Animals and sampling
The study was carried out in the Breeding Station of the Witold Stefański Institute of Parasitology, Academy of Sciences in Kosewo Górne (Region of Warmia and Mazury; Poland; N: 53° 48′; E: 21° 71 23′). Samples of heart and oesophagus tissue were obtained from five 1-year-old male fallow deer slaughtered for commercial purposes. All samples (10 g of weight) were gently broken up in physiological saline solution using a blender and filtered through gauze and the resulting sediments were collected. The basic morphology (shape, size) of any cysts present was examined in wet mounts under an inverted Olympus IX50 microscope fitted with a camera. The samples were preserved for molecular investigation in sterile H2O in Eppendorf tubes and stored at − 72 °C.
DNA extraction
Individual sarcocysts were detected in muscles by light microscopy. Total DNA was extracted from 10 individual sarcocysts using the NucleoSpin®Tissue kit (Macherey–Nagel, Germany) according to the manufacturers’ instructions. The DNA was eluted in 50 μl of distilled water. The extracted DNA was stored at − 20 °C for PCR assay.
Two different PCR protocols were employed for the molecular examination of tissue cysts
In the first protocol, DNA amplification was performed using PCR targeted at a 18S rDNA sequence of approximately 900 bp in accordance with More et al. [20]. The following specific primers were used: SarcoFext (GGT GAT TCA TAG TAA CCG AAC G) and SarcoRext (GAT TTC TCA TAA GGT GCA GGA G).
Reactions were performed in a final volume of 25 µl reaction mixture with 1 unit of Taq DNA polymerase per reaction (Thermo Fisher Scientific) under the following conditions (final concentrations): 1× reaction buffer supplied with the DNA polymerase, 1.25 mM MgCl2, 200 mM dNTPs, 0.2 M of each primer and the following cycler program: initial denaturation (95 °C, 5 min), followed by 40 cycles of denaturation (94 °C, 40 s), annealing (59 °C, 1 min), elongation (72 °C, 1 min) and a final extension at 72 °C (10 min).
The PCR products were analyzed by electrophoresis in 1.5% agarose gel and stained with GelRed (Nucleic Acid Gel Stain, Biotium). PCR products 900 bp in size were cut out from the gel and purified using Clean-up Product Purification Kits (A&A Biotechnology, Poland) according to the manufacturer’s instructions. DNA concentration was estimated using a NanoDrop ND-1000 Spectrophotometer (NanoDrop Technologies, USA).
The PCR amplicons were then ligated into pGEM-T easy cloning vector (Promega). Escherichia coli strain XL-1 Blue MRF electrocompetent cells (Promega) were used for cloning. Positive clones (six) were identified by colony PCR with primers directed against vector sequences outside the multi-cloning site. The clones found to contain inserts were used for further examination. Positive plasmids were purified using GeneAll Exprep Plasmid SV mini (GeneAll, Korea) according to the manufacturer’s instructions.
The ssu rRNa gene products concentration was measured using a NanoDrop ND-1000 Spectrophotometer (NanoDrop Technologies, USA) and was then sequenced (Genomed, Poland). The sequence information obtained from all the isolated clones was assembled using Vector NTI Advance 10 software (Invitrogen, Scotland). The complete sequences were checked against sequences published in GenBank using BLAST (https://www.ncbi.nlm.nih.gov/BLAST/).
In the second protocol, the ssu rRNA gene was amplified by PCR using the primers for Sarcocystis [ERIB1 5′-ACC TGG TTG ATC CTG CCA G-3′, Primer1L 5′-CCA TGC ATG TCT AAG TAT AAG C-3′, Primer3H 5′-GGC AAA TGC TTT CGC AGT AG-3′] [21] in a 25 μl final reaction volume with one unit of Taq DNA polymerase/reaction (Thermo Fisher Scientific). The following final concentrations of reagents were used: 1× reaction buffer, 1.5 mM MgCl2, 250 μM dNTPs, 0.5 μM of each primer. PCR was started with initial denaturation (95 °C, 5 min), followed by 35 cycles of denaturation (95 °C, 30 s), annealing (55.5 °C, 30 s), elongation (72 °C, 1 min) and a final extension at 72 °C (10 min).
PCR products were analyzed and sequenced as described above. The purified products were then sequenced (Genomed, Poland).
Phylogenetic analyses
The evolutionary history was inferred using the Maximum Likelihood method based on the Kimura 2-parameter model [22]. The tree with the highest log likelihood (− 2638.89) is shown. The percentage of trees in which the associated taxa clustered together is shown next to the branches. Initial tree(s) for the heuristic search were obtained by applying the Neighbor-Joining method to a matrix of pairwise distances estimated using the Maximum Composite Likelihood (MCL) approach. A discrete Gamma distribution was used to model evolutionary rate differences among sites [four categories (+G, parameter = 0.3385)]. The tree is drawn to scale, with branch lengths measured in the number of substitutions per site. The analysis involved 100 nucleotide sequences. All positions with less than 95% site coverage were eliminated. That is, fewer than 5% alignment gaps, missing data, and ambiguous bases were allowed at any position. There were a total of 789 positions in the final dataset. Evolutionary analyses were conducted in MEGA7 [23].