Chromatin diminution is defined as the elimination of DNA during the differentiation of early embryonic cells into pre-somatic cells. While it was first observed in the nematode Parascaris equorum, it also been identified in other parasitic nematodes, hagfish and copepods. In the copepod Mesocyclops edax, up to 90% of genomic DNA is eliminated during chromatin diminution. It was previously shown that the eliminated DNA contained highly repetitive heterochromatic sequences. Here, we digested pre- and post-diminution DNA with BamHI and produced small libraries of clones from each. Analyses revealed no decrease in low copy numbered sequences, such as transposable elements. Rather, both libraries are found to be surprisingly similar in all aspects analysed. Further comparison also demonstrated similarity of our libraries with the DNA sequences eliminated from Cyclops kolensis. Consequently, we suggest that M. edax eliminates portions of euchromatic DNA, in addition to the previously characterized satellite sequences.

Chromatin diminution is defined as the elimination of DNA during the differentiation of early embryonic cells into pre-somatic cells. While it was first observed in the nematode Parascaris equorum, it also been identified in other parasitic nematodes, hagfish and copepods. In the copepod Mesocyclops edax, up to 90% of genomic DNA is eliminated during chromatin diminution. It was previously shown that the eliminated DNA contained highly repetitive heterochromatic sequences. Here, we digested pre- and post-diminution DNA with BamHI and produced small libraries of clones from each. Analyses revealed no decrease in low copy numbered sequences, such as transposable elements. Rather, both libraries are found to be surprisingly similar in all aspects analysed. Further comparison also demonstrated similarity of our libraries with the DNA sequences eliminated from Cyclops kolensis. Consequently, we suggest that M. edax eliminates portions of euchromatic DNA, in addition to the previously characterized satellite sequences.

Chromatin diminution is defined as the elimination of DNA during the differentiation of early embryonic cells into pre-somatic cells. While it was first observed in the nematode Parascaris equorum, it also been identified in other parasitic nematodes, hagfish and copepods. In the copepod Mesocyclops edax, up to 90% of genomic DNA is eliminated during chromatin diminution. It was previously shown that the eliminated DNA contained highly repetitive heterochromatic sequences. Here, we digested pre- and post-diminution DNA with BamHI and produced small libraries of clones from each. Analyses revealed no decrease in low copy numbered sequences, such as transposable elements. Rather, both libraries are found to be surprisingly similar in all aspects analysed. Further comparison also demonstrated similarity of our libraries with the DNA sequences eliminated from Cyclops kolensis. Consequently, we suggest that M. edax eliminates portions of euchromatic DNA, in addition to the previously characterized satellite sequences.

The segregation of progenitor somatic cells from those of the primordial germ cells that sequester and retain elevated levels of DNA during subsequent developmental events, poses an interesting, alternative pathway of chromosome behavior during the reproductive cycle of certain species of cyclopoid copepods and several other organisms. Separation of maternal and paternal chromosome sets during very early cleavages (gonomery) is often a feature following marked elevations of DNA levels in germ cells for some of these species. Here, we report on the accumulation of large amounts of DNA in germ line nuclei of both female and male juveniles and adults of a freshwater copepod, Mesocyclops edax (Forbes, 1890). We also report the robust uptake of 3H-thymidine by germ cells prior to gametogenesis in this species. By using cytophotometric analysis of the DNA levels in both germ line cells and somatic cells from the same specimens we demonstrate that germ cell nuclei accumulate high levels of DNA prior to the onset of gametogenesis. These elevated amounts coincide with the levels of heterochromatic DNA discarded during chromatin diminution. A new model is proposed of major cytological events accompanying the process of chromatin diminution in M. edax.

Adult somatic nuclear DNA contents are reported for eleven cyclopoid species (Megacyclops latipes, Mesocyclops edax, M. longisetus, M. ruttneri, M. leuckarti, M. woutersi, Macrocyclops albidus, Cyclops strenuus, Acanthocyclops robustus, Diothona oculata, Thermocyclops crassus) and for the harpacticoid Tigriopus californicus and range from 0.50 to 4.1 pg DNA per nucleus. These diploid genome sizes are consistent with previously published values for four Cyclops species (0.28-1.8 pg DNA per nucleus), but are strikingly smaller than those reported for marine calanoids (4.32-24.92 pg DNA per nucleus). We discuss three explanations, none of them exclusive of another, to account for the smaller size and range of cyclopoid genome sizes relative to calanoid genome sizes: (1) higher prevalence of chromatin diminution in the Cyclopoida, (2) phylogenetic structure or older age of the Calanoida relative to Cyclopoida and (3) nucleotypic selection that may influence life history variation and fitness. Measurements of genome size were made on Feulgen stained, somatic cell nuclei, using scanning microdensitometry which is well suited to the sparse and heterogeneous populations of copepod nuclei. The importance of measuring large numbers of nuclei per specimen, possible sources of variation associated with cytophotometric measurements, and appropriate use of internal reference standards and stoichiometry of the Feulgen stained nuclei are discussed.

Genome sizes for 36 species of cyclopoid copepods were determined by DNA-Feulgen cytophotometry of nuclei from adults collected from diverse habitats and locales in North America, South America, Europe, and Asia. Genome sizes are small, show a 20-fold range (C = 0.10-2.02 pg DNA), and vary in a discontinuous fashion. The genomes of cyclopoid copepods are remarkably small and constant within each species, unlike the large and variable genomes of marine calanoid species. These differences may reflect the evolutionary antiquity of marine copepods in relation to marine, brackish, and freshwater copepods, as well as differences in mechanisms used to modulate genome size. The small genome sizes of contemporary cyclopoids provide substantive evidence of evolutionary constraint, possibly favouring small genomes, rapid replication rates and accelerated development as adaptive strategies for survival in often fragmented, stressful, and changing habitats.