Schematical description of the most parsimonious series of hypothetical rearrangements that would be needed to reconcile the observed marker order and the position of the centromeres. These rearrangements are based on the assumption that the centromere in PCA was positioned telomeric to marker A. This conclusion is drawn exclusively from the constraint imposed by the maximum parsimony. The inversions are indicated by brackets. The inversions not present in Fig. 2 have been specifically introduced to account for the paradoxal position of the centromere. In AGE and SSC the centromere is positioned at the boundary between chromosome IX and the chromosome segment brought there by an interchromosomal rearrangement. We cannot exclude the possibility that the centromere of these two species has originated from a different chromosome. The orientation of the chromosomes has been reported to match the orientation reported in Fig. 2.