Chromosome pairing in polyploid cells.

The homologues in polyploid metaphases of air-dried suspensions of cultured cells are usually seen closely paired as diplochromosomes ("endore duplicated" cells) or unpaired as randomly distributed chromosomes ("poly ploid" cells). More often we find another configuration, a mixture of paired and unpaired chromosomes in the same metaphase, and describe here the conditions under which it appears. Materials and methods Chromosome pairs were counted in colchicine-arrested metaphases of 75 polyploid cells in air-dried suspensions (Moorhead et al. 1960) of cultured human amnion cells (Weinstein et al. 1956, Bodian 1956) from 13 normal births, grown at 37 • Ž as primary cultures for 3-4 days or as secondary cultures (one passage) for up to 21 days and in 67 polyploid cells from 3-day microcultures (Arakaki and Sparkes 1963) of leukocytes from 24 healthy adults and children. The counts were made on prints enlarged 7 • ~ from 35mm photomicrographs of the metaphases magnified 430 • ~ or 1250 • ~ ; chromosomes were considered pairs if they were diplochromosomes or apparent homologues by morphology and proximity (separated by not more than one other chromo some). The chromosome pairs in the one polyploid that was not tetraploid , a quintupleploid cell from a leukocyte culture, were included as the propor tionally reduced number of a tetraploid. Results Inspection of the prints of both amnion and leukocyte metaphases revealed three configurations of homologues in polyploid cells: homologues closely paired as diplochromosomes, none of the homologues clearly paired, or some of them paired (Fig. 1). Frequency distributions of the pair counts in the two populations of polyploids were assembled: Median pair counts of five polyploid cells, ranked in groups from greatest to least to minimize distortion due to sample size, were matched with the sum of the polyploids comprising the group. The distributions illustrate the variable degrees of homologue pairing in the poly ploids from both sources and a central tendency toward infrequent pairs (Fig. 2). Thus, there were mixtures of paired and unpaired homologues in