Quantitative karyotyping and dual-color FISH mapping of 5S and 18S-25S rDNA probes in the cultivated Phaseolus species (Leguminosae)

Double-color fluorescence in situ hybridization (FISH) followed by DAPI cou nterstaining allowed the chromosomal assignment of 5S and 18S-25S rRNA gene s in the four cultivated Phaseolus species; P. vulgaris,P. coccineus,P. acu tifolius, and P. lunatus (all: 2n = 2x = 22). The rRNA gene loci display va riation between species as reflected in differences of signal size and (or) number. From one to three pairs of 5S sites and one to seven pairs of 18S- 25S sites were found in the diploid complements of the four taxa studied. I ntraspecific variation was studied in P. vulgaris, and it is shown that the number of 18S-25S rDNA sites differs between cultivars. Cytogenetic mappin g was complemented by karyotype analyses. Each of the four cultivated Phase olus species exhibits a characteristic heterochromatin endowment, with P. a cutifolius var. latifolius having the highest amount of C-band material. Qu antitative karyotyping in combination with cytogenetic mapping allowed the identification of homeologous chromosomes in the different species.