RESUMO
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.
Assuntos
Cromossomos/genética , Retroelementos/genética , Transcrição Gênica , Zea mays/genética , Mapeamento Cromossômico , DNA de Plantas/genética , Heterocromatina/genética , Análise de Sequência de DNARESUMO
A maize line expressing Cre recombinase as well as the recipient line without the transgene were assayed for evidence of ectopic recombination within the maize genome. Such a test is valuable for understanding the action of Cre as well as for its use to recombine two target lox sites present in the chromosomes. Pollen examination and seed set tests of material expressing Cre provided no evidence of ectopic recombination, which would be manifested in the production of translocations or inversions and result in pollen abortion and reduced seed set. Root-tip chromosome karyotype analysis was also performed on material with and without Cre expression. Chromosomal aberrations in Cre+ material were not observed above the background level.
Assuntos
Aberrações Cromossômicas , Integrases/metabolismo , Recombinação Genética/genética , Proteínas Virais/metabolismo , Zea mays/genética , Hibridização in Situ Fluorescente , Integrases/genética , Cariotipagem , Mutagênese/genética , Pólen/genética , Sementes/genética , Transgenes/genética , Proteínas Virais/genética , Zea mays/enzimologiaRESUMO
The complete sequence of rice centromere 8 reveals a small amount of centromere-specific satellite sequence in blocks interrupted by retrotransposons and other repetitive DNA, in an arrangement that is strikingly similar in overall size and content to other centromeres of multicellular eukaryotes.