Localization and transcription of a retrotransposon-derived element on the maize B chromosome.
Dispensable chromosomes in addition to the normal complement in diverse taxa are called B chromosomes. The maize B chromosome is discernible in mitotic chromosome spreads as a small compact chromosome composed mainly of heterochromatin. Although much of this chromosome consists of repetitive elements common to the A chromosomes, several sequences specific to the B chromosome have been identified. In the work described here we used the sequence from a B-specific RAPD (random amplification of polymorphic DNA) marker, pBGBM18.2, to isolate another DNA element, StarkB, present in many copies on the B chromosome. StarkB was mapped to the third and fourth blocks of distal heterochromatin using translocation breakpoints and fluorescent in-situ hybridization (FISH). Sequence analysis revealed that StarkB is composed of fragments from the A genome as well as B-specific sequences. The StarkB element is much larger than the other B-specific elements and is not present in large tandem arrays. Different copies of StarkB varied by small insertions, deletions, and duplications as well as single-nucleotide polymorphisms. Reverse transcriptase PCR showed that portions of the StarkB element are expressed. Using the LTR divergence of retroelements interrupting the B-specific sequences, the minimum age of the StarkB repeat array and, by inference, of the B chromosome, was estimated to be 2 million years.
Lamb
Jonathan C
JC
Division of Biological Sciences, University of Missouri-Columbia, Columbia, MO 65211, USA.
Riddle
Nicole C
NC
Cheng
Ya-Ming
YM
Theuri
James
J
Birchler
James A
JA
D002874:Chromosome Mapping; D002875:Chromosomes / Q000235:genetics*; D018744:DNA, Plant / Q000235:genetics; D006570:Heterochromatin / Q000235:genetics; D018626:Retroelements / Q000235:genetics*; D017422:Sequence Analysis, DNA; D014158:Transcription, Genetic; D003313:Zea mays / Q000235:genetics*
D018744:DNA, Plant; D006570:Heterochromatin; D018626:Retroelements