MEIOSIS IN ORNITHOGALUM VIRENS (LILIACEAE): MEIOTIC TIMING AND SEGREGATION OF H3-THYMIDINE LABELED CHROMOSOMES

Abstract

The timing of meiosis and the segregation of tritium-labeled chromosomes has been studied in Ornithogalum virens. Tritiated thymidine was administered to the inflorescences by a wick feeding process. The length of time required for labeled cells to pass from premeiotic DNA synthesis to the tetrad stage of meiosis is approximately 4 days. The labeled chromosomes display a semiconservative segregation pattern when labeled one division prior to meiosis; however, chromatid exchange events occur prior to, or during meiosis, or both. These exchange events are most likely a combination of crossing over between homologous chromosomes and sister chromatid exchange. The reductional separation of the centromere region at the first meiotic division is confirmed. The observations do not support Sax's two plane theory of crossing over, but are consistent with the theory that chiasmata are the cytological consequences of genetic exchange.