Chromosoma (Berl.) 81,673-690 (1981) CHROMOSOMA Springer-Verlag 1981 The Distribution of Repetitive DNAs Between Regular and Supernumerary Chromosomes in Species of Glossina (Tsetse): A Two-step Process in the Origin of Supernumeraries Amanda Amos 1 and Gabriel Dover Department of Genetics, University of Cambridge, Cambridge, CB2 3EH, England U.K. ; 1 present address: MRC Clinical and Population Cytogenetics Unit Western General Hospital, Crewe Road, Edinburgh, Scotland U.K. Abstract. Several species of tsetse fly within the Morsitans and Fusca subgen- era of Glossina contain supernumerary (B) chromosomes. Previous studies on the meiotic behaviour of chromosomes (Southern and Pell, 1973) and the C-band patterns (Jordan et al., 1977) have indicated a close similarity between the Y chromosome and the supernumeraries. The distributions of the highly abundant families of DNA (satellite DNAs) between the auto- somes, sex chromosomes and B chromosomes of G.m. morsitans, G. austeni and G. paIlidipes have been examined by in situ hybridisation. In addition, the organisation and sequence homologies of satellite DNAs have been exam- ined by restriction enzymes and heterologous hybridisations in in situ and "Southern" transfer conditions. The majority of satellite sequences that are homologous between species are distributed in several different arrange- ments between A and B chromosome telomeres with minority sequences at some centromeric and intercalary locations. There is no extensive satellite DNA similarity between the Y and B chromosomes. We suggest that the Y and B chromosome associations and synchronous allocycly during meiosis are the result of extensive heterochromatinisation of these two chromosome types, that is probably a reflection of two separate stages involved in the generation of the B chromosomes in the genus. The independent evolution of satellites and supernumeraries is discussed. Introduction Supernumerary (B) chromosomes are a perpetual curiosity in that their biology and evolution are but little understood. Phenotypic effects, where observed, take place often in unusual circumstances, For example, in some plant genera a B chromosome disturbance of meiotic chromosome pairing takes place in interspecific hybrids but not in the parental genotypes (Dover and Riley, 1972; Evans and Macefield, 1972). B chromosomes are also able to interact with and release genetic variability residing on the A chromosomes although they 0009-59t5/81/0081/0673/$03.60