Chromosomal location of the active NORs in theSteropleurus martorelli complex

The chromosomal location of the active NORs has been analyzed by a silver impregnation procedure in theSteropleurus martorelli complex. A ‘primary NOR’, which is always present at the first meiotic prophase, has been found in each of the four described races. In addition to this, all races possess one or two ‘secondary NORs’ which are less active than the former and can be occasionally shown. Usually only one of the two homologous chromosomes has been found to be involved with nucleolus organisation.These results are discussed in relation to hypotheses on the chromosome differentiation of this species complex.