CYTOGENETIC STUDIES ON THE SUBFAMILY HYPOPTOPOMATINAE (PISCES, SILURIFORMES, LORICARIIDAE) .3. ANALYSIS OF 7 SPECIES

Cytogenetic studies carried out on nine species belonging to five gene ra of the subfamily Hypoptopomatinae showed that this group has a rela tively constant diploid number, 2n = 54, with only one species having 2n = 72 chromosomes. Nevertheless, the karyotypic formulae, NOR positi on and C-band pattern are very different among species and sometimes a mong local populations, with species having undifferentiated sex chrom osomes and species having the XX/XY or the ZZ/ZW mechanisms. The popul ation structure of species belonging to the subfamily Hypoptopomatinae suggests that many chromosome rearrangements have been fixed in the d ifferent species and populations due to their geographic isolation and these karyotypic differences may be very important today for species definition.