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Megacyclops gigas, a freshwater microcrustacean (Copepoda: Cyclopoida), undergoes endoreduplication during early embryogenesis, a process during which DNA synthesis during the cell cycle occurs without subsequent mitosis and cytokinesis. This kind of cell cycle, an endocycle, results in a doubling of the nuclear DNA content. By the 4-cell embryonic stage, two endocycles have occurred, increasing the initial 2C level of DNA at 7.5 pg DNA per nucleus to the 8C level at 30 pg DNA per nucleus. These 8C cells proliferate to produce additional cells at the 8C level in the 8-cell embryo. DNA contents were measured using Feulgen Image Analysis Densitometry. Re-entry into the typical 2C – 4C cell cycle has begun by the 6th cleavage division, with some cell lineages containing 2C amounts while others contain 4C amounts. This mixture of cells with different DNA contents persists until the ~ 500-cell stage. All adult somatic cells contain the 2C amount of DNA. Some half anaphase chromosome figures at the 64-cell stage contain ~ 5 or ~ 9 pg DNA per nucleus, indicative of aneuploidy resulting from imperfect mitoses. The large clutch sizes of M. gigas may compensate for such an error prone process. Endoreduplication of the kind found in M. gigas is rare in that (1) it involves endocycles in all cells of the organism in a particular developmental stage, rather than being limited to specific tissues and (2) the endoreduplicated cells undergo depolyploidization. Overall, genomic stability in the M. gigas population appears to be unaffected. We hypothesize that the role of endoreduplication in this species is associated with a combination of increased demands for cellular differentiation and protein transcription related to the unusually large body size of M. gigas and its habitat type characterized by extremely low temperatures. 

Megacyclops gigas, a freshwater microcrustacean (Copepoda: Cyclopoida), undergoes endoreduplication during early embryogenesis, a process during which DNA synthesis during the cell cycle occurs without subsequent mitosis and cytokinesis. This kind of cell cycle, an endocycle, results in a doubling of the nuclear DNA content. By the 4-cell embryonic stage, two endocycles have occurred, increasing the initial 2C level of DNA at 7.5 pg DNA per nucleus to the 8C level at 30 pg DNA per nucleus. These 8C cells proliferate to produce additional cells at the 8C level in the 8-cell embryo. DNA contents were measured using Feulgen Image Analysis Densitometry. Re-entry into the typical 2C – 4C cell cycle has begun by the 6th cleavage division, with some cell lineages containing 2C amounts while others contain 4C amounts. This mixture of cells with different DNA contents persists until the ~ 500-cell stage. All adult somatic cells contain the 2C amount of DNA. Some half anaphase chromosome figures at the 64-cell stage contain ~ 5 or ~ 9 pg DNA per nucleus, indicative of aneuploidy resulting from imperfect mitoses. The large clutch sizes of M. gigas may compensate for such an error prone process. Endoreduplication of the kind found in M. gigas is rare in that (1) it involves endocycles in all cells of the organism in a particular developmental stage, rather than being limited to specific tissues and (2) the endoreduplicated cells undergo depolyploidization. Overall, genomic stability in the M. gigas population appears to be unaffected. We hypothesize that the role of endoreduplication in this species is associated with a combination of increased demands for cellular differentiation and protein transcription related to the unusually large body size of M. gigas and its habitat type characterized by extremely low temperatures.