ARI Volume 2 Number 1.pdf - Zoo-unn.org

Cytogenetic variations in Clarias species 277of 2n = 54. Furthermore, sex chromosomes wereobserved as ZW heteromorphic pair in femalehybrid karyotypes. The ZZ chromosome pair inmale hybrids was similar to that found in male C.gariepinus and male H. longifilis.Hartley (1987) divided the species of thesub-order: Salmonidea into two groups (categoriesA and B) based on chromosome number,chromosome arm number, and the distribution ofone-armed and two-armed chromosomes.Category A karyotypes were widespread both interms of geographic distribution and number ofspecies, while category B karyotypes wererestricted to member of the genera Salmo andOncorhynchus. In category A, it was assumedthat tetraploidy followed arrangement of thediploid ancestral karyotype of 2n = 50, NF = 60(the lowest diploid and chromosome arm numberfound in the smolts) and that the resultingancestral tetraploid salmon had 2n = 100, NF =120 karyotype. Hartley (1987) observed that aseries of pericentric inversion and centric fusionshad reduced chromosome number andchromosome arm number through Brachymystazlenok Gunter, 1866, (2n = 94, NF = 116) andHucho hucho Gunter, 1866, (2n = 82, NF = 114)to the category A karyotypes (2n = 80, NF = 100)and finally to the category B karyotypes (2n = 60,NF = 104). He concluded that by the time thecategory B karyotypes were achieved, centricfusions had played a far greater role in theevolution of the vast majority o the salmonidskaryotypes than pericentric inversions, althoughthe latter had played important role in theevolution of the Atlantic salmon karyotype.The karyotypic evolution of tenNeotropical cichlids showed that the chromosomeevolution of the group was more conservative thandivergent (Feldberg and Bertollo, 1985). Allspecies had a chromosome number of 2n = 48,though with some differences in chromosomemorphology. Pericentric inversions were probablythe main event that led to the karyotypes morecommonly found in this group. On the basis offundamental number (FN), four general groupsreflected the occurrence of a progressive numberof chromosome rearrangements. Group one withFN = 48, (represented by Chaetobranchopsisaustralis Eigenmann and Ward 1907), group twowith FN = 50 – 52 (comprising Geophagusbrasiliensis Heckel, 1840; G. surinamensis Heckel,1840 and Gymnogeophagus balzanii Ribeiro,1918), group three with FN = 54 (represented byCrenicichla lacustris Ribeiro, 1918, C. lepidotaHenkel, 1840 and C. vittala Heckel, 1840 and toBatrachops semifasciatus Heckel, 1840), groupfour with FN = 58 – 60 (represented by Astronotusocellatus Swainson, 1839 and Cichlasoma facetumSwainson, 1839) (Feldberg and Bertollo, 1985).Despite the constancy of chromosome number, 2n= 48, a few distinctive characteristics which areproducts of structural rearrangements ofchromosomes that occurred during their karyotypicevolution can be seen in some species or group ofspecies.Ichthyologists and fish taxonomists in theclassification of related species have often employknowledge from karyotype. For instance, thelarval forms of most species are morphologicallyidentical and quite different from the adult forms.Because of the resemblance among larval forms,their identification if often difficult. Morphologicalobservations are inadequate vis-à-vis theidentification of the species, except where thedevelopmental stages can be followed, which isusually difficult and not documented for many fishspecies. In such cases, Sola et al. (1981) notedthat the knowledge of the karyotype and theirdiversified forms might provide a precisediagnostic criterion. Oliveria et al. (1988) basedthe classification of Neotropic Ostrariophysi onkaryotypic data and noted that 2n = 50 tend toprevail among Ostrariophysi. This character wasalso seen among the Otophysi (2n = 50). TheCypriniformes have a second modal number of 2n= 100, often seen among the families ofCyprinidae and Catostanidae. Yu et al. (1987)proposed that the cypriniformes might haveoriginated from 2n = 50 ancestors. Thecharaciphysi and characiformes differ from thecypriniforms by having almost 2n = 54. Amongthe characiformes, several groups may becharacterized based on their chromosome number.The characiforms, erythrinidae and lebiasinidae aretypified by chromosome number of less than 2n =54 while the Serrasalminae are characterized bychromosome number of more than 2n = 54.From the foregoing review, karyotypicinformation appears to be very important indiscriminating species. The technical andinterpretation problems often encountered by fishcytologists should not be employed as a base forjudging the validity of karyotypic information vis-àvisclassification of related and unrelated species.The identification of artificial (diploidy and/orpolyploidy) and natural hybrids, and of sexdetermining chromosomes is currentlycytogenetically accomplished. For a properdiscrimination of the species, information from fishmolecular biology, anatomy, anatomy, physiologyethology and ecology is highly desirable. Thisstudy further enriches the information on fishcytogenetic by investigating into the karyotypicvariations among members of the Pisces familyClariidae of Anambra river, Nigeria usingleucocytes culture techniques.