A comparative study of retrotransposons in the centromeric regions of A and B chromosomes of maize.
Bacterial Artificial Chromosomes (BACs) derived from the B chromosome, based on homology with the B specific sequence, were subcloned and sequenced. Analysis of DNA sequence data indicated the presence of 23 common retroelements, as well as novel sequences of B chromosome origin. Generally, where the same retrotransposon type was observed in both A and B chromosomes, there were more copies per unit of sequence in the B centromeric region (the major site of B repeat) than in the A centromere, except for Huck-1. Based on previous estimates of the age of the major burst of transposition into the maize genome, the oldest retrotransposons (Ji-6 and Tekay, approximately 5.0 and 5.2 million years ago, respectively) were found in the B centromere region only, while the next two oldest (Huck-1 and Opie-1) were found in both the A and B sequences. Phylogenetic analysis of Opie retroelements from both A and B centromeres indicated that some of the B Opie centromeric sequences share a more recent common ancestor with A Opie retroelements than they do with other B Opie centromeric sequences. These results imply that the supernumerary maize B chromosome has coexisted with the A chromosomes during that period of transposition. They also support the hypothesis that the B chromosome had its origins from A chromosome elements, or that alternative origins, such as being donated to the maize genome in a wide species cross, preceded six million years ago, because the spectrum of retrotransposons in the two chromosomes is quite similar.
Theuri
J
J
Division of Biological Sciences, University of Missouri, Columbia, MO 65211, USA.
Phelps-Durr
T
T
Mathews
S
S
Birchler
J
J
D002503:Centromere / Q000235:genetics* / Q000648:ultrastructure; D002874:Chromosome Mapping / Q000379:methods; D022202:Chromosomes, Artificial, Bacterial; D032461:Chromosomes, Plant / Q000235:genetics*; D004251:DNA Transposable Elements / Q000235:genetics; D018626:Retroelements; D003313:Zea mays / Q000235:genetics*
D004251:DNA Transposable Elements; D018626:Retroelements