TIMING OF CHROMATIN DIMINUTION IN THE FREE-LIVING FRESH-WATER CYCLOPIDAE (COPEPODA)

Chromatin diminution, the fragmentation and elimination of chromosome regions during early embryogenesis, occurs in some species of free-liv ing, fresh-water copepods. We studied the variation in timing of this developmental trait by examining durations of cell stages in several g enera of Cyclopidae. Chromatin diminution varied in timing within and among genera, and ranged from the 8-64-cell stage. The timing of chrom atin diminution, as measured by the number of hours post fertilization , differs considerably within and among genera. An unusually long cell -stage duration relative to that of other cell stages is indicative of the stage during which chromatin diminution occurs or the preceding s tage. Thus, the relative length of cell-stage durations within a speci es is a reliable assay for the presence or absence of chromatin diminu tion.