Centromere emergence in evolution

Evolutionary centromere repositioning is a paradox we have recently discove red while studying the conservation of the phylogenetic chromosome IX in pr imates. Two explanations were proposed: a conservative hypothesis assuming sequential pericentric inversions, and a more challenging assumption involv ing centromere emergence during evolution. The complex evolutionary history showed by chromosome IX did not allow us to clearly distinguish between th ese two hypotheses. Here we report comparative studies of chromosome X in t wo lemur species: the black lemur and the ringtailed lemur. The X chromosom e is telocentric in the black lemur and almost metacentric in the ringtaile d lemur. The marker order along these chromosomes, however, was found to be perfectly colinear with humans. Our data unequivocally point to centromere emergence as the most likely explanation of centromere repositioning.