Genome intermixing and sister chromatid exchange in newly-formed HeLa-3T3 Hybrid cells

Mouse and human genomes occupy distinct regions within hybrid interphase nuclei following division of HeLa-3T3 heterokaryons. With subsequent cell division the proportion of interphase cells displaying separation of human and mouse genomes decreases. Examination of several hundred hybrid colonies revealed a linear relation between the log of the fraction of interphase cells with separated genomes and the log of clone size. This indicates that there is a constant probability that separated genomes will intermingle at each mitosis. Human and mouse chromosomes can also occupy distinct sectors in metaphase spreads derived from heterokaryons. Computer analysis of the distribution of chromosomes within 548 hybrid metaphases showed that mouse and human chromosomes are randomly intermixed within several divisions and before the onset of rapid chromosome loss. Sister chromatid exchange (SCE) rates were also measured in mass populations of newly-formed HeLa-3T3 hybrid cells. For most hybrid metaphases there was not significant change in SCE rates within the human chromosome set. In a small minority of hybrid metaphases, characterized by asynchronous condensation of chromosome sets, there was a 50-fold increase in SCE. However, chromosomes are progressively lost from all hybrid cells. Thus, the two processes examined in the present studies, the distribution of human chromosomes at metaphase and SCE, are not implicated in the preferential loss of human chromosomes from HeLa-3T3 cells.