Proposed sequence of events leading to present-day Ty1 elements in the subtelomeric regions of chromosomes I and VIII (I andVIII, respectively). (Black arrowheads) Telomeres; (black circles) centromeres. Chromosome sequences left of the centromere are depicted as thin lines. Chromosome sequences right of the centromere are depicted as a narrow black box for chromosome I, a narrow gray box for chromosome VIII, and a narrow white box for the chromosome involved in a reciprocal exchange with chromosome I. Open arrowheads labeleda, b, or c indicate target-site sequences.