Replication patterns of repetitive DNA sequences on the W chromosome are altered during development of the chick...

A novel method was developed to study developmental changes in the replication pattern of repetitive DNA sequences on the W chromosome (W-DNA) of the female chick embryo. The amount of total nuclear DNA and W-DNA as well as 5-bromodeoxyuridine (BrdU) incorporation was successively measured on the same cells using multiparametric microfluorometry. With this method we first examined the possibility of changes in replication patterns of W-DNA during development. Measurements were conducted on various heterogeneous cell populations obtained from whole embryo on Day 0.4 and Day 1, and from pectoral muscle, neural tube, liver, and oogonium on Day 9. Parameters of W-DNA replication, duration, and timing were found to vary according to the stage of embryonic development. Developmental features of these changes were further studied on specific cell types during their critical developmental processes. In scutate scale dermis, the W-DNA replication duration showed a characteristic lengthening from around 0.45C during Day 5 through Day 7.4 to 0.9C during Day 7.7 through Day 7.9 and shortening to 0.37C during Day 8.1 through Day 12. Transient lengthening in W-DNA replication duration was also observed in erythrocytes; 0.65C ->1.0C ->0.6C during Day 0.9 through Day 2.17. Timing also shifted earlier in accord with changes in the duration. Replication rate of whole genome DNA was monitored by measuring BrdU incorporation on respective cells and found, to a large extent, comparable to that of W-DNA. The data suggest that a link might be operative between replication patterns of genes and the developmental program.