Real chromatid deletions versus gaps

Abstract

The validity of previous scoring of radiation-induced chromatid deletions detected at metaphase has been brought into serious question by Revell in recent years. He finds in Vicia that only 1 20 to 1 100 of the apparent chromatid deletions are real, the remaining large fraction are achromatic lesions or “gaps”. Here, this problem was studied in Tradescantia microspores, γ-irradiated in late interphase (S or perhaps G 2), by analysis of the chromatid types of aberrations at metaphase and at anaphase. A real chromatid deletion will release its fragment at anaphase, the gap will not. The scoring validity of metaphase-detected chromatid deletions was established by observation, at the same dose, of an equal frequency of free fragments at anaphase. Metaphase scoring of all other aberration types was similarly confirmed, and yield per rad agreed with previous metaphase scoring. The frequency of gaps was at the most but a third that of the confirmed real chromatid deletions, actually probably less. (Radiation was delivered at low intensity (50 rad/h), which, on Revell's hypothesis, should have maximized the ratio of gap to chromatid deletion production.) Reasons for the 60- to 300-fold disagreement ( 1 3 versus 20 1 to 100 1 ) in scoring of gaps in Vicia and in Tradescantia are considered. Differences in microscopic detection, staining methods, and stages irradiated are unlikelily as major contributors to the discrepancy; intrinsic species or possibly cell type differences are left as the most likely, though unexplained, reasons.