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Comparative study of four Mystus species (Bagridae, Siluriformes) from Thailand: insights into their karyotypic diversity
expand article infoPun Yeesin, Phichaya Buasriyot§, Sukhonthip Ditcharoen§, Patcharaporn Chaiyasan§, Chatmongkon Suwannapoom|, Sippakorn Juntaree, Sitthisak Jantarat, Sucheela Talumphai#, Marcelo de Bello Cioffi¤, Thomas Liehr«, Alongklod Tanomtong§, Weerayuth Supiwong
‡ Prince of Songkla University, Pattani, Thailand
§ Khon Kaen University, Khon Kaen, Thailand
| University of Phayao, Phayao, Thailand
¶ Khon Kaen University, Nong Khai, Thailand
# Roi Et Rajabhat University, Roi Et, Thailand
¤ Universidade Federal de São Carlos, São Carlos, Brazil
« Friedrich Schiller University, Jena, Germany
Open Access

Abstract

Karyotypes of four catfishes of the genus Mystus Scopoli, 1777 (family Bagridae), M. atrifasciatus Fowler, 1937, M. mysticetus Roberts, 1992, M. singaringan (Bleeker, 1846) and M. wolffii (Bleeker, 1851), were analysed by conventional and Ag-NOR banding as well as fluorescence in situ hybridization (FISH) techniques. Microsatellite d(GC)15, d(CAA)10, d(CAT)10 and d(GAA)10 repeat probes were applied in FISH. The obtained data revealed that the four studied species have different chromosome complements. The diploid chromosome numbers (2n) and the fundamental numbers (NF) range between 52 and 102, 54 and 104, 56 and 98, or 58 and 108 in M. mysticetus, M. atrifasciatus, M. singaringan or M. wolffii, respectively. Karyotype formulae of M. mysticetus, M. atrifasciatus, M. singaringan and M. wolffii are 24m+26sm+4a, 26m+24sm+2a, 24m+18sm+14a and 30m+22sm+6a, respectively. A single pair of NORs was identified adjacent to the telomeres of the short arm of chromosome pairs 3 (metacentric) in M. atrifasciatus, 20 (submetacentric) in M. mysticetus, 15 (submetacentric) in M. singaringan, and 5 (metacentric) in M. wolffii. The d(GC)15, d(CAA)10, d(CAT)10 and d(GAA)10 repeats were abundantly distributed in species-specific patterns. Overall, we present a comparison of cytogenetic and molecular cytogenetic patterns of four species from genus Mystus providing insights into their karyotype diversity in the genus.

Keywords

Chromosomes, fluorescence in situ hybridization (FISH), karyotype, Mystus

Introduction

Bagridae are the largest family of Thai catfishes, with six genera (Bagrichthys Bleeker, 1857, Batasio Blyth, 1860, Hemibagrus Bleeker, 1862, Mystus Scopoli, 1777, Pseudomystus Jayaram, 1968, and Sperata Holly, 1939) and 28 species in Thailand. They play an important role in the national economic value of the country, as they are kept in aquaria and contribute heavily to the aquaculture industry. Most species of the genus Mystus are booming in aquaculture, with some of them being kept in aquaria (Vidthayanon 2005). However, several species in this family are rather morphologically similar especially during the juvenile stage that may pose difficulties for their identification. Mystus is a poorly diagnosed group, and they are morphologically similar and diagnostic characteristics are usually subtle (Ng 2003; Ferdous 2013).

Cytogenetic studies on Thai bagrids are quite scarce; as yet only conventional cytogenetics have been applied to determine chromosome numbers and karyotype complements. Therefore, their chromosomal evolution is not clear, even though from family Bagridae up to 45 species have been karyotyped so far. The diploid chromosome number (2n) varies between 2n = 44 [Coreobagrus brevicorpus Mori, 1936)] and 2n = 80 [Batasio fluviatilis (Day, 1888)]. The fundamental number (number of chromosome arms, NF) varies between 64 [for M. tengara (Hamilton, 1822) and M. vittatus (Bloch, 1794)] and 116 [for Horabagrus brachysoma (Günther, 1864) and H. nigricollaris Pethiyagoda et Kottelat, 1994] (Arai 2011).

Focusing on the genus Mystus, chromosomal diversity and chromosomal variations among populations can be found. The so far reported 2n for diploid chromosome numbers varies between 50 and 58 chromosomes and for NF from 64 to 110 (Table 1). Intra-specific variations of 2n were reported in M. mysticetus Roberts, 1992 (2n = 50, 52) (Donsakul 2002; Supiwong et al. 2014a, b) and M. vittatus (Bloch, 1794) (2n = 50, 54, 58) (Das and Srivastava 1973; Manna and Prasad 1974; Tripathi and Das 1980; Rishi 1981; Sharma and Tripathi 1986; Khuda-Bukhsh and Barat 1987; John et al. 1992; Choudhury et al. 1993; Ramasamy et al. 2010). The cytogenetic characterization of a species could be applied to other fields such as systematics, but also economic interests, as breeding practices of organisms by using chromosome set management (Na-Nakhon et al. 1980), strain improvement (Sofy et al. 2008) and brood stock selection (Mengampan et al. 2004).

Table 1.

Comparative cytogenetics of Mystus genus (2n = diploid chromosome number, m = metacentric, sm = submetacentric, st = subtelocentric, a = acrocentric, t = telocentric, NOR = nucleolar organizer regions, NF = fundamental number, and – = not available).

Species 2n NF Karyotype NOR Locality Reference
Mystus albolineatus Roberts, 1994 56 108 28m+6sm+12st+10a - Thailand (Ayutthaya) Donsakul (2000)
M. atrifasciatus Fowler, 1937 54 92 30m+8sm+16a - Thailand (Nakhon Phanom) Magtoon and Donsakul (2009)
54 96 24m+18sm+12st/a Thailand (Bueng Kan) Supiwong et al. (2014 a, b)
54 104 24m+26sm +4a/t 2 Thailand (Maha Sarakham) Present study
M. bleekeri (Day, 1877) 56 90 20m+14sm+10st+12a - India (Jammu) Sharma and Tripathi (1986)
56 102 32m+14sm+10a - India Chanda (1989)
M. bocourti (Bleeker, 1864) 56 104 24m+18sm+6st+8a - Thailand (Nong Khai) Donsakul (2000)
56 100 22m+22sm+12st/a - Thailand (Sing Buri) Supiwong et al. (2013a, 2014a, b)
M. cavasius (Hamilton, 1822) 58 102 18m+16sm+10st+14a 2 India (Jammu) Sharma and Tripathi (1986); Rishi et al. (1994)
58 108 18m+22sm+8t - India (Orissa) Tripathi and Das (1980)
58 102 14m+26sm+4st+14a - India (Bihar) Khuda-Bukhsh et al. (1980)
M. gulio (Hamilton, 1822) 58 102 30m+12sm+2st+14a 2 India (West Bengal) Das and Khuda-Bukhsh (2007b)
Female 58 108 12m+34sm+4st+8t - India (Orissa) Choudhury et al. (1993)
Male 58 110 13m+33sm+4st+8t - India (Orissa) Choudhury et al. (1993)
M. multiradiatus Roberts, 1992 54 98 30m+10sm+4st+10a - Thailand (Kanchana-buri) Magtoon and Donsakul (2009)
54 96 18m+24sm+12st/a - Thailand (Maha Sarakham) Supiwong et al. (2014 a, b)
M. mysticetus Roberts, 1992 50 92 28m+14sm+8a - Thailand (Ayutthaya) Donsakul (2002)
52 100 26m+22sm+4st/a - Thailand (Maha Sarakham) Supiwong et al. (2014a, b)
52 102 26m+24sm+2a 2 Thailand (Bueng Kan) Present study
M. ngasep Darshan et al., 2011 56 90 12m+22sm+8st+14t - India (Manipur) Sing et al. (2013)
M. singaringan (Bleeker, 1846) 56 94 24m+14sm+10st+8a - Thailand (Nakhonsawan) Donsakul (2001)
56 98 24m+18sm+14a 2 Thailand (Sing Buri) Present study
M. tengara (Hamilton, 1822) 54 64 10m+44a - India (Haryana) Nayyar (1966)
Female 54 101 9m+38sm+7a - India Rishi (1973)
Male 54 102 10m+38sm+6a - (Haryana) Rishi (1973)
Female 54 97 25m+18sm +11a - India Rishi and Rishi (1981)
Male 54 98 26m+18sm +10a - (Haryana) Rishi and Rishi (1981)
M. vittatus (Bloch, 1794) 54 108 22m+26sm+6st - India (Orissa) Tripathi and Das (1980)
54 108 22m+20sm+12st - India (Jammu) Sharma and Tripathi (1986)
58 110 10m+30sm+12st+6t - India (Orissa) Choudhury et al. (1993)
54 108 20m+24sm+10st - India Rishi (1981)
50 64 14m+36a - India Das and Srivastava (1973)
58 104 16m+10sm+20st+12a - India (West Bengal) Manna and Prasad (1974)
54 106 28m+22sm+2st+2a 2 India (Orissa) Khuda-Bukhsh and Barat (1987); John et al. (1992)
54 78 6m+18sm+30a - India (Tamilnadu) Ramasamy et al. (2010)
M. wolffii (Bleeker, 1851) 58 100 26m+10sm+6st+16a - Thailand (Tak) Donsakul (2000)
58 108 30m+22sm+6a 2 Thailand (Nakhon Sri Thammarat) Present study

Conventional cytogenetics may be sufficient to identify intra- and interspecific variations and is an inexpensive approach. However, it has restrictions, and accordingly the use of molecular cytogenetic analyses plays an increasing role for more precise characterization of the structure of genomes, including that of fishes. Especially, fluorescence in situ hybridization (FISH) for mapping of repetitive DNA sequences provided important contributions to the characterization of biodiversity and evolution in divergent fish groups (Cioffi and Bertollo 2012), especially as some microsatellite repeats are species-specific (Cioffi et al. 2015). To date, there are only three studies within Bagridae using such FISH techniques, all performed by our group (Supiwong et al. 2013a, 2014a, b).

In the present study, chromosomal structures and genetic markers for Thai populations of M. atrifasciatus Fowler, 1937, M. mysticetus, M. singaringan (Bleeker, 1846) and M. wolffii (Bleeker, 1851) (Fig. 1A–D) were for the first time analysed by cytogenetics and molecular cytogenetics.

Figure 1.

Specimens and karyotypes from conventional staining and Ag-NOR banding techniques of Mystus atrifasciatus (A, E, I), M. mysticetus (B, F, J), M. singaringan (C, G, K) and M. wolffii (D, H, L); arrows indicate NOR carrying chromosomes. Scale bars: 2 cm (A–D); 5 µm (E–L).

Material and methods

Ten males and ten females of each species were collected from the Chi (Maha Sarakham Province), Songkhram (Bueng Kan Province), Chao Phraya (Sing Buri Province) and Pak Phanang Basins (Nakhon Sri Thammarat Province), Thailand from 2016–2018. The procedures followed ethical protocols as approved by the Institutional Animal Care and Use Committee of Khon Kaen University, based on the Ethics of Animal Experimentation of the National Research Council of Thailand ACUC-KKU-15/2559. Preparation of fish chromosomes from kidney cells was done as previously reported (Supiwong et al. 2012; Pinthong et al. 2015). The chromosomes were stained with Giemsa solution for 10 minutes. Ag-NOR banding was performed by applying two drops of 2% gelatin to the chromosomes, followed by four drops of 50% silver nitrate (Howell and Black 1980). Metaphases were evaluated according to the chromosome classification of Levan et al. (1964). Chromosomes were classified as metacentric (m), submetacentric (sm), subtelocentric (st) or acrocentric (a). Fundamental number, NF (number of chromosome arm) was obtained by assigning a value of two to metacentric and submetacentric chromosomes and one to subtelocentric and acrocentric chromosomes. The chromosome sizes were calculated applying the method of Tanomtong (2011).

Microsatellites d(GC)15, d(CAA)10, d(CAT)10 and d(GAA)10 repeat probes (Kubat et al. 2008) were directly labeled by Cy3 at 5´ ends during synthesis (Sigma, St. Louis, MO, USA). FISH under high stringency conditions on mitotic chromosome spreads (Pinkel et al. 1986) was performed as previously reported (Supiwong et al. 2017b; Yano et al. 2017). The evaluation was done on an epifluorescence microscope Olympus BX50 (Olympus Corporation, Ishikawa, Japan).

Results

Diploid number, fundamental number and karyotype of Mystus atrifasciatus, M. mysticetus, M. singaringan and M. wolffii

The four studied Mystus species have different diploid chromosome numbers (2n) and fundamental numbers (NF) as follows: the 2n (NF) were 52 (102), 54 (104), 56 (98) and 58 (108) in M. mysticetus, M. atrifasciatus, M. singaringan and M. wolffii, respectively. The karyotypes of M. atrifasciatus (24m+26sm+4a), M. mysticetus (26m+24sm+2a), M. singaringan (24m+18sm+14a) and M. wolffii (30m+22sm+6a) were species-specific (Fig. 1E–H; Table 1). Differentiated sex chromosomes between male and female specimens could not be identified in all analyzed species.

Chromosome markers in Mystus atrifasciatus, M. mysticetus, M. singaringan and M. wolffii

One single pair with NOR-bearing chromosomes was present in all four species analyzed. NOR positions were observed at regions adjacent to the telomere of the short arm of the chromosome pairs 3 (metacentric), 20 (submetacentric), 15 (submetacentric), and 5 (metacentric) in M. atrifasciatus, M. mysticetus, M. singaringan and M. wolffii, respectively (Figs 1I–L, 2). The typical diversity of chromosome shapes and sizes among the four analyzed species is shown in Fig. 2 and Table 2. Karyotypic complements comprise most bi-armed and few mono-armed chromosomes revealed in M. atrifasciatus, M. mysticetus and M. wolffii whereas in M. singaringan, there are several pairs of both bi-armed and mono-armed chromosomes. Chromosome sizes are classified as large (L), medium (M) and small (S) in each species as follows: 18L+36M in M. atrifasciatus, 4L+30M+18S in M. mysticetus, 16L+36M+4S in M. singaringan, and 30L+26M+2S in M. wolffii.

Figure 2.

Idiograms representing shapes and sizes of chromosomes (haploid set) of A Mystus atrifasciatus B M. mysticetus C M. singaringan and D M. wolffii; arrows indicate NOR carrying chromosomes.

Table 2.

Cytogenetic and FISH studies on four Mystus fishes in Thailand (2n = diploid chromosome number, NF = fundamental number or number of chromosome arm, m = metacentric, sm = submetacentric, a = acrocentric, NOR = nucleolar organizer region, I = interstitial site, T = telomere, W = whole chromosome).

Mystus Species 2n NF Chromosome type Ag-NOR pair (type) Microsatellite patterns
m sm a (GC)15 (CAA)10 (CAT)10 (GAA)10
M. atrifasciatus Fowler, 1937 54 104 24 26 4 3 (m) T&I T&I T&I I
M. mysticetus Roberts,1992 52 102 26 24 2 20 (sm) T&I T&I T&I T&I
M. singaringan (Bleeker, 1846) 56 98 24 18 14 15 (sm) T&I T&I T T&I
M. wolffii (Bleeker, 1851) 58 108 30 22 6 5 (m) W&T T W&T T&I

Patterns of microsatellite repeats in the genomes of Mystus atrifasciatus, M. mysticetus, M. singaringan and M. wolffii

The mapping of d(GC)15, d(CAA)10, d(CAT)10 and d(GAA)10 microsatellites showed different hybridization signals among the species. The repeats of d(GC)15 and d(CAA)10 are abundantly distributed in the telomeric regions of several pairs and in interstitial sites of some chromosomes in M. atrifasciatus, M. mysticetus, M. singaringan. In contrast, in M. wolffii, d(GC)15 repeats are dispersed throughout all chromosomes, while d(CAA)10 repeats are accumulated at telomeric positions of some chromosome pairs with more density in only one pair. The d(CAT)10 repeats in M. atrifasciatus and M. mysticetus display high accumulations at the telomeric regions of almost all chromosomes and interstitial sites in some pairs whereas they have high accumulations at only the telomeric regions of almost all chromosomes in M. singaringan, and highly distributed in some chromosome pairs in M. wolffii. The d(GAA)10 repeats are abundantly distributed at interstitial and telomeric regions of several chromosome pairs in M. mysticetus, M. singaringan and M. wolffii, while they are highly accumulated in some chromosome pairs of M. atrifasciatus (Fig. 3; Table 2).

Figure 3.

Metaphase chromosome plates showing d(GC)15, d(CAA)10, d(CAT)10 and d(GAA)10 microsatellites mapping on chromosomes of Mystus atrifasciatus (A, E, I, M), M. mysticetus (B, F, J, N), M. singaringan (C, G, K, O) and M. wolffii (D, H, L, P). Scale bars: 5 µm.

Discussion

Diploid chromosome numbers, fundamental numbers and karyotypes of M. atrifasciatus, M. mysticetus, M. singaringan and M. wolffii

The diploid chromosome numbers (2n) in all analyzed species confirmed previous cytogenetic studies (Donsakul 2000, 2001; Magtoon and Donsakul 2009; Supiwong et al. 2014a, b), except for M. mysticetus with 2n=50 reported in a previous study (Donsakul 2002) and 52 in the present one. In agreement with the literature, 2n in the genus Mystus ranges between 50 and 58 chromosomes (Arai 2011; Table 1). The possible mechanisms that promoted intra- and interspecific karyotype diversification are biogeographic barriers, small population, limited gene flow (Galetti Jr. et al. 2000). Although all studied species except M. mysticetus, had the same 2n as previous studies, the karyotypes were different, probably because of different sampling sites should be considered (Fig. 4). The predominant 2n in this genus is 56 chromosomes (five from 13 species) and may represent an ancestral character in this family (Sharma and Tripathi 1986). This is consistent with the hypothesis of Oliveira and Gosztonyi (2000) that 2n=56 could be a plesiomorphic character in the order Siluriformes. However, NF and karyotypes found in the present study differ from all previous reports (Donsakul 2000, 2001, 2002; Magtoon and Donsakul 2009; Supiwong et al. 2014a, b). These differences may be species-specific variations within populations, and/or misidentification of species, or different species in presumed species complexes. NF in Mystus vary from 64 to 110. Ghigliotti et al. (2007) suggested that species with a higher NF value are more advanced in evolutionary terms than such with lower one. That hypothesis can be described that primitive karyotype of fish possesses many acrocentric chromosomes (mono-arm chromosomes). During evolution, the mono-arm chromosomes changed to bi-arm chromosomes. The NF would be unaltered, but the 2n would decrease. Changes in NF appear to be related to the occurrence of pericentric inversions, which play a major role for karyotypic rearrangement in fishes and other vertebrates (King 1993; Galetti Jr. et al. 2000; Wang et al. 2010). Accordingly, from comparative analysis among the here studied four Mystus species, NF data and analyses of karyotypic complements indicate for that M. singaringan has the most primitive karyotype while M. wolffii has the most derivative karyotype. As often seen in fishes of this family, no heteromorphic sex chromosomes for males and females could be identified. Nonetheless it must be mentioned, that there are two species, M. gulio (Hamilton, 1822) and M. tengara (Hamilton, 1822), which have differentiated sex chromosome systems as XX/XY and ZZ/ZW, respectively (Arai 2011). Accordingly, differentiated sex chromosome system in this fish group seems to be a quite rare phenomenon.

Figure 4.

Map showing the comparison of sampling sites among present and previous studies.

Karyotypes of the genus Mystus in Thailand showed high diversification (Table 1). Seven species have been cytogenetically studied. The 2n ranged between 50 chromosomes in M. mysticetus (Donsakul 2002) and 58 chromosomes in M. wolffii (Donsakul 2000; present study). The predominant 2n is 56 chromosomes found in M. albolineatus (NF = 108, 28m+6sm+12st+10a) (Donsakul 2000), M. bocourti (NF = 100, 22m+22sm+12st/a; NF = 104, 24m+18sm+6st+8a) (Donsakul 2000; Supiwong et al. 2013a, 2014a, b) and M. singaringan (NF = 94, 24m+14sm+10st+8a; NF = 98, 24m+18sm+14a) (Donsakul 2001; present study). Our results showed differences among NFs and karyotypes in the studied species. Interestingly, M. mysticetus had two variants, 2n = 50 chromosomes (NF=92, 28m+14sm+8a), found in Ayutthaya Province, Central Thailand (Donsakul 2002), and 52 chromosomes (NF = 100, 26m+22sm+4st/a; NF=102, 26m+24sm+2a) found in Maha Sarakham and Bueng Kan Provinces, Northeast of Thailand (Supiwong et al. 2014a, b; present study) (Fig. 4). This variation may be caused by a rearrangement of chromosomes by centric fusion and pericentric inversion during chromosomal evolution in groups of populations separated by a geographic barrier.

Chromosome markers for M. atrifasciatus, M. mysticetus, M. singaringan and M. wolffii

Nucleolus organizer regions (NORs)

The localization of nucleolus organizer regions (NORs) is a simple method to determine chromosomal marker. NORs are specific positions on the chromosome that consist of tandemly repeated sequences of ribosomal genes (rRNA). In eukaryotes, each unit is composed of three genes coding for 18S, 5.8S and 28S ribosomal RNA (Sharma et al. 2002). Generally, most fishes have one pair of small NORs (single NOR) on chromosomes. However, some species of fishes have more than two NORs which may be caused by the translocation between some part of the chromosome with NORs and another chromosome (Sharma et al. 2002). Interspecific and intraspecific NOR polymorphism in the number of NORs per genome, in the chromosomal location of NOR sites, in the relative sizes of individual NORs, and in the number of active NOR sites per cell are commonly observed in fish, where the rDNA loci have been shown to be highly dynamic (Milhomem et al. 2013). Changes in chromosome number and structure can alter the number and structure of NOR as well. The pattern of NORs may be specific to populations, species and subspecies. Robertsonian translocations may cause losses of NOR. Species, which have limited gene exchange due to geographical isolation, have elevated karyotype numbers and NOR variation. (Yüksel and Gaffaroğlu 2008). The NOR is frequently used to compare variations as well as to identify and explain specifications. Therefore, it can be used as taxonomic and systematic characters in order to infer phylogenetic hypotheses of species relationships (Gold 1984; Amemiya and Gold 1990).

If these loci are active during the interphase before mitosis, they can be detected by silver nitrate staining (Howell and Black 1980). The single NOR-bearing chromosome pairs in the present study is consistent in M. cavasius (Hamilton, 1822) (Sharma and Tripathi 1986; Rishi et al. 1994), M. gulio (Das and Khuda-Bukhsh 2007b), and M. vittatus (Khuda-Bukhsh and Barat 1987; John et al. 1992). This character is a common characteristic found in many species in this family such as Bagrichthys majusculus Ng, 2002 (Supiwong et al. 2018), He. menoda (Hamilton, 1822) (Barat and Khuda-Bukhsh 1986), He. wyckii (Bleeker, 1858) (Supiwong et al. 2017c), Horabagrus brachysoma (Günther, 1864) (Nagpure et al. 2003), Ho. nigricollaris Pethiyagoda et Kottelat, 1994 (Nagpure et al. 2004), Pelteobagrus ussuriensis (Dybowski, 1872) (Kim et al. 1982), Pseudobagus vachelii (Ueno 1985), Pseudomystus siamensis (Regan, 1913) (Supiwong et al. 2013b), Rita rita (Hamilton, 1822) (Khuda-Bukhsh and Barat 1987) and Sperata seenghala (Sykes, 1839) (Sharma and Tripathi 1986; Das and Khuda-Bukhsh 2007a). However, only a single species, Tachysurus fulvidraco (Richardson, 1846), has two NOR carrying chromosome pairs (Zhang et al. 1992). In fishes, a single NOR carrying chromosome pair is considered as a primitive state (Milhomem et al. 2013). Many families such as Chaetodontidae (Supiwong et al. 2017a), Lutjanidae (Phimphan et al. 2017), Notopteridae (Maneechot et al. 2015), Scaridae (Kaewsri et al. 2014), Serranidae (Pinthong et al. 2013), share this character. Also, for fishes the location of NORs in a terminal position, as seen in the studied species, is also considered as a primitive characteristic (Vitturi et al. 1995).

Patterns of microsatellite repeats on the genomes of Mystus atrifasciatus, M. mysticetus, M. singaringan and M. wolffii

Repetitive DNAs like microsatellites can be used to spot genomic evolution as previously been reported for different fish groups (Cioffi et al. 2010; Cioffi and Bertollo 2012; Terencio et al. 2013; Yano et al. 2014; Cioffi et al. 2015; Moraes et al. 2017, 2019; Sassi et al. 2019). It is known from fossil records that there is a major evolutionary diversification in Siluriformes fishes; this has in parts already also been verified at chromosomal level.

Here, four bi- and tri-nucleotide microsatellite sequences were mapped on chromosomes of four Mystus species. The patterns of microsatellites d(GC)15 and d(CAA)10 repeats in three species in the present study (M. atrifasciatus, M. mysticetus, M. singaringan) are similar to those found in Channa micropeltes (Cuvier, 1831) (Cioffi et al. 2015). On the other hand, they are differences known for C. gachua (Hamilton, 1822), C. lucius (Cuvier, 1831), C. striata (Bloch, 1793) (Cioffi et al. 2015), Toxotes chatareus (Hamilton, 1822) (Supiwong et al. 2017b) and Asian swamp eel, Monopterus albus (Zuiew, 1793) (Supiwong et al. 2019). The pattern of microsatellite d(GC)15 repeats in M. wolffii is similar to that of C. lucius (Cioffi et al. 2015) and T. chatareus (Supiwong et al. 2017a). Interestingly, the patterns of microsatellite d(CAT)10 repeats in M. atrifasciatus, M. mysticetus and M. singaringan are similar to the patterns of the (CA)15 repeats on chromosomes of other species in the family Bagridae (Supiwong et al. 2013a, 2014b). Comparative study on four species showed that not only there are differences of 2n, NF and karyotype, but the patterns of microsatellite repeat on chromosomes also have difference among them. Thus, the cytogenetic data may be a tool for classification of fish species that there is similar morphology as the stripe Mystus (M. atrifasciatus and M. mysticetus).

From previous reports, it may be carefully deduced that most heterochromatin in fish genomes consist of microsatellites (Cioffi and Bertollo 2012). However, microsatellites have also been found in non-centromeric regions, many of them were located either near or within genes (Rao et al. 2010; Getlekha et al. 2016). Indeed, GC rich motifs are common in exons of all vertebrates (Chistiakov et al. 2006). Since higher recombination rates can be found near the telomeric region (Jensen-Seaman et al. 2004), it is possible that the physical proximity of microsatellite and rDNA repeats could favor the evolutionary spreading of both sequences together, despite the possibility of spreading some errors, too. Repetitive DNA sequences could act as primary driving forces in speciation (Biémont and Vieira 2006). These sequences are closely associated with heterochromatic regions, thus contributing to gene activation and structural maintenance of chromosomes (Dernburg et al. 1996). Therefore, great variations in the amount and position of these sequences could create fertility barriers by fostering the occurrence of chromosomal rearrangements (Cioffi and Bertollo 2012).

Indeed, the distribution of microsatellite motifs in fish genomes could be biased to some specific noncoding regions, as found in the Asian swamp eel, M. albus (Li et al. 2017). Finally, closely related fish species involved in recent speciation events could present a differential pattern in the distribution and quantity of microsatellite sequences on chromosomes, as demonstrated for naked catfishes (Supiwong et al. 2014b), channid fishes (Cioffi et al. 2015) and four Mystus in the present study.

Conclusions

The present research is the first report on NOR and microsatellites d(GC)15, d(CAA)10, d(CAT)10 and d(GAA)10 mapping in M. atrifasciatus, M. mysticetus, M. singaringan and M. wolffii. There are differences in the diploid chromosome number, the fundamental numbers, karyotypes, pairs having NORs, and patterns of microsatellite distributions on chromosomes. These results indicated that (molecular) cytogenetic data can be used for classification in related fish species and to explain karyotype diversification.

Acknowledgments

This work was financially supported by the Research Fund for DPST Graduate with First Placement Year 2015, Development Promotion of Science and Technology Talents project (DPST), the Post-Doctoral Training Program from Research Affairs and Graduate School (Grant no 59255), Khon Kaen University, and Unit of Excellence 2020 on Biodiversity and Natural Resources Management, University of Phayao (UoE63005), Thailand.

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