Karyotypic diversity and evolution of seven mahseer species (Cyprinidae) from India

A Current Update on the Distribution, Morphological Features, and Genetic Identity of the Southeast Asian Mahseers, Tor Species

Simple Summary

The mahseer, particularly the Tor species, is of significant cultural and economic importance as a high-value freshwater food fish in the Southeast (SE) Asian region. However, overexploitation of natural stocks because of high demand and their deteriorating habitat has resulted in a marked decline of Tor species in the wild. There are 13 Tor species that inhabit SE Asian rivers. All these species share distinct morphology, which is the presence of the median lobe. The unique characteristics, including body color, mouth position, and number of lateral scales, distinguish one species from another. Nonetheless, the taxonomy of the Tor species remains unstable and confusing, with contradictory data presented by different authors from different countries for a single Tor species. Therefore, in this review, we have gathered data for the SE Asian Tor species, outlining their distribution, morphology, and genetic identification. In addition, the present review also proposes a list of valid Tor species in the SE Asian region. The proposed list will serve as a standard and template for improving SE Asia’s Tor taxonomy, enhancing the study’s continuity, and a better understanding of specific Tor species.

Abstract

The king of rivers or mahseer comprises three genera: Tor, Neolissochilus, and Naziritor, under the Cyprinidae family. The Tor genus has been classified as the true mahseer due to the presence of a median lobe among the three genera. The Tor species are widely distributed across Southeast (SE) Asia, and 13 Tor species have been reported previously: Tor ater, Tor dongnaiensis, Tor douronensis, Tor laterivittatus, Tor mosal, Tor mekongensis, Tor putitora, Tor sinensis, Tor soro, Tor tambra, Tor tambroides, Tor tor and Tor yingjiangensis. However, the exact number of valid Tor species remains debatable. Different and unstandardized approaches of applying genetic markers in taxonomic identification and morphology variation within the same species have further widened the gap and ameliorated the instability of Tor species taxonomy. Therefore, synchronized and strategized research among Tor species researchers is urgently required to improve and fill the knowledge gap. This review is a current update of SE Asia’s Tor species, outlining their distribution, morphology, and genetic identification. In addition, the present review proposes that there are ten valid Tor species in the SE Asian region. This list will serve as a template and standard to improve the taxonomy of the SE Asian Tor species, which could serve as a basis to open new directions in Tor research.

Development and characterization of cell culture systems from Puntius (Tor) chelynoides (McClelland)

Puntius (Tor) chelynoides, commonly known as dark mahseer, is a commercially important coldwater fish species which inhabits fast-flowing hill-streams of India and Nepal. Cell culture systems were developed from eye, fin, heart and swim bladder tissues of P. chelynoides using explant method. The cell culture system developed from eye has been maintained towards a continuous cell line designated as PCE. The cells were grown in 25cm(2) tissue culture flasks with Leibovitz' L-15 media supplemented with 20 % fetal bovine serum (FBS) at 24°C. The PCE cell line consists of predominantly fibroblast-like cells and showed high plating efficiency. The monolayer formed from the fin and heart explants were comprised of epithelial as well as fibroblast-like cells, a prominent and rhythmic heartbeat was also observed in heart explants. Monolayer formed from swim bladder explants showed the morphology of fibroblast-like cells. All the cells from different tissues are able to grow at an optimum temperature of 24°C and growth rate increased as the FBS concentration increased. The PCE cell line was characterized using amplification of mitochondrial cytochrome oxidase subunit I (COI) & 16S rRNA genes which confirmed that the cell line originated from P. chelynoides. Cytogenetic analysis of PCE cell line and cells from fin revealed a diploid count of 100 chromosomes. Upon transfection with pEGFP-C1 plasmid, bright fluorescent signals were observed, suggesting that this cell line can be used for transgenic and genetic manipulation studies. Further, genotoxicity assessment of PCE cells illustrated the utility of this cell line as an in vitro model for aquatic toxicological studies. The PCE cell line was successfully cryopreserved and revived at different passage levels. The cell line and culture systems are being maintained to develop continuous cell lines for further studies.

Nucleotide variation and physical mapping of ribosomal genes using FISH in genus Tor (Pisces, Cyprinidae)

Molecular cytogenetic studies were carried out for localization of 18S and 5S ribosomal DNAs on chromosomes of three cyprinid fish species viz., T. khudree, T. mussullah and T. mosal mahanadicus using two color fluorescence in situ hybridization (FISH). All the species typically possessed 100 diploid chromosomes with minor variation in karyo-morphology. The 18S rDNA signals were observed on two pair of chromosomes in T. khudree and T. mussullah, and three pairs in T. mosal mahanadicus. The location of 18S signals also showed affinity to silver nitrate and chromomycin A3 staining. Similarly, variation in localization of 5S rDNA among the three species has been detected with the presence of FISH signals on one pair of chromosome in T. khudree and T. mussullah, and on two pairs in T. mosal mahanadicus. These molecular markers could be used as species specific markers for taxonomic identification and can further add in understanding the dynamics of genome organization and karyotypic evolution of these species. The 18S rDNA region was sequenced that generated 1811, 1810 and 1776 bp long 18S sequence in T. khudree, T. mussullah and T. mosal mahanadicus, respectively. The 18S rDNA sequence showed 95-98% identity among the subject species. Similarly, 5S sequencing generated 203 bp long fragments in these species with 100% identity in coding and 9.63% variability in non-transcribed spacer regions. The nucleotide sequence variations could be used for understanding the genetic diversity and will add new informative characters in comparative genomics. These results, in general, would enhance the value and interpretation of ecological assessment data for conservation of Tor species.

Nucleotide sequences and chromosomal localization of 45S and 5S rDNA in Neolissochilus hexagonolepis (Pisces, Cyprinidae), using dual-color fish

Dual-color fluorescence in situ hybridization (FISH) was performed to study the simultaneous localization of major (45S) and minor (5S) family ribosomal RNA genes on chromosomes of Neolissochilus hexagonolepis. The partial 45S (18S, ITS 1, 5.8S, ITS 2 and 28S) and complete 5S (coding and NTS) rDNA units were amplified, sequenced, analyzed, and mapped on the metaphase chromosomes. The complete 18S, 5.8S and partial 28S rDNAs were 1849 bp, 157 bp and 1819 bp long, respectively. Internal transcribed spacers, namely ITS 1 (828 bp) and ITS 2 (359 bp), showed significant nucleotide variations from other fish species listed in NCBI database. The 5S rDNA contained an identical coding region of 120 bp and a highly divergent, non-transcribed 81-bp spacer. The specimens of N. hexagonolepis showed six bright fluorescent signals of 18S, while the 5S signals were present only on one pair of chromosomes. Subsequent analyses between conventional Ag-NORs and 18S rDNA FISH strongly suggested the possible inactivation of one pair of NORs that was localized at a telomeric position of a submetacentric chromosome. The sequencing and chromosomal localization of 45S and 5S rDNAs may serve as a useful genetic marker in taxonomic classification as well as in phylogenetic and evolutionary studies.