A family of dispersed repeats in the genome of Vicia faba: structure, chromosomal organization, redundancy modulation, and evolution.
A family of repeated DNA sequences of about 1200 bp in length and bordered by well-conserved, 18 bp inverted repeats (VfB family) was found in the nuclear genome of Vicia faba. The structure, chromosomal organization, redundancy modulation and evolution of these sequences were investigated. They are enriched in A+T base pairs (about 40% G+C) and lack any obvious internally repeated motif. A 64%-73% nucleotide sequence identity was found when pairwise comparisons between VfB sequences were carried out (average 69%). Direct repeats were not found to flank the inverted repeats that border these DNA sequences. The results obtained by hybridizing VfB repeats to Southern blots of V. faba genomic DNA digested with EcoRI indicated that these DNA elements are interspersed in the genome. The appearance of bands in these Southern blots and comparison of the structure of the sequences that flank different VfB elements showed that these repeats might be part of other, longer repeated DNA sequences. A high degree of dispersion throughout the genome was confirmed by cytological hybridization, which showed VfB sequences to be scattered along the length of all chromosomes and to be absent or rare only at heterochromatic chromosomal regions. These sequences contribute to intraspecific alterations of genomic size. Indeed, dot-blot hybridizations proved that their redundancy, which is positively correlated with the overall amount of nuclear DNA in each accession, varies between V. faba land races (27x10(3)-230x10(3) copies per 1C DNA). Southern blot hybridization of VfB repeats to restriction endonuclease-digested genomic DNAs of V. faba, V. narbonensis, V. sativa, Phaseolus coccineus, Populus deltoides, and Triticum durum revealed nucleotide sequence homology of these DNA elements, whatever the stringency conditions, only to the DNAs of Vicia species, and to a reduced extent to the DNAs of V. narbonensis and V. sativa compared with that of V. faba. It is concluded that VfB repeats might be descended from mobile DNA elements and contribute to change genomic size and organization during evolution.
Frediani
M
M
Dipartimento di Agrobiologia e Agrochimica, Università della Tuscia, Via S. Camillo de Lellis, I-01100 Viterbo, Italy.
Gelati
M T
MT
Maggini
F
F
Galasso
I
I
Minelli
S
S
Ceccarelli
M
M
Cionini
P G
PG
D001482:Base Composition; D020029:Base Pairing; D001483:Base Sequence; D002874:Chromosome Mapping; D002875:Chromosomes / Q000235:genetics* / Q000648:ultrastructure; D018744:DNA, Plant / Q000235:genetics*; D004276:DNA, Satellite / Q000235:genetics*; D019143:Evolution, Molecular; D007887:Fabaceae / Q000145:classification / Q000235:genetics*; D018745:Genome, Plant; D008969:Molecular Sequence Data; D010946:Plants, Medicinal; D016133:Polymerase Chain Reaction; D016415:Sequence Alignment; D012689:Sequence Homology, Nucleic Acid; D013045:Species Specificity
D018744:DNA, Plant; D004276:DNA, Satellite