A Study of X-ray-induced Chromosomal Aberrations in Barley

Abstract

Observations are presented on chromosomal aberrations in root-tip mitoses and microsporogenesis in plants grown from x-rayed dormant seeds of barley. The study was made as part of a considerable number of experiments over a period of five years. In all tests x-radiation was applied at 34 KVP and 27 ma. Dosages ranged from 0 to 32, 000 r.It was concluded that chromosomes in the resting embryo ordinarily responded to x-radiation as though they were monopartite, because induced breaks were almost always chromosomal breaks rather than chromatid breaks. This conclusion was based on: 1) the high frequency of paired, apparently identical dicentric chromosomal bridges and acentric fragments at anaphase in somatic cells of root tips; 2) the presence of unlike ends on some of these dicentric chromosomes; 3) the relatively rare occurrence of single bridges in these somatic anaphases.The rarity of single bridges at anaphase in somatic cells, in contrast with the frequency of paired bridges (even at low dosages), also was interpreted as indicating that the fusion of sister chromatids at a common breakage point, or following splitting of a chromosome during seed germination, did not often occur. Unlike ends on a dicentric chromosome also would not be expected if broken ends of sister strands fused.Usually two acentric chromatinic fragments do not join. Evidence for this is the fact that acentric fragments usually occurred in pairs (ranging in size from minute dots to rods as long as a normal chromosome) rather than as V's or rings as would be the case if their broken ends united. Thus, it seems that the centromere plays an important part in the union of broken ends of chromosome fragments.The observations suggest that acentric fragments are able to undergo one division, but do not persist beyond the subsequent resting stage. This opinion is based on the fact that these fragments usually occurred in pairs at somatic metaphase and anaphase, as though a piece of chromatin had reduplicated. If these fragments went into the telophase stage, returned to the condensed state and redivided again, quadruplet fragments should have been seen occasionally, but were not. However, it is true that most of the experimental observations were believed to have been made on the first cell division following radiation.At anaphase the acentric fragments were distributed freely in the cytoplasm, although they seemed to occur most frequently in the region of the equatorial plate. There was no evidence of any attraction between the fragments and the pole.Evidently a centric fragment attached itself as often to an acentric as to another centric fragment. This conclusion is based on the fact that there was a reasonably close parallelism between the frequencies of pairs of dicentric bridges in root tip cells (resulting from a union between two centric fragments) and interchanges in pollen mother cells (resulting from a union between a centric and an acentric fragment). If this relationship is proved to be true, it should be possible to predict fairly accurately the frequency of spikes with interchanges in their microsporocytes from the frequency of bridges in cells of root tips taken from seeds given the same x-ray treatment.Evidence is presented (see Discussion) which indicates that ordinarily, broken ends of chromosomes unite shortly after the breakage takes place. That is, most of the results were consistent with the theory that exchanges between irradiated chromosomes are usually the result of simultaneous breakage and fusion. However, a few observations seem to be accounted for most logically if it is assumed that delayed attachments between fragments occasionally do occur.Observations at meiosis in 3, 509 spikes revealed that the most common aberration was a ring-of-four (in 371 spikes), resulting from a simple reciprocal translocation.