THE TAXONOMIC STATUS OF DIGRAMMA (PSEUDOPHYLLIDEA: LIGULIDAE) INFERRED FROM DNA SEQUENCES

Sequence and Haplotype Analyses of Ligula intestinalis in Acanthobrama marmid (Cyprinidae) in Turkey

PURPOSE

Ligulosis caused by Ligula intestinalis adversely affects the fisheries carried out in the lakes and ponds, causing economic losses in the fish industry. In this study, it was aimed to reveal the molecular characterization of L. intestinalis isolates obtained from woodfish (Acanthobrama marmid) in Keban Dam Lake in Elazig province of Turkey by using mt-CO1 gene sequences and to determine the genetic differences and haplotypes between the isolates.

METHODS

In the examination made in terms of L. intestinalis, the intestine of the fish was opened with the help of fine-tipped scissors, the contents were allowed to come out, and the parasites were taken into a petri dish containing phosphate buffered saline (PBS). Then, L. intestinalis plerocercoids were taken into 15 ml falcon tubes containing 70% ethanol and stored at - 20 °C until further analysis. From each isolate, total gDNA was extracted from the plerocercoids. A partial (480 bp) mt-CO1 gene was amplified by PCR and sequenced unidirectionally. The final size of the trimmed sequences was 392 bp for 43 sequences. Sequence and haplotype analyses were performed, followed by phylogenetic analyses.

RESULTS

All isolates were confirmed as L. intestinalis by BLAST analysis. In addition, 87 nucleotide mutation positions were determined among 43 CO1 gene sequences. As a result of the haplotype network performed for the mt-CO1 gene region of L. intestinalis isolates; arranged in a star-like configuration with the main haplotype (Hap05), separated from other haplotypes by 1-6 mutation steps, and 29 haplotypes were identified, covering 13.9% (6/43) of the total isolates. Also, 75 variable (polymorphic) sites were determined, 52 of which were parsimony informative sites.

CONCLUSIONS

The molecular characterization of L. intestinalis in woodfish (A. marmid) was identified for the first time in Turkey.

Infection Status with Plerocercoid of Ligulid Tapeworm in Cyprinid Fish from Three Lakes in Republic of Korea

We have investigated freshwater cyprinid fish for its current infection status with plerocercoid of ligulid tapeworm in the Republic of Korea. A total of 6,049 (517 Euiam Lake and 4,071 Hoengseong Lake in Gangweon-do and 1,461 Chungju Lake in Chungcheongbuk-do) fish were examined by dissecting the peritonial cavity between March 2015 and December 2016. Totally 45 (0.74 %) fish in of 5 (26.3 %) species (8 Squalidus japonicus coreanus, 6 Squalidus gracilis majimae, 7 Opsariichthys uncirostris amurensis, 15 Zacco platypus and 9 Erythroculter erythropterus) were infected with plerocercoids of ligulid tapeworm. The infection density with plerocercoids in Erythroculter erythropterus was 12 – 26 per fish infected in Erythroculter erythropterus, and 1 – 2 in other 4 fish species. The plerocercoid was ivory-white and 26.2 – 57.8 cm long. The prevalence value in this survey was 0.9 % (45/6,049). The genetic analysis in this study was conducted to identify plerocercoid species. Based on genetic analysis with data in GenBank, these plerocercoids were identified as the L. intestinalis.

Comparative mitogenomics supports synonymy of the genera Ligula and Digramma (Cestoda: Diphyllobothriidae)

Background

After observing differences in the number of reproductive complexes per proglottid within the genus Ligula, the genus Digramma was erected. However, the validity of Digramma has been previously questioned due to a low variability in the cox1, nad1 and ITS rDNA sequences between the two genera. We undertook a study to greatly increase the amount of sequence data available for resolution of this question by sequencing and characterizing the complete mitogenomes of Digramma interrupta and Ligula intestinalis.

Results

The circular mtDNA molecules of Digramma interrupta and Ligula intestinalis are 13,685 bp and 13,621 bp in size, respectively, both comprising 12 PCGs, 22 tRNA genes, two rRNA genes, and two mNCRs. Both mitogenomes exhibit the same gene order and share 92.7% nucleotide identity, compared with 85.8–86.5% to the most closely related genus Dibothriocephalus. Each gene from D. interrupta and L. intestinalis is almost of the same size, and the sequence identity ranges from 87.5% (trnD) to 100% (trnH, trnQ and trnV). NCR2 sequences of D. interrupta and L. intestinalis are 249 bp and 183 bp in length, respectively, which contributes to the main difference in length between their complete mitogenomes. A sliding window analysis of the 12 PCGs and two rRNAs indicated nucleotide diversity to be higher in nad5, nad6, nad2, nad4 and cox3, whereas the most conserved genes were rrnL and rrnS. Lower sequence identity was also found in nad2, nad4, nad5, nad6 and cox3 genes between the two diphyllobothriids. Within the Diphyllobothriidae, phylogenetic analysis indicated Ligula and Digramma to be most closely related to one another, forming a sister group with Dibothriocephalus.

Conclusions

Owing to higher nucleotide diversity, the genes nad2, nad4, nad5, nad6 and cox3 should be considered optimal candidates to use as molecular markers for population genetics and species identification between the two closely related species. The phylogenetic results in combination with the comparative analysis of the two mitogenomes, consistently support the congeneric status of L. intestinalis and D. interrupta.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2910-9) contains supplementary material, which is available to authorized users.

Integrating molecular and morphological approaches for characterizing parasite cryptic species: implications for parasitology

Herein we review theoretical and methodological considerations important for finding and delimiting cryptic species of parasites (species that are difficult to recognize using traditional systematic methods). Applications of molecular data in empirical investigations of cryptic species are discussed from an historical perspective, and we evaluate advantages and disadvantages of approaches that have been used to date. Developments concerning the theory and practice of species delimitation are emphasized because theory is critical to interpretation of data. The advantages and disadvantages of different molecular methodologies, including the number and kind of loci, are discussed relative to tree-based approaches for detecting and delimiting cryptic species. We conclude by discussing some implications that cryptic species have for research programmes in parasitology, emphasizing that careful attention to the theory and operational practices involved in finding, delimiting, and describing new species (including cryptic species) is essential, not only for fully characterizing parasite biodiversity and broader aspects of comparative biology such as systematics, evolution, ecology and biogeography, but to applied research efforts that strive to improve development and understanding of epidemiology, diagnostics, control and potential eradication of parasitic diseases.

Ligula intestinalis (Cestoda: Pseudophyllidea): an ideal fish-metazoan parasite model?

Since its use as a model to study metazoan parasite culture and in vitro development, the plerocercoid of the tapeworm, Ligula intestinalis, has served as a useful scientific tool to study a range of biological factors, particularly within its fish intermediate host. From the extensive long-term ecological studies on the interactions between the parasite and cyprinid hosts, to the recent advances made using molecular technology on parasite diversity and speciation, studies on the parasite have, over the last 60 years, led to significant advances in knowledge on host-parasite interactions. The parasite has served as a useful model to study pollution, immunology and parasite ecology and genetics, as well has being the archetypal endocrine disruptor.

Advances and Trends in the Molecular Systematics of the Parasitic Platyhelminthes

The application of molecular systematics to the parasitic Platyhelminthes (Cestoda, Digenea and Monogenea) over the last decade has advanced our understanding of their interrelationships and evolution substantially. Here we review the current state of play and the early works that led to the molecular-based hypotheses that now predominate in the field; advances in their systematics, taxonomy, classification and phylogeny, as well as trends in species circumscription, molecular targets and analytical methods are discussed for each of the three major parasitic groups. A by-product of this effort has been an ever increasing number of parasitic flatworms characterized genetically, and the useful application of these data to the diagnosis of animal and human pathogens, and to the elucidation of life histories are presented. The final section considers future directions in the field, including taxon sampling, molecular targets of choice, and the current and future utility of mitochondrial and nuclear genomics in systematic study.