DIFFERENTIAL REPLICATION AND DNA ELIMINATION IN THE POLYTENE CHROMOSOMES OF EUPLOTES-CRASSUS

The transposon-like Tec elements of Euplotes crassus are precisely exc ised during formation of polytene chromosomes in the developing macron ucleus. To determine whether all Tec elements exhibit identical develo pmental timing of excision, we used polymerase chain reaction to visua lize amplification and diminution at numerous randomly selected Tec in sertion sites. Two classes of sites are evident. Early replicating sit es show one or more rounds of amplification and diminution (correspond ing to excision) and frequently occur within macronuclear-destined seq uences. Late replicating sites do not undergo diminution until chromos ome fragmentation and are predominantly associated with eliminated seq uences, We conclude that the previously described clustering of macron uclear-destined sequences in the micronuclear genome allows for their differential replication at the polytene stage and results in targetin g of these sequences for transcriptional activation and highly specifi c deletion and chromosome fragmentation processes.