KARYOTYPES OF THE MOST PRIMITIVE CATFISHES (TELEOSTEI, SILURIFORMES, DIPLOMYSTIDAE)

Karyotypes of Diplomystes camposensis and Diplomystes nahuelbutaensis were the same diploid number (n = 56). The chromosome formula for D. c amposensis was 16 metacentric + 24 submeracentric + 8 subrelocentric 8 telocentric chromosomes and for D, nahuelbutaensis was 14 metacentr ic + 26 submetacentric + 8 subtelocentric + 8 telocentric chromosomes. In contrast, the differences in the chromosomal C-banding patterns be tween these species was large. For instance, chromosome pairs 5, 6, an d 7 of D. nahuelbutaensis showed heterochromatic centromeres and pairs 23, 24, 27, and 28 were entirely heterochromatic. Diplomystes campose nsis showed conspicuous C-banded blocks in pairs 7, 24, and 25 (chromo some pair 7 had one heterochromatic arm, chromosome pair 24 was entire ly heterochromatic, and chromosome pair 25 had heterochromatin close t o centromere). Comparison with other ostariophysan karyotypes (e.g. gy mnotiforms, characiforms, and cypriniforms), does not allow any conclu sions about the plesiomorphic catfish condition, because the karyotype s of the outgroups are too variable. A synapomorphy shared by characif orms, gymnotiforms, and diplomystid catfishes is the presence of more metacentric to submetacentric than subtelocentric to telocentric chrom osomes. Cypriniforms are more primitive because they have more subtelo centric to telocentric than metacentric to submetacentric chromosomes.