Gonomery and chromatin diminution in Mesocyclops longisetus (Copepoda)

Abstract

The segregation of progenitor somatic cells from those of the primordial germ cells during early cleavage divisions occurs in certain copepods exhibiting the phenomenon of chromatin diminution during early embryogenesis. These species provide an interesting alternative to the usual reproductive strategy of other species of freshwater cyclopoid copepods. Levels of DNA for the soma and germ cells of Mesocyclops longisetus have been determined for individual nuclei by using Feulgen-DNA cytophotometry to monitor changes of DNA amounts during gametogenesis and early cleavage stages of embryogenesis. Germ cell nuclei of both female and male adults contain marked elevations of DNA, far in excess of expected 4C DNA level for their replication prior to meiosis. The elevated amounts of DNA in these germ cells are equivalent to the elevated DNA content found during the gonomeric divisions observed in embryos. Following the gonomeric divisions there is roughly a 40% loss of germ cell heterochromatin during the chromatin diminution stages of embryogenesis. The role of this excised DNA remains unclear.