MEIOSIS IN HOLOCENTRIC CHROMOSOMES - KINETIC ACTIVITY IS RANDOMLY RESTRICTED TO THE CHROMATID ENDS OF SEX UNIVALENTS IN GRAPHOSOMA-ITALICUM(HETEROPTERA)

We have determined the number and location of the nucleolar organizing regions in spermatocytes of Graphosoma italicum (2n = 12A + XY male/X X female) by means of silver impregnation, chromomycin A(3)/distamycin A staining and fluorescence in situ hybridization. The identification of only one nucleolar organizing region located at one of the X chrom osome ends has provided a suitable cytological marker to analyse the s egregation of this univalent and that of the XY pseudobivalent during the first and second meiotic divisions respectively. Our results clear ly show that at first meiotic metaphase the chromatids of the X chromo some are orientated with their long axes perpendicular to the polar ax is. Although the kinetic activity is restricted to only one end in bot h X chromatids during the first meiotic division, both ends of the sam e chromatid have the same probability of showing such kinetic activity . in this sense, we also report that the chromatid segregation may be initiated either at the same sister chromatid ends or at opposite ends in each chromatid. Thus, this indicates a sex chromatid independence as regards to the chromatid segregation during the first meiotic divis ion. Throughout the second meiotic division both ends of the X chromat id are involved with the same probability in the end-to-end associatio n to conform the XY pseudobivalent. This also implies a random localiz ation of the kinetic activity at the ends opposite to those involved i n the end-to-end association.