Karyotype Analysis of Four Vicia Species using In Situ Hybridization with Repetitive Sequences
Moving bands amongst the beans
Over 25 years ago I suggested that the genus Vicia would be an ideal subject for study of plant genome evolution. The features that had caught my attention were the seven-fold range in genome size and the different chromosome numbers within the genus. These features have also been noted by Alice Navr�tilov� and colleagues at Cesk� Budejovice, Czech Republic (pp. 921-926). They have previously described two repetitive satellite DNAs, VicTR-A and VicTR-B in Vicia genomes. The copy number of the A-satellite varies between zero and approx. 106 in different species. There is some correlation between genome size and copy number suggesting that acquisition and further amplification of this repetitive DNA may be associated with genome expansion. B-satellite copy number varies between 100 and 106 but there is no correlation between copy number and genome size. Both satellites have proved useful in studying aspects of chromosome organization. The authors have used FISH (hybridization of fluorescently labelled DNA to metaphase chromosomes) and PRINS (amplification of DNA sequences in situ, using a PCR-based technique) to study the position of satellite bands in relation to established chromosome markers such as the rRNA genes. Three features of the results are very clear. First, these satellites provide unequivocal identification of each chromosome within each species, i.e. they can be used to establish the karyotype. Secondly, in species that possess the A-satellite it is always located at chromosome ends, whereas B-satellite bands occur throughout the karyotype. Thirdly, the banding patterns are not conserved between species so that they cannot be used to identify homologous chromosomes. This raises the possibility that the B-sequence may have been mobile in evolution and that there has been interspecific variation in the extent of amplification at different chromosomal locations. The genus Vicia thus continues to be highly suitable for study of genome evolution.