XX/XY heteromorphic sex chromosome systems in two bullhead catfish species, Liobagrusmarginatus and L. styani (Amblycipitidae, Siluriformes).

Karyotypes and chromosomal characteristics of two species of bullhead catfish of the genus Liobagrus, namely L. marginatus and L. styani were examined by means of conventional (Giemsa and CMA(3) staining) and molecular (FISH with telomeric, 5S and 18S rDNA probes, respectively) cytogenetic techniques. The diploid numbers of L. marginatus and L. styani were 2n = 24 and 30 respectively. The karyotypes were: L. marginatus, 20m+2sm+2st in females and 19m+2sm+2st+1a in males; L. styani, 16m+ 10sm+4st in both sexes. The karyotypes of the two species and the delayed-pairing of synaptonemal complexes in meiosis of L. marginatus suggested the presence of heteromorphic XX/XY sex chromosome systems. The results of (TTAGGG)(n)FISH and CMA(3)-staining suggested that Robertsonian fusion might play an important role in chromosome differentiation among these species of Liobagrus. The 5S rDNA situated on the sex chromosomes of L. marginatus and 18S rDNA on the sex chromosomes of L. styani were located by FISH, and the origins of heteromorphic sex chromosomes via chromosomal reversion or unequal crossing over of heterochromatin were hypothesized.