Evidence for chromosome endoreduplication in Eudorina californica, a colonial alga.

Several nuclear events seen during the cleavage period in Eudorina suggest that chromosome endoreduplication, proportional to the number of cells to be produced, may occur in the gonidia prior to cleavage. Presumably the DNA concentration is reduced to the haploid level during rapid, successive divisions of the cleavage period. To test this hypothesis, I determined DNA content of gonidia as they grew from 4 mum to 38 mum in diameter between cleavage periods. During growth from 4 mum to 8 mum in diameter, the DNA concentration remained at the haploid level of 0.17 pg/cell. As gonidia in 64 cell colonies continued to grow from 8 mum to 33 mum in diameter, their DNA concentrations increased 60-fold. Analysis of the Eudorina DNA by equilibrium centrifugation in CsCl showed only 2 bands with buoyant densities of 1.721 g/cm3 and 1.699 g/cm3, presumed to be nuclear and chloroplast, respectively, on the basis of labelling with 3H-thymidine and 3H-adenine. The 8:2 ratio of the two bands did not change with increase in cell size and no other bands were detected, suggesting that both nuclear and chloroplast DNAs were synthesised proportionately prior to the cleavage period.