Abstract
Chromosome behavior in meiosis I was studied in six partially sterile hybrids between varieties of cultivated rice. Diakinesis and metaphase were normal in the sterile hybrids, with no evidence of translocations or absence of pairing to account for the sterility. In two of the hybrids chromosome bridges without fragments occurred at anaphase and telophase in a low but regular frequency. It was concluded that such bridges may be the result of delayed terminalization of chiasmata in pairs of chromosomes which are structurally different at some point. Although rice is not well suited to a study of pachytene and detailed analysis of chromosome pairing was not possible, figures which appeared to be chromosome loops typical of inversions were found in five of the seven hybrids. It was concluded that some evidence of structural differentiation in the parental chromosomes existed and that partial sterility in intervarietal hybrids of cultivated rice is probably due primarily to cryptic structural hybridity, possibly arising from included inversions.
