Cytogenetic analyses of the stingless bee Partamona helleri collected in the state of Bahia, Northeast Brazil revealed the chromosome numbers n = 18 in the haploid males and 2n = 35 in the diploid females. All karyotypes displayed one large acrocentric B chromosome, which differs from the minute B chromosomes previously described in the populations from southeastern Brazil. Giemsa staining, C-banding and DAPI/CMA3 fluorochrome staining also revealed a remarkable interpopulational divergence regarding both the regular karyotype and the B chromosomes. The B chromosomes found in the samples from Jequié, Bahia, were entirely heterochromatic, while those found in Cravolândia, Bahia, displayed a euchromatic portion at the telomeric end of the long arm. CMA3 labeling sites varied from seven to eight between the two localities in Bahia, due to the presence of an extra GC-rich block in the karyotype of the samples from Jequié. This is the first report of a large B chromosome in P. helleri and reveals the occurrence of a geographic differentiation within this species.

Although many species of the genus Trigona have been taxonomically described, cytogenetic studies of these species are still rare. The aim of the present study was to obtain cytogenetic data by conventional staining, C banding and fluorochrome staining for the karyotype characterization of the species Trigona fulviventris. Cytogenetic analysis revealed that this species possesses a diploid chromosome number of 2n = 32, different from most other species of this genus studied so far. This variation was probably due to the centric fusion in a higher numbered ancestral karyotype, this fusion producing the large metacentric chromosome pair and the lower chromosome number observed in Trigona fulviventris. Heterochromatin was detected in the pericentromeric region of the first chromosome pair and in one of the arms of the remaining pairs. Base-specific fluorochrome staining with 4'-6-diamidino-2-phenylindole (DAPI) showed that the heterochromatin was rich in AT base pairs (DAPI+) except for pair 13, which was chromomycin A3 (CMA3) positive indicating an excess of GC base pairs. Our data also suggests that there was variation in heterochromatin base composition.