he analysis of cells from nine individuals showed the presence of, at least, two cytotypes which we named C1 and C2. Karyotypes of cytotypes C1 and C2 obtained by arranging the chromosomes in order of decreasing size are shown in Figure 1. Cytotype C1,2n=46, found in six individuals,was composed of 6 medium size metacentrics, 2 subtelocentrics and 38 acrocentrics.Elements of the metacentric and subtelocentric series were classified as homologous pairs. Similar morphology and slight differences in chromosome size prevented identification of the homologous in the acrocentrics series. Cytotype C2,2n=45, with one large metacentric plus 5 medium size metacentrics, one subtelocentric,and 38 acrocentrics chromosome,was found in three individuals. In the cytotype C1, major ribosomal DNA cistrons are located in the terminal position of the short arms of the first metacentric as determined by silver nitrate impregnation (Figure 1b) and FISH with the 18S rDNA
probe (data not shown). In cytotype C2, silver staining revealed only one positive
signal in the single chromosome of the pair 1 (Figure 1d) whereas FISH with the 18S
rDNA probe hybridized in two different chromosomes:one coincident with the Ag-NOR signal in the single chromosome N° 1 and the other in one acrocentric
chromosome (Figure 2a). The differences observed in the cytotype C2 in relation to the cytotype C1 (loss of one element of the metacentric pair N° 1,loss of one of the
chromosomes of the subtelocentric pair N° 4,and appearance of a large metacentric element) could be explained by: a) a fusion of one of the subtelocentrics of the pair N°