Evidence for Subchromatid Organization in Marsupial Chromosomes

The exposure of mitotic or meiotic cells to ionizing radiation or certain drugs produces chromosome aberrations which appear as side-arm bridges at anaphase . The aberration has been so named because the bridge apparently involves a chromosomal strand smaller than either of the anaphase chromosome arms, i .e ., a subchromatid exchange (Nebel, 1936 ; Swanson, 1947 ; Crouse, 1954 ; LaCour and Rutishauser, 1954) . Ostergren and Wakonig (1954), on the other hand, challenged the contention that side-arm bridges are formed by subchromatic exchanges . These investigators were unable to observe chromatid breaks in the second mitosis following irradiation in Allium cepa, as might be expected if half chromatid breaks had occurred before replication . They proposed that the side-arm configurations were pseudochiasmata resulting from "stickiness" of the chromosomal matrix. A similar interpretation was made by Kihlman and Hartley (1967) in Vicia faba . Peacock (1961), however, reported that such aberrations induced in chromosomes of Vicia faba did express as chromatid breaks in the subsequent division . Peacock's observations are supported by Heddle (manuscript in preparation) who also found chromatid aberrations in Vicia faba following an intervening period of DNA replication after irradiation. More recently, Whissel and Heddle (personal communication) observed the induction of side-arm bridges in rat kangaroo cells with X-irradiation and have found that such aberraBRIEF NOTES