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.

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