Discovery of B Chromosomes in Drosophila Melanogaster That Causes Female Specific 4th Chromosome Nondisjunction. Elisabeth Bauerly¹, Stacie Hughes¹, R. Scott Hawley^1,2. 1) Stowers Institute for Medical Research, Kansas City, MO; 2) Department of Molecular and Integrative Physiology, Kansas University Medical Center Kansas City, Kansas 66160, USA.

   B chromosomes are small, nonrecombiŽnant chromosomes that are not required for the viability of a species and are transmitted in a non-Mendelian manner. B chromosomes have been hypothesized to be a mechanism used to drive evolution, as they create cells with novel karyotypes and similar chromosomes in humans are linked to a number of developmental abnormalities. We have identified a strain of Drosophila melanogaster, which carried the meiotic mutant matrimony (mtrm), in which we frequently observe 12 to 15 B chromosomes, a number that is much higher than what is typically seen in similar organisms. These B chromosomes can be stably maintained in an otherwise completely wild type background at about half the amount as what it is seen in the original mutant strain, despite the fact that they cause a significant amount of 4th chromosome nondisjunction in females (but not in males). As shown by fluorescent in situ hybridization they are largely, if not entirely, composed of 4th chromosome heterochromatic sequences and indirect immunofluorescence indicates that they contain centromeres. Presumably as a consequence of their heterochromatic content, these chromosomes show a significant effect on position effect variegation. Insights on these B chromosomes in a highly tractable genetic system such as Drosophila melanogaster could provide insight on chromosome formation and chromosome segregation, as well as side effects that arise when extra or unstable chromosomes are present.