Analysis of DNA Levels During Gonomery in Early Cleavage Divisions of the Freshwater Copepod Mesocyclops Edax

Abstract

Gonomery, the separate grouping of maternal and paternal chromosomes during the first few mitoses following fertilization (Fig. 1) and chromatin diminution, the fragmentation and elimination of whole chromosome regions during specific stages of early cleavage in all primordial cells of the soma, occur in several species of copepods, ascarid nematodes, ciliated protozoa, Japanese hagfish 9 and a few other invertebrates. Because of the marked loss of DNA from all presumptive somatic cells by selective chromatin diminution during the 4th cleavage division in M. edax, it was of interest to determine DNA levels in chromosomes during gonomery in anaphases preceding diminution (Fig. 1). It was also important to estimate the DNA content of sperm as an index of the relative contribution by the male genome at fertilization in this species.Mature males and egg-carrying females were fixed in 3:1 methanol/acetic acid for 3-5 mins, swollen in 45% acetic acid for 2-3 mins, squashed, frozen in liquid N2 for coverslip removal, thawed in 2 changes of absolute ETOH and air dried.