Unusual augmentation of germline genome size in Cyclops kolensis (Crustacea, Copepoda): further evidence in support of a revised model of chromatin diminution.

Embryonic chromatin diminution, the selective excision of large amounts of heterochromatic DNA from presomatic cell lineages, provides an example of an unusually large augmentation of the germline genome and raises questions regarding the source of the increased amount of DNA and its relevance to the biology of the organism. DNA levels in adult germ cell nuclei of the copepod Cyclops kolensis were determined by DNA-Feulgen cytophotometry and compared with those of somatic nuclei of adults and both pre- and postdiminuted embryos from the same mothers. Almost 75 pg DNA/nucleus is excised by diminution, resulting in the return of each generation to the approximately 1 pg DNA/nucleus level found for adult soma. To account for the increase in DNA levels of germ cells observed here, we propose alternative hypotheses to the original model of chromatin diminution: (1) repetitive endocycles or (2) proliferation of genetic elements. Specific tests for these hypotheses using next-generation sequencing and quantitative cytophotometry, as well as the functional significance of germ cell DNA augmentation to the copepod, are discussed.