere family Nannochoristidae [11]. However, phylogenomic analyses of both nucleotide and amino acid sequences of 1478 protein-coding genes robustly and congruently lead to the conclusion that both Siphonaptera and Mecoptera are monophyletic [38]. Nevertheless, the results of a phylogenetic analysis using large-scale transcriptomic data provide strong support that fleas and mecopterans together are the sister groups of flies, although based on these results it is not possible to resolve whether Siphonaptera is a sister group to the monophyletic Mecoptera [12]. These controversial results show that the phylogeny of fleas among insects has proved challenging to resolve.

The results of the phylogenomic analysis performed in the present study support the hypothesis that the order Siphonaptera is monophyletic (Figs. 2, 3). They also revealed a sister relationship between Siphonaptera and orders of Diptera + Mecoptera + Megaloptera + Neuroptera. However, in the current study we did not establish the monophyly of Mecoptera, which is consistent with current decades-long controversy on the monophyly of Mecoptera involving two families of Boreidae and Nannochoristidae [10, 39, 40]. In the present study, we analyzed nine Mecopteran species, including Boreus elegans in the family Boreidae and Nannochorista philpotti of the family Nannochoristidae. N. philpotti and seven other Mecopteran species clustered together to form a clade that also includes Diptera, Megaloptera and Neuroptera, whereas B. elegans was in a separate clade even though it is closely related to a clade containing all members of the orders Diptera, Mecoptera, Megaloptera and Neuroptera with strongly support in all analyses (Bpp = 1.0; UFBoot = 99) (Figs. 2, 3). These results and those of several previous studies [5, 11,12,13] have provided insights into the phylogenetic position of the order Siphonaptera within holometabolan insects. However, they also contradict results from a few other studies [10,11,12]. One shortcoming of the current study is that not all lineages of fleas were included in the analyses. Therefore, further study involving more mt genomes of fleas representing all Siphonapteran families is needed to reassess the phylogeny of these families within holometabolous insects.

Conclusions

The complete mt genome of P. irritans and complete coding sequences of the C. canis mt genome were obtained and annotated, the mt genomes of P. irritans and C. canis were compared and a phylogenetic analysis of the mt datasets was performed. This analysis revealed a clear genetic distinctiveness, demonstrating that P. irritans and C. canis are distinct species, and provided a robust phylogenetic tree that fleas are an order-level monophyletic classification. These mt genomes provide novel molecular markers for studying the taxonomy and phylogeny of fleas in the future.