Twenty to Ten: Centromere Loss and Retention during the Descent of Maize from a Tetraploid Ancestor

Although centromere function is highly conserved in eukaryotes, centromere sequences are highly variable. Only a few centromeres have been sequenced in higher eukaryotes because of their repetitive nature, thus hindering study of their structure and evolution. Conserved single-copy sequences in pericentromeric regions of sorghum and maize were found to be diagnostic characteristics of adjacent centromeres. By analyzing comparative map data and centromeric/pericentromeric sequences of sorghum, maize and rice, the major evolutionary events related to centromere dynamics were discovered for the maize lineage after its divergence from its common ancestor with sorghum. (i) Remnants of ancient centromeric/pericentromeric regions were found for the 10 lost ancestral centromeres. (ii) Five cases of long distance, intra-chromosome movement of centromeres/pericentromeres were detected in the retained centromeres, with inversion the major process involved. (iii) The 12 major chromosome rearrangment events that led to maize chromosome number reduction from 20 to 10 were uncovered. (iv) Besides chromosome insertion near (but not always into) other centromeres, translocation and fusion were also found to be important mechanisms underlying grass chromosome number reduction. (v) Comparison of chromosome structures confirms the polyploid event that led to the tetraploid ancestor of modern maize.