The cyst-forming protozoan parasite Neospora caninum is one of the main causes of bovine abortion worldwide and is of great economic importance in the cattle industry. Recent studies have revealed extensive genetic variation among N. caninum isolates based on microsatellite sequences (MSs). MSs may be suitable molecular markers for inferring the diversity of parasite populations, molecular epidemiology and the basis for phenotypic variations in N. caninum, which have been poorly defined. In this study, we evaluated nine MS markers using a panel of 11 N. caninum-derived reference isolates from around the world and 96 N. caninum bovine clinical samples and one ovine clinical sample collected from four countries on two continents, including Spain, Argentina, Germany and Scotland, over a 10-year period. These markers were used as molecular tools to investigate the genetic diversity, geographic distribution and population structure of N. caninum. Multilocus microsatellite genotyping based on 7 loci demonstrated high levels of genetic diversity in the samples from all of the different countries, with 96 microsatellite multilocus genotypes (MLGs) identified from 108 N. caninum samples. Geographic sub-structuring was present in the country populations according to pairwise FST. Principal component analysis (PCA) and Neighbor Joining tree topologies also suggested MLG segregation partially associated with geographical origin. An analysis of the MLG relationships, using eBURST, confirmed that the close genetic relationship observed between the Spanish and Argentinean populations may be the result of parasite migration (i.e., the introduction of novel MLGs from Spain to South America) due to cattle movement. The eBURST relationships also revealed genetically different clusters associated with the abortion. The presence of linkage disequilibrium, the co-existence of specific MLGs to individual farms and eBURST MLG relationships suggest a predominant clonal propagation for Spanish N. caninum MLGs in cattle.
Citation: Regidor-Cerrillo J, Díez-Fuertes F, García-Culebras A, Moore DP, González-Warleta M, Cuevas C, et al. (2013) Genetic Diversity and Geographic Population Structure of Bovine Neospora caninum Determined by Microsatellite Genotyping Analysis. PLoS ONE 8(8): e72678. https://doi.org/10.1371/journal.pone.0072678
Editor: Ana Paula Arez, Instituto de Higiene e Medicina Tropical, Portugal
Received: November 22, 2012; Accepted: July 19, 2013; Published: August 6, 2013
Copyright: © 2013 Regidor-Cerrillo et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was funded by the Spanish Ministry of Economy and Competitiveness (Ref. AGL2010-22191/GAN). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Neospora caninum is a cyst-forming obligate intracellular parasite that is phylogenetically related to Toxoplasma gondii and has been recognised as a worldwide cause of neuromuscular disease in dogs and a major cause of abortion in cattle [1-3]. Advances in research concerning the N. caninum life cycle have confirmed that canids (dog, coyote, dingo and gray wolf) are intermediate and definitive hosts and that cattle and other ungulates, including sheep, goat, water buffalo, bison and deer, are natural intermediate hosts [1-3]. There is experimental evidence that the N. caninum-life cycle can be maintained by domestic dogs and cattle. Dogs can become infected by ingesting N. caninum-infected bovine tissues, and oocysts are shed in the faeces after sexual reproduction presumably in intestinal cells, which in turn can orally infect cattle [2-6]. Based on results observed previously for T. gondii, sexual crosses of N. caninum in canids could produce new isolates with distinct biological characteristics by recombination and reassortment of genetic material between two different parental genomes . N. caninum can also be transmitted transplacentally to the foetus by a primary infection of a dam with oocysts during pregnancy (exogenous transmission) or the recrudescence of a chronic infection in the dam (endogenous transmission), causing fetopathy [8,9]. Endogenous vertical transmission is recognised as the main route of transmission in cattle [2,3,9], but it has been suggested that N. caninum infection is not likely to persist in the absence of horizontal transmission  and recent extensive serological analyses have demonstrated significant post-natal infection rates in cattle . The extent to which each route of transmission contributes to bovine abortion can vary among herds, with different epidemiological and control implications . In addition, a wildlife cycle for N. caninum has been described involving white-tailed deer (Odocoileus virginianus) and coyote (Canis latrans) as intermediate and definitive hosts, respectively [12,13].
Suitable genetic discriminatory methods for differentiating parasites at the strain or isolate levels are necessary to investigate the intra-species diversity and epidemiological and clinical aspects associated with neosporosis. Molecular genotyping techniques have been used to study the taxonomy, evolutionary mechanisms and phylogenetic relationships, population genetic structure and epidemiology of many organisms, including protozoan parasites [14-19]. These molecular methods could be used to establish the importance of domestic and sylvatic life cycles in neosporosis, clarify the role of horizontal transmission in spreading and perpetuating infection and abortion, determine the geographic distribution of isolates and evaluate the emergence of new N. caninum isolates with specific host preferences and virulence. These topics have been poorly investigated in N. caninum, likely due to a lack of suitable molecular tools. Several different studies have recently shown the discriminatory power of microsatellite (MS) markers to genetically identify N. caninum isolates from diverse hosts and geographic origins [20-25]. The use of MS markers to genotype N. caninum in clinical samples was facilitated by the development of nested-PCR [23,26]. Mini- and microsatellite markers have been broadly applied to investigate the epidemiology and population structure of diverse protozoan parasites, including T. gondii [19,27-30]. In the present study, we evaluated these MS markers used in multilocus as a genetic tool for large-scale population genetic analyses. Nine MS markers were evaluated using a panel of 11 N. caninum-derived reference isolates from around the world, 96 N. caninum bovine clinical samples and one ovine clinical sample collected from four countries, including Spain, Argentina, Germany and Scotland, on two continents over a 10-year period. These markers were used to investigate the genetic diversity of N. caninum at global and country level, geographic distribution of N. caninum genotypes and the genetic relatedness between country populations and population structure of N. caninum in cattle.
Materials and Methods
Parasites and Clinical DNA Samples
The Neospora DNA samples included in this study were obtained from isolates maintained in a laboratory and clinical samples from contaminated tissues collected during the necropsy of ruminant abortions, resulting in a total of 108 samples from various regions of the world (Figure S1, Table S1). The DNA samples from 11 N. caninum isolates that had been obtained from dogs with clinical Neospora infection (NC-1, Nc-Liv, Nc-Bahia and Nc-GER1), oocysts shed from a naturally infected dog (Hammondia heydorni-Berlin-1996; later referred to as Hh-Berlin isolate), bovine abortions, stillbirths or calves with neurological signs (Nc-SweB1, Nc-PV1, KBA1 and KBA2), a clinically healthy calf (Nc-Goiás 1) and a naturally infected sheep (named as Nc-Sheep) were used in this study. The majority of these isolates have been previously genotyped [20,31]. This worldwide group of samples served as the reference and out-group for comparison (Figure S1, Table S1).
Microsatellite multilocus genotype (MLG)-data of 10 N. caninum samples from asymptomatic-congenitally infected calves from different provinces in Spain (Nc-Spain1H-Nc-Spain10) collected from 2003–2006 were included in this study [21,32]. The remaining DNA samples were selected from infected tissues from abortions that were submitted to the Animal Health Department at the Complutense University of Madrid and the Agricultural Research Center of Mabegondo in Spain, the Instituto Nacional de Tecnología Agropecuaria of Balcarce in Argentina, the Institute of Epidemiology, Friedrich-Loeffler-Institut in Wusterhausen in Germany and the Moredun Research Institute in Scotland for routine diagnosis or further analysis. Prior to genotyping, the clinical DNA samples were analysed using nested N. caninum-specific ITS-1 PCR . The PCR-positive samples were further analysed using MS genotyping. The DNA samples with five or more out of nine MS marker products were selected (Table S1).
Clinical samples of Spanish origin.
A total of 53 DNA samples from the infected tissues of the aborted bovine foetuses collected primarily from Holstein-Friesian cattle between 2001 and 2010 were included in this study (Table S1). A total of 41 bovine samples were collected from an area of approximately 10,417 Km2 in Galicia (north-west Spain), which is one of the main cattle-producing regions in Spain; 11 bovine samples were collected from the Madrid, Navarra and Zaragoza provinces (central and north-east Spain) covering a geographic area of 343 Km2 and 7,097 Km2 for the central and north-east regions, respectively, and 1 ovine sample from Jaén province (southern Spain). These areas were located 285 to 556 Km apart in an area of 385,106 Km2. Of these, a total of 18 samples were genotyped in a previous study  (Figure S1, Table S1).
Clinical samples of Argentinean origin.
A total of 16 DNA samples obtained from bovine foetuses between 2004 and 2008 were included in this study (Table S1). The foetuses originated from the humid pampas, the main cattle-producing area of Argentina, including the province of Buenos Aires, south of Santa Fe and Cordoba (Figure S1). Most of samples were collected from Buenos Aires province in an area of 149,146 Km2 and the farms for sampling were approximately 53 to 491 Km apart. The cattle were commercial beef (primarily Aberdeen Angus, Hereford and crossbreeds) and dairy (Holando Argentino and Jersey) breeds.
Clinical samples of German and Scottish origin.
DNA samples obtained from nine bovine foetuses of the Holstein-Friesian breed collected in Germany between 2000 and 2009 were included in this study. The samples originated from different states, including Bavaria, Lower Saxony, North rhine-Westphalia, Schleswig-Holstein, Rhineland-Palatinate, Brandenburg and Hesse, comprising a geographic area of 233.657 Km2 and from farms located approximately 10 to 556 Km apart (Figure S1, Table S1).
A total of nine DNA samples were obtained in Scotland from bovine abortions between 2007 and 2009. The foetuses originated from south-west Scotland, in the Dumfries and Galloway region of 6.426 Km2 (Figure S1, Table S1). The farm and breed origin was not available.
Multiplex Multilocus Nested PCR Microsatellite Genotyping
N. caninum genotyping was performed in ITS-1 PCR-positive clinical samples using the nine MS markers MS4, MS5, MS6A, MS6B, MS7, MS8, MS10, MS12 and MS21 that were previously described . The targeted DNA sequences were first amplified by two rounds of multiplex PCRs for the MS4, MS5, MS7 and MS21 markers and the MS6A, MS6B, MS8, MS10 and MS12 markers, respectively. These multiplex PCRs were performed using approximately 200 ng of DNA as a template. The multiplex PCR-amplified products were diluted 1:5 in water and used for a second round of amplifications (nested-PCR) with separate internal primers for each marker. All of the external and internal primers, including the 6-FAM dye labelled reverse primers, and PCR conditions were previously described . The size of the 6-FAM-labelled PCR products for all of the MSs was determined using a 48-capillary 3730 DNA analyzer (Applied Biosystems, Foster City, CA, USA) with Gene Scan-500 (LIZ) Size Standards (Applied Biosystems) at the Unidad Genómica del Parque Científico de Madrid and the GeneMapper1 V 3.5 Software (Applied Biosystems) as previously described .
MS7 and MS10 amplifications were also performed with non-labelled reverse primers for sequencing with the Big Dye Terminator v 3.1 Cycle Sequencing Kit (Applied Biosystems) and a 3730 DNA analyser (Applied Biosystems) at the Unidad Genómica del Parque Científico de Madrid. The sequences were analysed using BioEdit Sequence Alignment Editor v.7.0.1 (Copyright_ 1997–2004 Tom Hall, Ibis Therapeutics, Carlsbad, CA, USA).
Negative controls, including reactions without a template and extraction of bovine DNA negative for Neospora by ITS-1 PCR, were included in each round of PCR. In each round of amplifications, a N. caninum (Nc-Liv) reference isolate for which the sizes of the nine MS loci had been determined by sequencing was included as an internal control .
Allele assignment was performed according to the sizes determined by capillary electrophoresis and sequencing of the MS10 and MS7 markers. Allele designation in this study was performed according to the length of the variable repeat motifs identified in the MS sequences based on previous analyses [20,23,26] (Table S2).
The seven-loci MS profiles (MS6B and MS21 loci were excluded; see below) obtained from 108 samples infected with N. caninum were used for subsequent analyses.
Genetic and genotypic diversity within populations.
We measured the allele number (A), allele frequency and allele richness (Ar) per locus in whole and separate parasite country populations using FSTAT v.18.104.22.168 ( ; http://www2.unil.ch/popgen/softwares/fstat.htm). The allele frequency illustrated the composition and distribution of alleles in whole and separate country populations. The Ar was corrected for unequal sample size by standardising the allelic richness to the smallest sample size in each dataset of 7 individuals. Genetic polymorphisms were measured by Nei’s unbiased genetic diversity within samples (He) using GDA software v.1.1 ( ; http://www.eeb.uconn.edu/people/plewis/software.php).
Genotypic diversity was calculated as the number of unique MLGs divided by the total number of individuals genotyped. Missing alleles were not considered in the genotype comparisons. Genotypic diversity was estimated for the entire population and within defined parasite populations.
Genetic distance (F-statistic) and linkage disequilibrium of the populations.
The GDA package was used to calculate Nei’s genetic distance (D) between each group of N. caninum samples from different geographic locations [34,35]. The FSTAT package version 22.214.171.124  was used for pairwise comparisons of the FST index based on Weir and Cockerham’s θ estimator  between each group of parasite populations. Significance testing was performed by comparing the values obtained after 10,000 random permutations of the dataset.
Multilocus linkage disequilibrium (LD) amongst seven-loci genotypes was assessed using a standardised index of association (IAS), implemented in the LIAN v 3.5 web interface (; http://pubmlst.org/perl/mlstanalyse/mlstanalyse.pl?site=pubmlst&page=lian &referer=pubmlst.org). Because LIAN cannot analyse MLGs with missing data, only MLGs with complete allelic data for the seven markers were used for analyses. The presence of LD was assessed for whole and separate populations in: the complete dataset and a reduced dataset of unique MLGs, where duplicate MLGs (found in more than one sample) were only included once.
The eBURST software was used to explore the genetic relationships within entire dataset . The eBURST software generates networks composed of MLGs represented as dots, whose diameter is proportional to the number of identical genotypes, linked to their single-locus (SLV- 6 shared loci of 7) and double-loci (DVL- 5 shared loci of 7) variants by lines. Because the missing data are not identified, only MLGs with complete allelic data were included.
Principal component analysis (PCA).
A seven-loci MLG dataset from each population was used for PCA . Similarity comparisons between individual MLGs were performed using the allele-sharing coefficient as determined with the Excel Microsoft-Toolkit. The Microsoft Excel plug-in software GenAlEx6  was used to generate a covariance matrix and perform PCA. Graphics showing the axes for each analysis were generated in Excel software.
Genetic relatedness based on the proportion of shared alleles (DAS) and Cavalli-Sforza distances among the sample MLGs was calculated from the complete dataset by applying POPULATIONS v. 126.96.36.199 software (http://bioinformatics.org/~tryphon/populations/). An unrooted Neighbor-Joining (NJ) tree was constructed with the DAS matrix using POPULATIONS software. This analysis was performed with bootstrap values for 1,000 replicates. The NJ tree was visualised with FigTree v.1.3.1 (http://tree.bio.ed.ac.uk/software/figtree/).
Allele Description: Chromosomal Location, Allelic Profile, In Vitro Stability of Microsatellite Markers and N. caninum Microsatellite Genotyping
The nine MS markers initially selected for N. caninum genotyping were located on six chromosomes (II, VIIa, VIII, IX, X and XII) based on a BLAST search in the ToxoDB database (http://toxodb.org/toxo/) (Table S2). Three MS pairs were located in the same chromosome: MS6A and MS6B (178 bp apart) on chromosome X, MS4 and MS5 (at >1,200 Kb) on chromosome IX and MS7 and MS21 (at >608 Kb) on chromosome VIIa (Table S2). Most of the MSs are located in inter-genic non-coding regions and only the MS10 marker is located in an intron sequence and the predicted sequence of the open reading frame NCLIV_036590 (Table S2).
The number and size of the alleles at each MS marker are also displayed in Table S2. The number of alleles ranged from 4 to 12 alleles at most of the loci. MS10 was the most polymorphic marker with 37 alleles. Only two alleles were identified for the MS21 marker, which was excluded from subsequent MLG-data analyses. Within the MS7 marker, two types of alleles were clearly defined by a single nucleotide polymorphism (SNP) -2 bp from the TA repetitive motif that results in an additional TA repeat for the two unique alleles identified as 9.1 and 10.1 (Table S2). Correlation between the repeat length and the number of alleles at a locus was determined for the MSs (r2= 0.9288, p<0.0001 by linear regression analysis in GraphPad Prism v.5 software).
The influence of in vitro passages of the parasite on the stability of the repetitive motif for each MS was evaluated in a stock of N. caninum DNA from the NC-1 isolates routinely maintained in 7 different research laboratories. The unique MS variation among NC-1 samples was detected on the MS6B marker from a unique NC-1 sample lacking one AT repeat (Table S1).
The nine MS markers evaluated were initially used to genotype N. caninum in this study (Table S1). More than one allele was sporadically detected in <1% (7/937 loci examined) of the DNA extracted from the clinical bovine samples. Secondary alleles were recognised as peaks with similar fluorescence intensity than the predominant peak in the electropherogram traces, varying by a single repeat (Table S1). Because double alleles were limited to a unique locus per sample and the N. caninum stages in intermediate hosts are haploid, the duplicity in alleles most likely reflects a transitory shift in the MSs or artefact due to sample processing more than mixed infections. Thus, the dominant allele was selected to generate a single MLG at each sample (Table S1). The genotypic diversity in the complete dataset was high (0.89), with 96 distinct MLGs in the 108 N. caninum samples (Table S1). This high genotypic diversity was maintained even after the MS6B and MS21 alleles were excluded from the MLG analysis. Because the moderately polymorphic MS6B might act as a “dumb” allele in this N. caninum population and MS6B is physically linked to MS6A, MS6B was also excluded from further MLG analyses. Full seven-loci genotypes were obtained in 76% of the samples, as 15.7% and 8.3% of the samples were missing data for a single locus or two loci, respectively.
MS and Genotypic Diversity in Bovine N. caninum Populations from Different Countries
The MLGs were grouped according to geographic origin (including MLGs from the worldwide reference group) to investigate the genetic diversity in bovine N. caninum from different regions. High levels of diversity were observed among the populations for most of the markers according to allele frequencies (Figure S2). Predominant allele frequencies and allele distributions, involving the identification of unique alleles in each geographical population, were distinct in each country (Figure S2). The number of alleles (A), allelic richness (Ar) and Nei’s genetic diversity (He) values per locus and geographic origin are shown in Table S3. The mean Ar and He were moderately-high in all of the population samples, varying from 4.23 to 5.95 and from 0.62 to 0.80, respectively (Table 1). These results indicate a high level of genetic diversity in bovine N. caninum in all of the regions studied. The worldwide reference population displayed the highest Ar and He values (Table 1), likely because the worldwide dataset includes MLGs for N. caninum from different hosts and diverse geographic origins, and therefore a higher genetic diversity would be expected in this N. caninum population.
|Population||Na||Number of alleles (A)b||Allele richness (Ar)b||Genetic diversity(He)b||Genotypic diversityc|
|Worldwide*||11||6.42 ± 2.57||5.95 ± 2.17||0.80 ± 0.16||1.00 (11/11)c|
|Argentina||16||4.86± 2.61||4.26 ± 2.01||0.62 ± 0.31||1.00 (16/16)|
|Scotland||9||4.29 ± 1.50||4.23 ± 1.48||0.70 ± 0.11||0.89 (8/9)|
|Germany||9||4.86± 1.68||4.79 ± 2.05||0.76 ± 0.14||1.00 (9/9)|
|Spain||63||8.57 ± 4.28||5.01 ± 1.60||0.67 ± 0.21||0.83 (52/63)|
Genotypic diversity was similarly high across all of the populations from different countries (Table 1). All of the individual samples from the worldwide, Argentinean and German populations had unique MLGs, and only two Scottish samples showed identical MLGs (SCOT-08-4 and SCOT-08-7). Of the 63 samples from the Spanish population, 52 different MLGs were found (0.83). Identical profiles in the Spanish population were essentially observed in samples originating from the same herd (Table S1), and minor genotypic diversity can be attributed to sampling bias. Only a single pair of identical MLGs (SP-05-GAL-31 and GER-05-6) was identified between the four country populations and worldwide out-group. These genotypic data suggested a degree of geographic sub-structuring between populations from different countries.
Population Differentiation in Bovine N. caninum Populations from Different Countries
Population differentiation was estimated between geographic regions using Nei’s genetic distance (D) and pairwise FST analysis (Table 2). Significant genetic differentiation was observed between the Spanish and German or Scottish populations, and between the Argentinean and German or Scottish populations (D> 0.4; pairwise FST values > 0.1). A lower level of genetic differentiation was observed between the Scottish and German populations (D= 0.3678; pairwise FST= 0.0421). However, the pairwise FST value estimated between these populations was not significant, which is likely due to the limited sample size from both populations. Notably, when the Spanish and Argentinean populations were compared, minimal genetic differentiation was also observed (D= 0.2167; pairwise FST= 0.0757). Geographical association of N. caninum MLGs was examined by overall structure PCA analysis (Figure 1A). The two axis account for 49% of variation and, although geographical populations are partially resolved, they showed a trend to be clustered into different quadrants, indicating geographical sub-structuring. Thus, the majority of German and Scottish MLGs were clustered in the far lower left quadrant, whereas the majority of Spanish MLGs were distributed in the far right quadrants together with several Argentinean MLGs. The remaining Argentinean MLGs were distributed in the higher right quadrant (Figure 1A). Clustering analysis of the MLGs using the NJ algorithm from DAS genetic distances also revealed different main branches grouping MLGs with mixed geographical origin (Figure 1B). Nevertheless, a more in depth evaluation showed a large number of Scottish, Argentinean and Spanish MLGs grouped within the particular branches. In addition, a mix of Argentinean and Spanish MLGs were associated in one of the main branches in accordance to PCA analysis. However bootstrap values in NJ dendrogram only showed significance towards the branch extremities, hampering to discern main genetically different groups associated with the geographical origin in N. caninum (Figure 1B). NJ nodes receiving statistical support by bootstrapping differentiated clusters of MLGs from the same region (e.g. KBA1 and KBA2) or herd origin, as well as of MLGs from distinct geographical locations. These results indicate that factors other than geographic origin and distance may influence the level of genetic differentiation observed between parasite populations. A clear MLG association with the date of collection of samples was not inferred from these clustering analyses (data not shown).
(A) Graphics represent the genetic relationships between the MLGs from each country population based on a covariance matrix. Colours indicate geographic origin (see legend). The proportion of variation in the dataset for each axis is indicated in parentheses. (B) Unrooted NJ tree inferred from allele shared genetic distances. Each tip represents a single MLG. Woldwide MLGs are identified by isolate name. Colour of circles in terminal branches indicates geographic origin (see legend). Percentage bootstrap values were generated from 1,000 replicates. Bootstrap values ≥70% are shown in black circles. Scale bar represents branch lengths. NJ tree based in Cavalli-Sforza distances depicted similar topologies (data not shown).
Linkage Disequilibrium in Bovine N. caninum Populations
LD estimations were determined for each country population (Table 3). Multilocus analysis showed significant LD in the Argentinean, Spanish and Scottish populations (IAS> 0.1; p≤ 0.009). LD was slightly reduced in the Spanish population when the analysis was performed with unique MLGs, but with a higher level of statistical significance (IAS= 0.0872; p< 0.0001). These results suggest a non-random association between the MS loci in these parasite populations and support the possibility of genetic isolation. By contrast, significant LD was not detected among the German N. caninum MLGs. However, established LD estimations should be considered with caution due to the limited number of complete MLGs for some of the populations included in the analyses (<10). LD could be a consequence of the physical linkage of the polymorphic loci used for genotyping, such as MS4 and MS5. Complementary analyses of LD between all pairs of loci ruled out the influence of this MS pair in multilocus LD (data not shown).
|Population||N°a||Index of associationb (IAS)||VD||L||VD > Lc||P|
The relationship between MLGs was examined using eBURST software to explore the occurrence of SLVs and clonal clusters that may explain LD in country populations (Figure 2). When the whole dataset was visualised as a population snapshot, 4 main clonal SLV groups (with n≥ 3 MLGs) were detected, exclusively connecting Spanish MLGs. Interestingly, less stringent analysis considering DLVs revealed relationships among 53 (80%) MLGs from 59 samples, distributed within nine groups (Figure 2). Three main clusters (n≥ 3 MLGs) were recognised. The major cluster comprised 32 MLGs, primarily of Spanish (n= 26) and Argentinean (n= 4) origin with one German MLG and the Nc-Bahia (Brazilian) MLG (Figure 2, Group 1). The second group associated 5 MLGs distributed throughout Europe without apparent geographical connexion and grouping two Scottish MLGs, one German MLG and the Hh-Berlin (German) and Nc-Spain 5H (Spanish) MLGs (Figure 2, Group 2). Moreover, four Argentinean MLGs were associated in the third cluster (Figure 2, Group 3). In addition, 21 MLGs were singletons, lacking of association with other MLGs (data not shown). These eBURST findings suggest the presence of different clusters that comprise MLG variants that result more from high mutation rates than from recombination events. Notably, when the MLG origin at the herd level was recorded, a high number of clusters was shaped by MLGs as SLVs and DLVs from the same herd (e.g. MLG20 and MLG41-43) independently of time of collection, origin of the cattle and the circulation of genetically divergent MLGs at the same herd (e.g. Herd 3 and Herd 1-Table S1). These results indicate evidence of clonal propagation of N. caninum in these herds (Figure 2). Clonal expansion may also explain LD and resembles with the dynamics of N. caninum infection in domestic hosts. However, the moderate IAS values obtained in the geographic populations and the complexity of the relationships between the MLGs in Group 1 did not discard some level of genetic exchange.
Each complete MLG is represented by a dot and the genotype number assigned in Table S1 (Numbers 1-11 represent worldwide MLGs; 12-64 Spanish MLGs; 65-80 Argentinean MLGs; 81-88 Scottish MLGs; and 89-95 German MLGs). MLG dots were also coloured according to their geographical origin (see legend). The dot diameter is proportional to the number of samples with identical MLG (see legend). Single locus variants (SLV) are connected by black lines and double loci variants (DLV) by blue lines. MLGs clusters (n=9) are represented. Main MLGs clusters (n=3) are showed by clear circles. Number and name in the squares identifies the herd origin and worldwide isolates, respectively (see Table S1). Note that singletons were excluded from the snapshot representation (worldwide n=7; Spanish n=7; Argentinean n=2; Scottish n=3; and German n=2).
Microsatellite Markers as Molecular Tools for N. caninum Genotyping
MS amplification of the clinical samples in this study was performed using nested-PCR developed by Pedraza-Díaz et al. , with slight modifications for the multiplexing of the first PCR reactions and increasing throughput. MS amplification was not achieved for all of the clinical samples and complete or nearly complete MS profiles were only obtained from approximately 50% of the ITS-1 PCR-positive DNA samples (data not shown), likely due to the fact that majority of the samples were extracted from aborted foetuses after mummification or autolysis in which the integrity of the DNA could be compromised . Amplification failure led to a significant reduction in the sample size of the complete MLGs. For this reason, incomplete MS profiles were also included in some of these analyses.
Genotyping of a substantial number of samples also allowed us to define the suitability of these markers for the population genetic analysis of N. caninum. Novel alleles were found in most of the MS markers after analysis of the complete dataset, particularly for the most polymorphic marker MS10. This finding confirmed and increased the high discriminatory power for these markers established in previous studies [20,21,23,26]. The level of polymorphism in MSs is expected to be directly associated with the underlying mutation rate that is several orders of magnitude higher than that in other DNA sequences . Several factors have been suggested to contribute to variations in the mutation rate of MSs such as the number of repeat motifs and the MS sequence [40,41], which were also associated with the level of polymorphism showed by MSs in this study. Nevertheless, the stability of these MS markers was confirmed when identical MS profiles were found for the NC-1 isolates collected from different laboratories. NC-1 has been maintained at the rapidly replicating tachyzoite stage in different cell cultures for undetermined successive passages and prolonged periods of time since its isolation in 1988 . With regards to the principal variation (mutation) mechanism in MS sequences, it has been suggested to be DNA replication slippage, being the stepwise mutation model the simplest which assumes changes in only a single repeat unit [40-42]. Most of the alleles for each MS varied in length through the successive addition of one repeat unit following a normal distribution. In addition, MLGs sporadically showed double alleles that only differ by a single repeat unit, indicating a transient mutation/shift and a mutational pattern based on the stepwise mutation model in these MSs.
Multilocus analysis of MSs confirmed the great discriminatory power of these markers for the large-scale genotyping of N. caninum, as suggested in previous studies [20,21,23,26]. Extensive diversity was detected in N. caninum, with 96 MLGs identified in 108 N. caninum samples typed even when the multilocus analysis was limited to seven loci. High levels of genotypic diversity were also determined for the phylogenetically related T. gondii isolates when five MS markers were applied to French and French Guiana parasite populations or seven MSs were used to genotype 271 T. gondii isolates with different geographic origins [30,43]. These results strengthen the value of these MSs as robust, reproducible, highly polymorphic and stable molecular markers for epidemiological and population genetic studies of N. caninum at global and local scale.
Multilocus Microsatellite Analysis Suggests Geographic Sub-structuring and a Predominant Clonal Population Structure of N. caninum
Similar average Ar and He values were found in populations originating from different countries, indicating high genetic diversity in N. caninum within each population. Similarly, high genotypic diversity was also observed for each N. caninum country population. Indeed, detection of identical MLGs was basically restricted to samples from individual herds confirming the high diversity of N. caninum on a smaller regional level.
Marked differences in the predominant allele frequencies and unique alleles specific to each population were observed, suggesting a high level of genetic differentiation between these geographic populations. Moderately high estimated pairwise FST and D values demonstrated a clear genetic separation between the Spanish and Argentinean populations, on one side, and the German and Scottish populations, on the other side. However, less genetic divergence was observed between the Spanish and Argentinean populations. Population segregation by NJ analysis was partially associated with the geographic origin by country, although dendrogram analysis based on these data was unstable. NJ analysis based on MLGs may failure in represent the complex population structure of N. caninum and detect the association between genetic divergence and geographical origin with accuracy . When overall structure for N. caninum was also explored by PCA analysis, results revealed similar relationships for these country populations, showing a closer relationship between the Spanish and Argentinean populations and the German and Scottish populations. These results confirmed that there is a level of geographical sub-structuring between these populations. Considering the large distances and physical geographic barriers between these country populations - the Atlantic Ocean and Pyrenees Mountains - geographic differentiation is expected. However, genetic isolation by geographic distance cannot explain the minor genetic differences observed between the Spanish and Argentinean populations, and likely between the German and Scottish populations. When the relationships between individual MLGs were visualized by eBURST analysis, we observed two main clonal groups, clustering mainly Spanish and Argentinean MLGs (Figure 2, Group 1 and 3). Furthermore, the 40% of Argentinean MLGs were associated as DLVs to Spanish MLGs within the main clonal cluster (Figure 2, Group 1), demonstrating a close genetic relationship between these MLGs. Because a high level of polymorphism was determined by the MSs used in this study, “homoplasy” between Spanish and Argentinean N. caninum MLGs was discarded a priori . A likely hypothesis for explaining these results is a recent MLG circulation, for example, migration of the parasite between country populations. Historical evidence indicates that Iberian cattle were introduced into South America by Spanish and Portuguese colonisers at the end of the 15th century. Iberian breeds were the founders of the adapted Creole breeds, unique South American bovines for more than 300 years until the introduction of new selected European and Zebu breeds [45,46]. By contrast to other apicomplexans, which have more mobile hosts (arthropod vectors and birds) as part of their life cycles, we can assume that the Argentinean N. caninum population may be engrossed by Spanish MLGs introduced via movement of cattle to generate a mixed population of N. caninum MLGs in Argentina. Domestic dogs, the definitive host of N. caninum, were simultaneously introduced with cattle and could also have been a vehicle for introducing N. caninum into South America or, more importantly, could have contributed to maintaining and spreading Spanish N. caninum MLGs inside South American herds. Therefore, the N. caninum clusters identified by eBURST could reflect mutation and genetic drift in geographically disconnected populations previously to introduction of new MLGs by host migration events.
Similar to the Spanish cattle breeds in South-America, the Holstein-Friesian breed originating from Holland-Germany was introduced worldwide into dairy herds during the second half of the 19th century and the first decades of the 20th century, including Spain . As a result, N. caninum MLGs originating from Germany could have been globally distributed. However close relationships between German MLGs associated with MLGs from different geographic populations were only sporadically established by eBURST (MLG18-93). Additional investigations using a higher number of N. caninum samples, representing different European and American locations and a higher range of intermediate and definitive hosts, are needed to confirm this hypothesis. Moreover, new polymorphic markers based on genes and intron sequences must be evaluated in order to establish the history and geographic dissemination events for N. caninum.
The level of LD estimated for each population in this study also support parasite migration events. The observed LD implies low genetic exchange between parasite populations in the following scenarios: 1) “epidemic-ephemeral clonality”, 2) “cryptic biological speciation”, and 3) “clonal population structure”. LD might also reflect physical separation in either space or time or both, or the recent introduction of genetically different populations due to immigration and insufficient time for random mating or prolonged maintenance by clonal propagation [48,49]. This last scenario may be reflected by the significant LD (> 0.1) estimated in the Argentinean, Scottish and Spanish populations where genetically divergent N. caninum MLGs co-exist and are maintained by clonal propagation. This hypothesis is consistent with the results obtained from eBURST showing that different clonal clusters, some co-existing in individual herds, were described in the Spanish MLG population. Thus, the eBURST analysis suggests the clonal as the predominant form of propagation of N. caninum MLGs in the Spanish herd.
This clonal model is also compatible with the life cycle of N. caninum in domestic hosts, where the parasite is predominantly transmitted in cattle via vertical transmission routes after the reactivation of infection during the pregnancy or oral consumption of oocysts, likely after the inbreeding in the dog [23,50]. Nevertheless, the moderate IAS values, the extensive genetic diversity showed by N. caninum with different clusters and the presence of singletons that are not associated to main cluster through eBURST analysis could be interpreted as the result of sexual recombination and genetic exchange. The life cycle of T. gondii and N. caninum allows clonal and sexual expansion in populations. Recent molecular studies in phylogenetically related T. gondii species have demonstrated different population patterns depending on the level of sexual recombination [19,43]. Recently, inbreeding in T. gondii has been associated with disease outbreaks due to the rapid amplification of a single clone into millions of oocysts and out-breeding as a potent mechanism to generate offspring with a range of phenotypic traits, including virulence . The horizontal transmission route associated with infection from a common point source and likely inbreeding has been associated to epidemic outbreaks of abortion due to N. caninum in cattle herds . Additional studies with larger intermediate and definitive host sample sizes and extensive MS genotyping would be valuable, particularly if these studies can be associated with additional information from serology and abortion data to provide insight into the importance of horizontal transmission, the level of sexual reproduction of the parasite and the frequency of out-breeding (genetic exchange), its influence on abortion rates, and the establishment of the actual population structure of N. caninum (clonal vs. panmitic).
A limitation of this study was the low number of N. caninum MLGs for some populations and representative values of the N. caninum samples from this parasite population. Despite increased efforts to obtain a representative number of N. caninum samples suitable for genotyping, the final parasite dataset was primarily limited to bovine abortion samples and N. caninum MLGs could be biased towards more virulent isolates in cattle. Samples from other domestic and wild intermediate and definitive hosts from different geographic regions should be included to establish the genetic diversity of N. caninum in these hosts and to investigate if there is host preference across N. caninum isolates and the global population structure of N. caninum. Additionally, the MS markers used in this study were physically distributed on 6 out of the 14 chromosomes of N. caninum. The seven MSs used in multilocus genotyping represent a suitable molecular tool for genetic studies in N. caninum according to the results obtained in this study. Future approaches should also be directed towards the identification of a new set of markers that represent the remaining 8 chromosomes and apicoplast DNA to improve this integrated method of N. caninum genotyping.
In summary, this work is the first extended study to infer the genetic diversity, geographic distribution and population structure in N. caninum from cattle using a set of seven MSs. The results demonstrate the suitability of these MSs as molecular tools to characterize the population structure of N. caninum. Multilocus MS analysis confirmed an extensive genetic diversity across N. caninum isolates from different countries and continents that is maintained at local level. Genetic differentiation between N. caninum geographic populations demonstrates that there is a degree of geographic sub-structuring by genetic isolation, although it could be influenced by parasite migration and host movement between countries. The results also support a predominant clonal propagation of N. caninum MLGs, at least in Spanish herds, but maybe under the presence of some degree of genetic exchange. Nevertheless, it has to be kept in mind that conclusions derived from the molecular analysis directly depend on the resolution of the markers and sampling used in the study. Extended analyses with an increased number of markers and representative samples from different hosts and geographic origins are needed to confirm the conclusions of this study.
Geographic distribution of the N. caninum samples included in this study. The expanded window shows the detailed geographic origins of the European N. caninum samples. Circle sizes represent the number of samples included for each country population as indicated by the number inside.
Allele frequency distribution of the seven microsatellite loci used for genotyping each country population. Fragment size and number of repetitive motifs were determined using Genescan software and sequencing. Frequencies were assessed based on the total number of samples included for each population. Allele lengths as number of repetitive motifs (or allele number assigned in Table S2) are indicated on the x-axis, while the y-axis shows the frequency of occurrence for each allele as a percentage.
Table S1. Multilocus microsatellite genotyping and origin of the N. caninum samples included in the study.
Table S2. Chromosomal location and summary of the alleles for each microsatellite marker.
We would like to thank our colleagues for donating the DNA samples and isolates used in this study. NC-1 isolate or NC-1 DNA samples were kindly provided by Dr. Boothroyd (Department of Microbiology and Immunology, Stanford University School of Medicine, USA), by Dra. Venturini (Inmunoparasitology and Parasitology Laboratory, National University of La Plata, Argentina), by Dr. Barragan (Center for Infectious Medicine, Karolinska Institutet, Sweden) and by Dr. Hemphill (Institute of Parasitology, University of Berne, Switzerland). We gratefully acknowledge the veterinarians involved in clinical samples collection.
Conceived and designed the experiments: JRC LMOM. Performed the experiments: JRC DPM AGC CC SPD. Analyzed the data: FDF JRC. Contributed reagents/materials/analysis tools: MGW FK GS MM. Wrote the manuscript: JRC LMOM. Commented on the manuscript: DPM MGW FK GS SPD.
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