CHROMOSOME-NUMBERS AND KARYOTYPIC VARIATION IN BUMBLE BEES (HYMENOPTERA, APIDAE, BOMBINI)

Chromosome number of 16 Bombus and one Psithyrus species were determin ed, and the total chromosome complement lengths of 11 species were mea sured. The modal haploid chromosome number of the Bombus species was 1 8 (range in n 12-19). These comprise seven subgenera which are well se parated phylogenetically, therefore, we infer that the ancestral chrom osome number of bumble bees is probably 18. Chromosomal fission and fu sion events probably account for the intra- and inter-subgeneric varia tion in haploid number found in Bombus. The pattern of variation sugge sts that changes in chromosome number probably followed, rather than p receded, differentiation into subgenera. The much higher haploid numbe r (26) found in Psithyrus (cleptoparasites) must be the derived state, presumably attained by chromosomal fission, since the total complemen t length of P. citrinus is not significantly greater than that of most of the Bombus species. The genus Psithyrus is considered to be a mono phyletic group and this conclusion is supported by the chromosome coun ts.