© 2017 The Japan Mendel Society Cytologia 82(4): 435–441 A First Chromosomal and Nucleolar Organizer Regions (NORs) Analyses of Chevey’s Sheetfish, Micronema cheveyi (Siluriformes, Siluridae) Krit Pinthong1, Weerayuth Supiwong2*, Baramate Simporn3, Supatcha Chooseangjaew4, Wanpen Kakampuy5 and Alongklod Tanomtong3 1 Department of Fundamental Science, Faculty of Science and Technology, Surindra Rajabhat University, Muang, Surin 32000, Thailand 2 Faculty of Applied Science and Engineering, Khon Kaen University, Nong Khai Campus, Muang, Nong Khai 43000, Thailand 3 Toxic Substances in Livestock and Aquatic Animals Research Group, Department of Biology, Faculty of Science, Khon Kaen University, Muang, Khon Kaen 40002, Thailand 4 Marine Shellfish Breeding Research Unit, Department of Marine Science, Faculty of Science and Fisheries Tech- nology, Rajamangala University of Technology Srivijaya, Trang campus, Trang 92150, Thailand 5 Faculty of Agriculture and Technology, Nakhon Phanom University, Muang, Nakhon Phanom 48000, Thailand Received April 19, 2016; accepted July 25, 2017 Summary A first report on nucleolar organizer regions (NORs) and karyological analyses in the Chevey’s sheetfish (Micronema cheveyi) from the Chao Phraya Basin, Thailand, was performed. The mitotic chromosome preparation was directly prepared from kidney cells of ten male and eight female specimens. Conventional stain- ing and Ag-NOR banding techniques were applied to stain the chromosomes. The results shown that the diploid chromosome number of M. cheveyi was 2n=78 and the fundamental numbers (NF) were 96 in both sexes. The karyotype composed of 4 large metacentric, 6 large submetacentric, 10 large acrocentric, 32 medium telocentric, and 26 small telocentric chromosomes. No heteromorphic sex chromosomes were revealed. In addition, the telo- meric NORs were clearly observed at the short arm adjacent to the telomere of the third chromosome pair (sub- metacentric chromosomes). The karyotype formula for M. cheveyi is as follows: m sm a t t 2n (diploid)78=L4 +L 6 +L 10 +M 32 +S 26 Key words Micronema cheveyi, Chromosome, Nucleolar organizer regions (NORs), Karyotype. Micronema is a small genus of the family Siluridae to determine chromosome number and karyotype com- native to the Southeast Asian region. There are cur- position has been carried out. Structure, number, and rently three recognized species in this genus including morphology of a nucleolar organizer region (NOR) may M. cheveyi, M. hexapterus and M. platypogon (Froese be specific to populations, species, and subspecies. NOR and Pauly 2015, Eschmeyer and Fong 2016). Species of is frequently used to compare variations, as well as to the genus Phalacronotus were formerly placed in this identify and explain specifications. Changes in chromo- genus (Eschmeyer and Fong 2016). In Thailand, there some number and structure can alter the number and are about 30 species of the family Siluridae which can structure of NOR. Robertsonian translocations may be classified into 10 genera including Belodontichthys, cause losses of NOR (Yüksel and Gaffaroğlu 2008). Up Ceratoglanis, Hemisilurus, Kryptopterus, Micronema, to the present, only 12 species have been cytogenetically Ompok, Phalacronotus, Pterocryptis, Silurichthys, and investigated reporting the diploid chromosome number Wallago (Ferraris 2007, Rainboth 1996, Vidthayonon (2n) ranging from 40 to 92 chromosomes including S. 2005). Chevey’s sheetfish, M. cheveyi (Fig. 1) is com- phaiosoma, 2n=40; S. schneideri, 2n=40 (Magtoon monly found in natural water bodies such as rivers and and Donsakul 2009); O. siluriodes, 2n=50 (Donsakul floodplains in Thailand. The M. cheveyi is a very fa- 1992, Gomontier et al. 2012); W. micropogon, 2n=56 vorite food fish of Thai people and it has a high market (Donsakul 1996); O. fumidus, 2n=60 (Magtoon and value (Vidthayonon 2005). Donsakul 2009); B. truncatus, 2n=62 (Donsakul 1996); Cytogenetic studies in Thai freshwater fishes are quit K. macrocephalus, 2n=62 (Magtoon and Donsakul scarce, in which only conventional technique reported 2009); K. bicirrhis, 2n=64 (Donsakul 1996); P. apogon 2n=64 (Supiwong et al. 2012), P. bleekeri, 2n=64 (Don- * Corresponding author, e-mail: [email protected] sakul 1992); W. attu, 2n=86 from India (Rishi and Singh DOI: 10.1508/cytologia.82.435 1983, Sharma and Tripathi 1984); 2n=88 from Thailand 436 K. Pinthong et al. Cytologia 82(4) Fig. 1. General characteristic of the Chevey’s sheetfish (Micronema cheveyi). Fig. 2. Metaphase chromosome plates and karyotypes of male (A) and female (B) Chevey’s sheetfish, (Micronema cheveyi), 2n=78, by conventional straining technique. Scale bars indicate 5 µm. (Donsakul 1996) and K. cryptopterus, 2n=92 (Donsakul breeding in this fish. 1992). The present cytogenetic study is the first report on Materials and methods karyotype analysis and chromosomal characteristic of the NORs of M. cheveyi. The knowledge gained can pro- Ten males and eight females of M. cheveyi were col- vide useful cytogenetic information for further study on lected from the Chao Phraya River, Sing Buri Province, taxonomy, evolutionary relationship, conservation, and central part of Thailand. The fish were transferred to 2017 A First Chromosomal and Nucleolar Organizer Regions Analyses of Chevey’s Sheetfish 437 Table 1. Review of fish cytogenetic reports in seven genera of the family Siluridae in Thailand. Species 2n NF Karyotype Ag-NOR Locality Reference Belodontichthys truncates 62 100 20m+10sm+8st+24a ̶ Thailand Donsakul 1996 Kryptopterus bicirrhis 64 98 20m+10sm+4st+30a ̶ Thailand Donsakul 1996 K. cryptopterus 92 110 8m+10st+74a ̶ Thailand Donsakul 1992 K. macrocephalus 62 98 24m+12sm+26a ̶ Thailand Magtoon and Donsakul 2009 Micronema cheveyi 78 96 4m+6sm+10a+58t 2 Thailand Present study Ompok bimaculatus 42 72 18m+12sm+12t (F) ̶ India Rishi 1976 41 70 17m+12sm+12t (M) ̶ India Rishi 1976 42 72 6m+24sm+12t (F) 2 India Khuda-Bukhsh and Das 2007 41 70 5m+24sm+12t (M) 2 India Khuda-Bukhsh and Das 2007 42 68 12m+14sm+16a/t ̶ India Sharma 2008 54 102 16m+26sm+6a+6t 2 India Verma et al. 2011 O. fumidus 60 82 20m+2sm+2st+36a ̶ Thailand Magtoon and Donsakul 2009 O. pabda 54 100 28m+10sm+8a+8t ̶ India Das and Kar 1977 42 ̶ ̶ ̶ India Datta et al. 2003 42 68 12m+14sm+16a/t ̶ India Sharma 2008 42 84 26m+10sm+6a 2 India Verma et al. 2011 O. pabo 54 102 36m+12sm+6t ̶ India Khuda-Bukhsh and Chanda 1989 O. siluriodes 50 88 34m+2sm+2st+12a ̶ Thailand Donsakul 1992 50 90 14m+20sm+6a+10t 2 Thailand Gomontier et al. 2012 Phalacronotus apogon 64 108 18m+20sm+6a+20t 2 Thailand Supiwong et al. 2012 P. bleekeri 64 92 20m+6sm+2st+36a ̶ Thailand Donsakul 1992 Silurichthys phaiosoma 40 46 2m+4sm+8st+266a ̶ Thailand Magtoon and Donsakul 2009 S. schneideri 40 50 6m+4sm+4st+26a ̶ Thailand Magtoon and Donsakul 2009 Wallago attu 86 106 12m+6sm+2a+66t ̶ India Rishi and Singh 1983 86 116 10m+12sm+8a+56t ̶ India Sharma and Tripathi 1984 88 110 16m+2sm+4st+66a ̶ Thailand Donsakul 1996 W. micropogon 56 86 26m+4sm+26a ̶ Thailand Donsakul 1996 Remarks: 2n=diploid chromosome number, NF=fundamental number (number of chromosome arm), m=metacentric, sm=submetacentric, st=subtelocentric, a=acrocentric, t=telocentric chromosome, F=female, M=male, NOR=nucleolar organizer region, and ̶=not available. laboratory aquaria and were kept under standard condi- tive cytogenetic data among species of the family Siluri- tion for three days prior to the experiments. Procedures dae in Thailand is displayed in the Table 1. In this fam- for fish chromosome were prepared by directly prepared ily, the 2n range from 40 chromosomes in S. phaiosoma from kidney cells modifying from Supiwong et al. and S. schneideri, to 92 chromosomes in K. cryptopterus (2012). The chromosome preparations were stained with and the NF range between 46 and 116. Ghigliotti et al. 10% Giemsa’s for 30 min and NORs were identified by (2007) assumed that species with a larger NF are more Ag-NOR staining (Howell and Black 1980). The meta- advanced in evolutionary terms. Such changes in chro- phase figures were analyzed according to the chromo- mosome arm number appear to be related to the occur- some classification following by Turpin and Lejeune rence of pericentric inversions, which are among the (1965) and Chaiyasut (1989). most common modifications contributing to karyotypic rearrangement in fishes and other vertebrates (Dover Results and discussion 1993, Galetti Jr. et al. 2000, Wang et al. 2010). The M. cheveyi was formerly placed in the Kryptopterus spe- Chromosome number, fundamental number and karyo- cies and the species of the genus Phalacronotus were type of M. cheveyi formerly placed in the Micronema (Eschmeyer and Fong Cytogenetic studies have been firstly performed on 2016). Thus, it seems to be that these genera (Kryptop- eighteen specimens of M. cheveyi from Thailand. The terus, Micronema, and Phalacronotus) have most closed results shown that the diploid chromosome number of relationship among them. Moreover, the cytogenetic data M. cheveyi was 2n=78, and the fundamental numbers are quite in agreement with their morphology. (NF, number of chromosome arms) were 96 in both All specimens investigated have no cytologically sexes (Fig. 2). The present cytogenetic study is also the distinguishable sex chromosome. This characteristic new record in this genus. There are different from other is similar to others in the Siluridae family of Thailand. species in the previous studies (Rishi 1976, Das and Kar However, some silurid catfishes in India such as O. bi- 1977, Rishi and Singh 1983, Sharma and Tripathi 1984, maculatus have differentiated sex chromosome system Khuda-Bukhsh and Chanda 1989, Donsakul 1992, 1996, as X1X1X2 in male and X1X1X2X2 in female (Khuda- Datta et al. 2003, Khuda-Bukhsh and Das 2007, Sharma Bukhsh and Das 2007).