Sub‐chromatid Rearrangements in Trillium Erectum. I. Origin and Nature of Configurations Induced by Ionizing Radiation

Abstract

Wilson, G. B. (Michigan State U., East Lansing.), A. H. Sparrow, and Virginia Pond. Subchromatid rearrangements in Trillium erectum. I. Origin and nature of configurations induced by ionizing radiation. Amer. Jour. Bot. 46(4): 309–316. Illus. 1959.—Microsporocytes of Trillium erectum were x‐irradiated with 25 r at various stages of meiotic prophase and at first metaphase. Analysis of these cells at the following first and second anaphase revealed that post‐pachytene irradiation produces 2‐side‐arm bridges which are indicative of half‐chromatid exchanges. The occurrence of these bridges and knowledge of the structure and spatial relationship of chromatid strands in T. erectum have led to certain conclusions regarding the target and the number of strands broken by a single event: (1) the most likely target for primary effects is the 4 associated half‐chromatids of a half‐bivalent. The results of irradiation experiments suggest that the half‐bivalent is effectively as well as structurally quadripartite at stages following pachytene. (2) Consideration of the configurations which would result from breakage and rejoining of 2, 3 or all 4 strands of the half‐bivalent indicates that only 2 of the 4 half‐chromatids are broken by a single event. Exchanges between 2 half‐chromatids of sister chromatids will produce two recognizable types of 2‐side‐arm bridges: one with a true dicentric half‐chromatid and one in which the bridge results merely from an interlocking of coils. Whether a 2‐side‐arm bridge appears at first or second meiotic anaphase is determined by the position and number of chiasmata between the point of breakage and the kinetochore. No 2‐side‐arm bridges have been detected at microspore anaphase following meiotic prophase irradiation. The types of configurations which might be expected at microspore metaphase as a result of broken 2‐side‐arm bridges are noted.

Cytology of Some Variegated Forms ofTrillium Grandiflorum(Liliaceae)

“…The bridge and fragment formation observed during this study are presumed to be a result of spontaneous chromatid and subchromatid breakage and reunion. Similar aberrations are well documented in various plants and animals, either of spontaneous nature or induced by temperature and radiation effects (DAR-LINGTON and KEFALLINOU 1957;WILSON, SPARROW and POND 1959;LEWIS and JOHN 1966;BRANDHAM 1970;JoNES and BRAMPTON 1971;CHINNAPPA and SEMPLE 1976).…”

Classification and relationships of induced chromosomal structual changes.

“…The most characteristic type is a single bridge, with fragments attached to it ('side-arm' bridges). The structure of those bridges can be seen with particular clarity in some plant materials (Wilson et al, 1959;Wilson and Sparrow, 1960) and from such studies it has been inferred that the exchange involves a longitudinal sub-unit of the chromatid. Kihiman and Hartley (1967) have challenged this concept of the sub-unit and come to the conclusion that these aberrations are in reality true chromatid aberrations disguised by the processes of chromosome contraction which precedes mitosis.…”

The Meiotic Consequences of Radiation Induced Chromosome Breaks in <i>Lathyrus sativus</i> and <i>Vicia ervilia</i>

“…But the number of detailed studies showing the various types of chromosomal aberrations resulting from irradiation of the flower buds at different meiotic stages, however, is limited (Haque 1953, Crouse 1954, Sax and King 1955, Mitra 1958and Wilson et al . 1959.…”