CYTOGENETIC STUDIES OF CHROMOSOME REPLICATION IN ZEA MAYS L.: I. Analysis of quantitative autoradiographic data using Principal Components Analysis

A quantitative, autoradiographic study of 3H-TdR uptake at 18 °C was undertaken in the nuclei of primary root meristems of a highly inbred stock of maize. Plots of the mean number of silver grains over nuclei vs. hours after removal from 3H-TdR (replication profiles) revealed fluctuation in DNA replication. Wilcoxon's signed-ranks test showed that some chromosome arms had replication profiles different from the profile of the nucleus. A Principal Components Analysis (PCA) was used to characterize empirically and further compare the replication of chromosome arms. The PCA results confirm the Wilcoxon's signed-ranks test results and also suggest unique replication behavior for some additional chromosome arms. To quantitatively compare the position of individuals in the scatter diagram of the first three factor axes from the PCA, the standardized Euclidean distance of each individual from the centroid was employed. Since some chromosome arms were found to be different from the others, at least limited autono...