High quality draft genome sequences of Mycoplasma agassizii strains PS6^T and 723 isolated from Gopherus tortoises with upper respiratory tract disease

Abstract

Mycoplasma agassizii is one of the known causative agents of upper respiratory tract disease (URTD) in Mojave desert tortoises (Gopherus agassizii) and in gopher tortoises (Gopherus polyphemus). We sequenced the genomes of M. agassizii strains PS6^T (ATCC 700616) and 723 (ATCC 700617) isolated from the upper respiratory tract of a Mojave desert tortoise and a gopher tortoise, respectively, both with signs of URTD. The PS6^T genome assembly was organized in eight scaffolds, had a total length of 1,274,972 bp, a G + C content of 28.43%, and contained 979 protein-coding genes, 13 pseudogenes and 35 RNA genes. The 723 genome assembly was organized in 40 scaffolds, had a total length of 1,211,209 bp, a G + C content of 28.34%, and contained 955 protein-coding genes, seven pseudogenes, and 35 RNA genes. Both genomes exhibit a very similar organization and very similar numbers of genes in each functional category. Pairs of orthologous genes encode proteins that are 93.57% identical on average. Homology searches identified a putative cytadhesin. These genomes will enable studies that will help understand the molecular bases of pathogenicity of this and other Mycoplasma species.

Introduction

The genus Mycoplasma, within the bacterial class Mollicutes (Tenericutes), contains over one hundred species, many of which are pathogenic to vertebrates [1]. An upper respiratory tract disease has been implicated in population declines in Mojave Desert tortoises (Gopherus agassizii) found in the desert southwest of the United States and gopher tortoises (Gopherus polyphemus) inhabiting forests of the U.S. southeast [2–4]. Pathogens associated with this disease include two Mycoplasma, Mycoplasma agassizii and Mycoplasma testudineum [5–7]. Due to conservation concerns regarding URTD, this disease and its associated pathogens have become a topic of research interest, though our understanding of the biology and progression of URTD is lacking [8, 9]. In particular, disease in tortoises is found with varying levels of morbidity, and one hypothesis for this finding is that there is genetic variation of M. agassizii associated with varying levels of virulence [8]. To understand better the amount of genomic differentiation occurring between M. agassizii collected from different tortoise host species, and to identify markers associated with virulence, we sequenced the M. agassizii genome from two strains, PS6^T and 723. This sequencing is part of a larger project to ultimately genetically detect variation in strains and their virulence from field-cultured samples.

Organism information

Classification and features

Mycoplasma agassizii has been isolated from multiple tortoise species, and was found to be pathogenic in Mojave Desert tortoises and gopher tortoises in North America, causing URTD [5, 6, 10]. In infected North American tortoises, M. agassizii is most often found in the nasal passages and choana, but can also be isolated from the trachea and lungs [10]. This microbe forms a close extracellular association with the nasal epithelium of its host, and severe infections can result in lesions [11]. Infected hosts experience clinical signs of disease including nasal exudate, possibly leading to lethargic behavior and loss of appetite [5, 11].

M. agassizii is coccoid to pleomorphic in shape, lacks a cell wall, and has a three-layer membrane (Table 1, Fig. 1). These microbes range in size under 1 μm [10, 11] and grow in culture at an optimal temperature of 30 °C, with an extremely slow growth rate [10, 12]. Mortality of M. agassizii occurs at temperatures above 37 °C [12], and it retains viability after prolonged periods of cold temperatures [6, 10], indicating that body temperatures experienced by its ectothermic hosts likely affect the microbe’s success over the seasons. In an experiment to detect co-infection patterns of M. agassizii with its close relative M. testudineum, there was some indication that the two species form a facilitative relationship in a host-context-dependent manner [13]. Preliminary microbiome data suggest that the presence of M. agassizii is associated with a shift in the microbial community composition in Mojave and Sonoran Desert tortoises (Gopherus morafkai) (CLW, FCS and CRT, unpublished data).

Table 1.

Classification and general features of Mycoplasma agassizii, strains PS6^T and 723

MIGS ID	Property	Term	Evidence codea
	Classification	Domain Bacteria	TAS [38]
		Phylum Firmicutes	TAS [39]
		Class Mollicutes	TAS [40]
		Order Mycoplasmatales	TAS [41, 42]
		Family Mycoplasmataceae	TAS [42]
		Genus Mycoplasma	TAS [10]
		Species Mycoplasma agassizii	TAS [10]
		Strains PS6T and 723	TAS [5, 6, 10, 20]
	Gram stain	Negative	NAS
	Cell shape	Coccoid to pleomorphic	TAS [10]
	Motility	Non-motile	TAS [10]
	Sporulation	Nonspore-forming	NAS
	Temperature range	Not reported
	Optimum temperature	30 °C	TAS [10]
	pH range; Optimum	Not reported
	Carbon source	Glucose	TAS [10]
MIGS-6	Habitat	Tortoise respiratory tract	TAS [10]
MIGS-6.3	Salinity	Not reported
MIGS-22	Oxygen requirement	Aerobic	TAS [10]
MIGS-15	Biotic relationship	Symbiont	TAS [11]
MIGS-14	Pathogenicity	Pathogenic	TAS [5, 6]
MIGS-4	Geographic location	North America	TAS [6, 10]
MIGS-5	Sample collection	1991 (PS6T), 1992 (723)	TAS [43]
MIGS-4.1	Latitude	Approx.: 36 N (PS6T), 26.4 N (723)	TAS [6, 10]
MIGS-4.2	Longitude	Approx.: 115 W (PS6T), 82.1 W (723)	TAS [6, 10]
MIGS-4.4	Altitude	Approx.: 800 m (PS6T), 0 m (723)	TAS [6, 10]

IDA Inferred from Direct Assay, TAS Traceable Author Statement (i.e., a direct report exists in the literature), NAS Non-traceable Author Statement (i.e., not directly observed for the living, isolated sample, but based on a generally accepted property for the species, or anecdotal evidence). These evidence codes are from the Gene Ontology project [44]

^aEvidence codes

Fig. 1.

Electron micrograph of ultrathin section of Mycoplasma agassizii strain PS6^T. Image from ref. [10], reproduced with permission from the publisher. Scale bar = 0.5 μm

The strains of M. agassizii that we have sequenced were isolated from two host species. Strain PS6^T was isolated from the upper respiratory tract of a Mojave Desert tortoise in the Las Vegas Valley, Nevada, USA [10], while strain 723 was obtained from an ill gopher tortoise in Sanibel Island, Florida, USA [6]. Strains were cultured in SP4 broth, and have been used in experiments to demonstrate their pathogenic effects on their tortoise hosts [5, 6].

To determine the placement of M. agassizii in the mycoplasmal phylogeny, all 16S rRNA gene sequences from the type strains of Mycoplasma species were obtained from the SILVA database [14] and aligned using MUSCLE 3.8.31 [15], and a phylogenetic tree was constructed using the maximum likelihood method implemented in MEGA7 [16] (Fig. 2). Consistent with prior results [17, 18], M. testudineum is a sister group of M. agassizii in the resultant tree, and the M. agassizii/M. testudineum clade is a sister group of Mycoplasma pulmonis, the agent of murine respiratory mycoplasmosis. All three species fall within the hominis group of Mycoplasma (see ref. [19] for group definitions). The 16S rRNA gene sequence from M. agassizii, strain PS6^T, is 99.8, 93.2 and 89.2% identical to those of M. agassizii strain 723, M. testudineum strain BH29^T, and M. pulmonis strain PG34^T, respectively.

Fig. 2.

Phylogenetic tree of the Mycoplasma genus based on 16S rRNA gene sequences showing the phylogenetic position of M. agassizii PS6^T and 723 (●). All 16S sequences from the Mycoplasma genus were obtained from the SILVA database [14]. Only sequences in the ‘The All-Species Living Tree’ Project (LTP), release 128, were retained. This dataset only contains sequences from type strains, designated with a superscripted “T”. Clostridium botulinum strain ATCC 25763 was also included in the dataset as outgroup. Sequences were aligned using MUSCLE 3.8.31 [15]. A phylogenetic tree was obtained using the maximum likelihood method implemented in MEGA7 [16], with 1000 bootstrap replicates. Species with available genomes at the NCBI Genomes database or the Genomes Online Database are represented in bold face. GenBank accession numbers are shown in parentheses. Bootstrap support values above 50% are represented. The scale bar represents a divergence of 0.05 nucleotide substitutions per nucleotide position

Genome sequencing information

Genome project history

Two strains of M. agassizii were selected for sequencing, strains PS6^T and 723, both isolated from tortoises with signs of URTD [5, 6, 10, 20]. Sequencing was conducted in October 2016. The Whole Genome Shotgun projects were deposited at DDBJ/ENA/GenBank under the accession numbers NQMN00000000 (strain PS6^T) and NQNY00000000 (strain 723). The versions described in this paper are the first versions. A summary of the information of both projects in compliance with MIGS version 2.0 [21] is shown in Table 2.

Table 2.

Project information

MIGS ID	Property	Term
MIGS-31	Finishing quality	High quality drafts
MIGS-28	Libraries used	Illumina Nextera XT
MIGS-29	Sequencing platforms	Illumina NextSeq500
MIGS-31.2	Fold coverage	38.51 (strain PS6T); 37.73 (strain 723)
MIGS-30	Assemblers	SPAdes 3.10.1
MIGS-32	Gene calling method	NCBI Prokaryotic Genome Annotation Pipeline 4.2
	Locus Tag	CJF60 (strain PS6T); CJJ23 (strain 723)
	GenBank ID	NQMN00000000 (strain PS6T); NQNY00000000 (strain 723)
	GenBank Date of Release	August 28, 2017 (strain PS6T); August 29, 2017 (strain 723)
	GOLD ID	–
	BIOPROJECT	PRJNA397947 (strain PS6T); PRJNA398096 (strain 723)
MIGS-13	Source Material Identifier	ATCC 700616 (strain PS6T); ATCC 700617 (strain 723)
	Project relevance	Animal parasite

Growth conditions and genomic DNA preparation

Freeze-dried M. agassizii strains were obtained from the ATCC in March 2011 (strain PS6^T) and May 2016 (strain 723). Strain PS6^T was cultured on SP4 media and re-pelleted in-lab prior to DNA extraction. Genomic DNA was extracted using the Qiagen DNeasy Blood and Tissue Kit protocol for Gram-negative bacteria and eluted with water. Extracted DNA was quantified on a Qiagen QIAxpert system and with Picogreen analysis.

Genome sequencing and assembly

Genome sequencing was conducted using the Illumina Nextera XT DNA Library Preparation Kit (Illumina, Inc., San Diego, USA) with the Illumina NextSeq500 platform (150 bp, paired-end) and 2 ng of starting genomic DNA at the Nevada Genomics Center (University of Nevada, Reno). Sequencing was performed in multiplex with multiple samples, using dual index sequences from the Illumina Nextera XT Index Kit, v2 (PS6 indices: index 1 N702, index 2 S510; 723 indices: index 1 N702, index 2 S511). A total of 349,251 and 332,967 read pairs were obtained for strains PS6^T and 723, respectively. Using Trimmomatic, version 0.36 [22], reads were trimmed to remove Nextera adapter sequences and low quality nucleotides from either end (average Phred score Q ≤ 5, four bp sliding window), and sequences trimmed to < 35 bp were removed. After trimming, 330,351 (PS6^T) and 305,002 (723) read pairs, and 16,438 (PS6^T) and 25,017 (723) single-reads (the pairs of which were removed) remained. De novo genome assembly was performed using SPAdes 3.10.1 [23], using as inputs the trimmed paired reads, and the trimmed single reads (assembly k-mer sizes 21, 33, 55, and 77, with read error-correction enabled and ‘--careful’ mode mismatch correction). After removing scaffolds of less than 500 bp, the final assemblies consisted of 8 (PS6^T) and 40 (723) scaffolds with a total length of 1,274,972 bp (PS6^T) and 1,211,209 bp (723), an average length of 159,372 bp (PS6^T) and 30,280 bp (723), and an N50 of 654,010 bp (PS6^T) and 56,701 bp (723). The coverage was 38.51× for the PS6^T assembly and 37.73× for the 723 assembly.

Genome annotation

Gene prediction was carried out using the NCBI Prokaryotic Genome Annotation Pipeline 4.2 [24]. For each predicted protein, (i) families were identified using the Pfam 31.0 [25] batch search tool (“gathering threshold” option), (ii) Clusters of Orthologous Groups categories were assigned using eggNOG-mapper [26] based on eggNOG 4.5.1 data [27], (iii) signal peptides were identified using the SignalP server 4.1 [28], and (iv) transmembrane helices were inferred using the TMHMM server v. 2.0 [29]. CRISPR repeats were identified using PGAP and CRISPRFinder [30].

Genome properties

The properties of both draft genomes are summarized in Table 3. The final assembly for strain PS6^T consisted of 8 scaffolds, with a total length of 1,274,972 bp, and a G + C content of 28.43%. PGAP [24] identified a total of 979 protein-coding genes, 13 pseudogenes, and 35 RNA genes. The assembly for strain 723 consisted of 40 scaffolds, with a total length of 1,211,209 bp, and a G + C content of 28.34%. A total of 955 protein-coding genes, 7 pseudogenes, and 35 RNA genes were identified. In both cases, the identified RNA genes include 3 rRNAs (one 5S, one 16S and one 23S), 3 ncRNAs and 29 tRNAs. PGAP identified no CRISPR repeats in any of the two genomes, and CRISPRFinder [30] identified 6 “questionable” repeats in the PS6^T genome and one “questionable” repeat in the 723 genome, but no “confirmed” repeats. The numbers of protein-coding genes in each COG category [31] were similar for both strains, and are summarized in Table 4.

Table 3.

Genome statistics

Attribute	Strain PS6T		Strain 723
	Value	% of Total	Value	% of Total
Genome size (bp)	1,274,972	100.00	1,211,209	100.00
DNA coding (bp)	1124,547a	88.20c	1072,218a	88.52c
DNA G + C (bp)	362,520	28.43c	343,241	28.34c
DNA scaffolds	8	100.00	40	100.00
Total genes	1027	100.00	997	100.00
Protein coding genes	979	95.33d	955	95.79d
RNA genes	35	3.41d	35	3.51d
Pseudo genes	13	1.27d	7	0.70d
Genes in internal clusters	–	–	–	–
Genes with function prediction	467b	47.70e	301b	31.52e
Genes assigned to COGs	581	59.35e	577	60.42e
Genes with Pfam domains	608	62.10e	607	63.56e
Genes with signal peptides	160	16.34e	150	15.71e
Genes with transmembrane helices	294	30.03e	288	30.16e
CRISPR repeats	0	–	0	–

^aProtein-coding sequences, not including stop codons

^bProteins not annotated as “hypothetical protein” by PGAP

^cRelative to genome size

^dRelative to total number of genes

^eRelative to protein-coding genes

Table 4.

Number of genes associated with general COG functional categories

Codea	Strain PS6T		Strain 723		Description
	Valueb	%age	Valueb	%age
J	101	10.32	101	10.58	Translation, ribosomal structure and biogenesis
A	0	0.00	0	0.00	RNA processing and modification
K	21	2.15	20	2.09	Transcription
L	66	6.74	60	6.28	Replication, recombination and repair
B	0	0.00	0	0.00	Chromatin structure and dynamics
D	4	0.41	5	0.52	Cell cycle control, Cell division, chromosome partitioning
V	33	3.37	29	3.04	Defense mechanisms
T	5	0.51	5	0.52	Signal transduction mechanisms
M	10	1.02	10	1.05	Cell wall/membrane biogenesis
N	0	0.00	0	0.00	Cell motility
U	11	1.12	9	0.94	Intracellular trafficking and secretion
O	28	2.86	31	3.25	Posttranslational modification, protein turnover, chaperones
C	34	3.47	34	3.56	Energy production and conversion
G	72	7.35	74	7.75	Carbohydrate transport and metabolism
E	27	2.76	26	2.72	Amino acid transport and metabolism
F	25	2.55	25	2.62	Nucleotide transport and metabolism
H	13	1.33	13	1.36	Coenzyme transport and metabolism
I	9	0.92	9	0.94	Lipid transport and metabolism
P	35	3.58	36	3.77	Inorganic ion transport and metabolism
Q	1	0.10	1	0.10	Secondary metabolites biosynthesis, transport and catabolism
R	0	0.00	0	0.00	General function prediction only
S	92	9.40	93	9.74	Function unknown
–	398	40.65	378	39.58	Not in COGs

Percentages are based on the total number of protein coding genes in the genome

^aCOG category code

^bNumber of genes in the category

Insights from the genome sequence

The small genome size and low G + C content of both M. agassizii genomes described here are consistent with those of other Mycoplasma genomes sequenced [18, 32, 33]. However, the M. agassizii genomes are significantly larger than the genome of M. testudineum, strain BH29^T (960,895 bp, 788 protein-coding genes; ref. [18]). The difference in the genome size of both sister species might account for the fact that M. agassizii is associated with URTD, whereas the link between M. testudineum and URTD is less clear [13]; i.e., genes present in M. agassizii but not in M. testudineum might be responsible for pathogenicity.

In spite of the fact that the two M. agassizii strains sequenced here were obtained from geographically distant locations (the Mojave Desert and Sanibel Island) and from different tortoise species (G. agassizii and G. polyphemus; refs. [5, 6, 10, 20]), the two genomes are very similar, exhibiting very similar sizes, numbers of genes (Table 3), functional composition (Table 4), and a high degree of synteny (Fig. 3a). A best-reciprocal-hit approach (based on BLASTP searches, E-value ≤10^− 10) identified 828 pairs of putative orthologs within both genomes. The sequences of proteins encoded by pairs of orthologous genes were aligned using ProbCons version 1.12 [34], and were 93.57% identical on average (median: 96.84%). In contrast, comparison of the genomes of M. agassizii strain PS6^T and M. testudineum strain BH29^T [18] revealed much less synteny (Fig. 3b) and protein identity (average: 54.78%, median: 54.71%).

Fig. 3.

Comparison of the genomes of M. agassizii strains PS6^T and 723 (a), and M. agassizii strain PS6^T and M. testudineum strain BH29^T (b). The figure was generated using Circoletto 07.09.16 [45], a web interface for Circos [46]. The relative order of scaffolds is unknown. For strain PS6^T, scaffolds are sorted by size

The 16S rRNA gene sequences of M. agassizii, strains PS6^T and 723, differed at 3 nucleotide positions (Fig. 4). Surprisingly, our 16S sequence for strain PS6^T and that obtained by Brown et al. (also for strain PS6^T; ref. [20]) exhibit 8 differences (4 point differences and 4 indels; Fig. 4). These differences may represent mutations accumulated since the isolation of the strain, or sequencing errors.

Fig. 4.

Comparison of the 16S rRNA gene sequences generated by Brown et al. [20] (M. agassizii strain PS6^T; GenBank accession: U09786) and in our study (M. agassizii strains PS6^T and 723). Asterisks represent identical sites

To initiate pathogenesis, Mycoplasma cells usually require adhering to the host mucosa. Adhesion mechanisms are relatively well understood in Mycoplasma pneumoniae and its close relatives, but poorly understood in other Mycoplasma species [35]. In a prior study, we searched all available Mycoplasma genomic data (nr database, including the genome of M. testudineum BH29^T) for homologs of M. pneumoniae cytadhesins P1, P30, P65, P40 and P90 and cytadhesin accessory proteins hmw1, hmw2 and hmw3, finding homologs only in species closely related to M. pneumoniae (Mycoplasma genitalium, Mycoplasma gallisepticum, Mycoplasma pirum, Mycoplasma alvi, Mycoplasma imitans, and Mycoplasma testudinis) [18]. Here, we expanded these analyses (BLASTP and TBLASTN searches; E < 10^− 5 and low-complexity regions filtering) to the two M. agassizii proteomes/genomes, also with negative results. In addition, none of the predicted M. agassizii proteins exhibit any of the Pfam domains present in the M. pneumoniae (domains “CytadhesinP1”, “Adhesin_P1”, “Cytadhesin_P30”, “MgpC” and “EAGR_box”). This could be attributed either to (i) M. pneumoniae adhesion proteins being specific to this species and its close relatives, or (ii) adhesion proteins evolving very fast, perhaps due to co-evolutionary races, precluding detection of homologs in distantly related species. The first possibility is supported by the fact that M. pulmonis, the most closely related known species to the M. agassizii/M. testudineum clade, exhibits adhesion mechanisms different from M. pneumoniae, lacking an attachment organelle [36]. In support of the second scenario, our analysis of orthologous sequences revealed poor protein conservation among the sister groups M. agassizii and M. testudineum.

We repeated our similarity searches using as query a list of known Mycoplasma adhesins, which we obtained by searching the text “Mycoplasma adhesin” in the UniProt database [37]. Our prior searches against the M. testudineum BH29^T proteome/genome failed to detect any significant hits. In the current study, we detected significant similarity between a Mycoplasma mobile protein annotated as a “Truncated adhesin protein” (UniProt ID: Q8L3E5_9MOLU) and the proteins CJF60_05070 (strain PS6^T, 3308 amino acids) and CJJ23_03020 (strain 723, also 3308 amino acids) of M. agassizii. CJF60_05070 and CJJ23_03020 are 92% identical. The C-terminal part of the M. mobile protein exhibits homology to three regions of the M. agassizii proteins (positions 958–1261, 1296–1597 and 1717–1924 of CJF60_05070; positions 956–1259, 1294–1595 and 1715–1922 of CJJ23_03020). A BLASTP search using CJF60_05070 as query sequence against the nr database identified a total of 17 hits, including three adhesion proteins (Table 5). Of note, the first hit is a M. testudineum protein (34%), which was not detected in our prior analyses [18]. Equivalent results were obtained using the CJJ23_03020 protein sequence as query (data not shown). The TMHMM server v. 2.0 [29] predicted both CJF60_05070 and CJJ23_03020 to contain a transmembrane domain at the N-terminal part of the protein (positions 7–29), and most of the protein (positions 30–3308) to be extracellular. Taken together, these observations point to these proteins as potential M. agassizii adhesins.

Table 5.

Results of a BLASTP search using CJF60_05070 as query against the nr database

Accession	Description	Total score	Query cover	E-value	Identity
WP_094254640.1	hypothetical protein [Mycoplasma testudineum]	1254	98%	0.0	34%
CAC13384.1	unknown; predicted coding region [Mycoplasma pulmonis]	683	98%	0.0	27%
WP_041363975.1	hypothetical protein [Mycoplasma pulmonis]	682	98%	0.0	26%
WP_011264623.1	Gli349 adhesion and gliding protein [Mycoplasma mobile]	310	67%	10−80	25%
CCY45197.1	fNIP repeat-containing protein [Clostridium sp. CAG:1193]	105	2%	2 × 10−4	38%
WP_015135277.1	hypothetical protein [Leptolyngbya sp. PCC 7376]	215	5%	3 × 10−4	34%
AET68682.1	conserved repeat protein [Desulfosporosinus orientis DSM 765]	58.5	3%	3 × 10−4	38%
OPH56032.1	hypothetical protein BC351_29535 [Paenibacillus ferrarius]	105	5%	4 × 10−4	35%
KRK80309.1	adhesion exoprotein [Lactobacillus nodensis DSM 19682 = JCM 14932 = NBRC 107160]	57.8	2%	5 × 10−4	40%
WP_081776155.1	hypothetical protein [Lactobacillus nodensis]	57.4	2%	6 × 10−4	40%
CCY44912.1	fNIP repeat-containing protein [Clostridium sp. CAG:1193]	55.1	3%	6 × 10−4	35%
WP_057878036.1	hypothetical protein [Lactobacillus paucivorans]	53.5	4%	0.010	30%
WP_066545473.1	hypothetical protein [Caryophanon tenue]	53.1	5%	0.012	29%
WP_081780332.1	hypothetical protein [Porphyromonas uenonis]	97.8	2%	0.150	37%
BAB92076.1	truncated adhesin protein [Mycoplasma mobile]	47.4	9%	0.770	24%

Conclusions

We have obtained draft genome sequences for M. agassizii, strains PS6^T and 723, both isolated from tortoises of the genus Gopherus with URTD. Both genomes exhibited a very small size and low G + C content, which is typical from Mycoplasma genomes. The two assemblies were very similar, in terms of synteny and protein sequences, in spite of the fact that they were obtained from different hosts and geographical locations. We identified a putative cytadhesin in both genomes. The new genomes will facilitate future studies that will help understand the molecular bases of pathogenicity of this and other Mycoplasma species.

Abbreviations

ATCC

American Type Culture Collection

BLAST

Basic local alignment search tool

COG

Clusters of Orthologous Groups

MIGS

Minimum information on the genome sequence

MRM

Murine respiratory mycoplasmosis

NCBI

National Center for Biotechnology Information

PGAP

Prokaryotic Genome Annotation Pipeline

URTD

Upper respiratory tract disease

Authors’ contributions

DAP, CLW, FCS and CRT conceived the work. CLW conducted laboratory work. DAP and RLT conducted bioinformatic analyses. DAP and CLW drafted the manuscript. All authors contributed to interpreting data and improving the manuscript. All authors read and approved the final manuscript.

Competing interests

The authors declare that they have no competing interests.

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