CHROMOSOME-STUDIES IN HYPOPTOPOMATINAE (PISCES, SILURIFORMES, LORICARIIDAE) .2. ZZ ZW SEX-CHROMOSOME SYSTEM, B-CHROMOSOMES, AND CONSTITUTIVE HETEROCHROMATIN DIFFERENTIATION IN MICROLEPIDOGASTER-LEUCOFRENATUS

Cytogenetic analysis of two local populations of microlepidogaster leu cofrenatus showed a basic diploid chromosome number (2N) of 54 in both populations. Some fishes were found to have a 2N = 55 or 56 chromosom es due to the presence of one or two large heterochromatic B chromosom es. Specimens of M. leucofrenatus from the Poco Grande stream had 24 m etacentrics, 24 submetacentrics, four subtelocentrics, and one submeta centric homomorphic pair in males and one submetacentric/subtelocentri c heteromorphic pair in females, whereas individuals of this species f rom the Marumbi River had 22 metacentrics, 24 submetacentrics, four su btelocentrics, two acrocentrics, and one submetacentric/subtelocentric heteromorphic pair in females. The occurrence of the heteromorphic pa ir in the females was due to the presence of an extra C-banded segment on the W chromosome. Ag-NORs in both populations were located interst itially on the short arm of the largest metacentric pair. The Poco Gra nde population had less constitutive heterochromatin than did the Maru mbi River population. The speciation process in this fish species is d iscussed on the basis of heterochromatin distribution.