Chromosome preparation from gill tissue of the tropical oyster (Crassostera belcheri Sowerby, 1871) collected from an oyster farm in Kantang District, Trang Province, southern Thailand was studied by conventional staining and Ag-NOR baining techniques. The result revealed that the diploid chromosome number is 2n=20 and the fundamental number (NF) is 40. The karyotype has revealed 10 large metacentric and 10 medium metacentric chromosomes. One pair of nucleolar organizer regions (NORs) was apparent on the short arm telomeric region of chromosome pair 10. Basic information on the cytogenetics of the C. belcheri may be useful for future studies on interspecific hybridization and polyploidy. The karyotype formula of C. belcheri is as follows: m m 10 10 2 (diploid) 20=L +M n

BackgroundThe genus Clarias (Clariidae, Siluriformes) contains at least 61 species naturally spread over vast regions of Asia, India and Africa. However, Clarias species have also been introduced in many different countries and represent the most widespread catfishes in the world. These fishes are also known as “walking catfishes” due to their ability to move over land. A large degree of chromosomal variation has been previously found in this family, mainly using conventional cytogenetic investigations, with diploid chromosome numbers ranging between 48 and 100. In this study, we analyzed the karyotype structure and distribution of four repetitive DNA sequences (5S and 18S rDNAs and (CA)15 and (GA)15 microsatellites) in three Clarias species (C. batrachus, C. gariepinus, C. macrocephalus), as well as in a probable natural hybrid of the two latter species from different Thailand river basins.ResultsClarias gariepinus and C. macrocephalus had 2n = 56 and 2n = 54, respectively, as well as karyotypes composed mainly by metacentric and submetacentric chromosomes. Their karyotypes differed in the number and location of 5S and 18S rDNA sites and in the degree of microsatellite accumulation. An intermediate chromosomal pattern incorporating those of the parental species was found in the probable hybrid, confirming its interspecific origin. Clarias batrachus had 2n = 104 chromosomes and its karyotype was dominated by mainly acrocentric elements, indicating that unusual multiple centric fissions were involved in its karyotype differentiation. The karyotype of this species presented an unexpected dispersion of ribosomal DNAs, possessing 54 and 12 sites of 5S and 18S rDNAs, respectively, as well as a high accumulation and differential distribution of both microsatellite repeats, representing ‘hot spots’ for chromosomal rearrangement.ConclusionBoth conventional and molecular cytogenetic markers were useful tools for demonstrating remarkable evolutionary dynamism and highlighting multiple chromosomal rearrangements and hybridization events correlated with the notable karyotypic diversity of these walking catfishes.

The Dascyllus genus consists of 11 species spread over vast regions of the Indo-Pacific, showing remarkable reductions in the diploid chromosome numbers (2n). The present study analyzed the karyotypes and other chromosomal characteristics of D. trimaculatus (2n = 48; 2st + 46a; NF = 50), D. carneus (2n = 48; 2st + 46a; NF = 50) and D. aruanus (2n = 30; 18m + 2st + 10a; NF = 50) from the Thailand Gulf (Pacific Ocean) and D. melanurus (2n = 48; 2st + 46a; NF = 50) from the Andaman Sea (Indian Ocean), employing conventional cytogenetic analyses and the chromosomal mapping of repetitive DNAs, using 18S and 5S rDNA, telomeric sequences and (CA)15, (GA)15, and (CAA)10 microsatellites as probes. The C-positive heterochromatin was found in the centromeric regions of most chromosomal pairs and 18S rDNA phenotypes were single in all species. However, in D. aruanus (2n = 30), which harbors nine metacentric pairs; the 5S rDNA sites were located in the centromeric region of the shortest one. The mapping of the telomeric sequences in D. aruanus revealed the presence of interstitial telomeric sites (ITS) in the centromeric region of four metacentric pairs, with one of these pairs also displaying an additional ITS in the long arms. Distinct chromosomal markers confirmed the reduction of the 2n by chromosomal fusions, highlighting the precise characterization of these rearrangements by the cytogenetic mapping of the repetitive DNAs.

Summary A new natural autotetraploid and the chromosomal characteristics of the dwarf snakehead fish (Channa gachua) from Kalasin, Nong Khai, Nong Bua Lam Phu and Udon Thani Provinces (four populations) in northeast Thailand were studied. Kidney cell samples were taken from 12 male and 12 female fish. Mitotic chromosome preparations were conducted using a blood cell culture technique as well as taken directly from kidney cells. Conventional and Ag-NOR staining techniques were applied to stain the chromosomes. The results showed that the autotetraploid chromosome number of C. gachua was 4n = 104, and the fundamental number (NF) was 112 in both sexes. The types of chromosomes were 4 large submetacentric, 2 large acrocentric, 16 large telocentric, 2 medium submetacentric, 68 medium telocentric and 12 small telocentric chromosomes. No strange-sized chromosomes related to sex were observed. The region adjacent to the centromere of chromosome pair 3 showed clearly observable secondary constriction (NORs). The karyotype formula for C. gachua is as follows:4n (

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The standardized karyotype and idiogram of the Bengal monitor lizard (Varanus bengalensis) from the Kalasin Province of Thailand were studied. Blood samples were taken from male and female lizards. The standard T-lymphocyte cell was cultured at 26 C for 96 h. The mitotic chromosomes were harvested by colchicine-hypotonic-fixation-air drying technique. Conventional staining, Ag-NOR banding and C-banding techniques were applied to stain the chromosome with 20% Giemsa s solution. The results showed that the number of diploid chromosome was 2n= 40, while the fundamental number (NF) was 54 in males and 55 in females. The types of macrochromosomes were 4 large metacentric, 4 small metacentric, 6 small submetacentric, 2 small telocentric macrochromosomes and 24 microchromosomes. The Z chromosome is the small telocentric chromosome, while the W chromosome is the small metacentric chromosome. In addition, the short arm pericentromeric region of chromosome pair 1 showed clearly observable nucleolar organizer regions (NORs). This data is the first report on polymorphism of NORs in V. bengalensis. The result showed that the heteromorphic of one male had a different size of NORs of chromosome pair 1 (1a1b), while one female had an equal size of both chromosome pair 1 with a homomorphic (1a1a). The karyotype formula is as follows: m m sm t 4 4 6 2 2 (40) =L +S +S +S +Z(t)W(m)+22 microchromosomes n

Pomacentrus (damselfishes) is one of the most characteristic groups of fishes in the Indo-Pacific coral reef. Its 77 described species exhibit a complex taxonomy with cryptic lineages across their extensive distribution. Periods of evolutionary divergences between them are very variable, and the cytogenetic events that followed their evolutionary diversification are largely unknown. In this respect, analyses of chromosomal divergence, within a phylogenetic perspective, are particularly informative regarding karyoevolutionary trends. As such, we conducted conventional cytogenetic and cytogenomic analyses in four Pomacentrus species (Pomacentrus similis, Pomacentrus auriventris, Pomacentrus moluccensis, and Pomacentrus cuneatus), through the mapping of repetitive DNA classes and transposable elements, including 18S rDNA, 5S rDNA, (CA), (GA), (CAA), Rex6, and U2 snDNA as markers. P. auriventris and P. similis, belonging to the Pomacentrus coelestis complex, have indistinguishable karyotypes (2n = 48; NF = 48), with a peculiar syntenic organization of ribosomal genes. On the other hand, P. moluccensis and P. cuneatus, belonging to another clade, exhibit very different karyotypes (2n = 48, NF = 86 and 92, respectively), with a large number of bi-armed chromosomes, where multiple pericentric inversions played a significant role in their karyotype organization. In this sense, different chromosomal pathways followed the phyletic diversification in the Pomacentrus genus, making possible the characterization of two well-contrasting species groups regarding their karyotype features. Despite this, pericentric inversions act as an effective postzygotic barrier in many organisms, which appear to be also the case for P. moluccensis and P. cuneatus; the extensive chromosomal similarities in the two species of P. coelestis complex suggest minor participation of chromosomal postzygotic barriers in the phyletic diversification of these species.

The present study aims to analyze concerned karyotyping and idiograming of the three-spot damselfish (Dascyllus trimaculatus