Size of circular chromatids and amounts of haploid DNA in the dinoflagellates Gyrodinium cohnii and Prorocentrum micans

Abstract

Chromosomal DNA of two dinoflagellate species, Gyrodinium cohnii and Prorocentrum micans, were studied electron microscopically. As suggested earlier, chromosomal DNA was shown to be composed of circular DNA molecules each representing a single chromatid of the polytenic chromosomes of these species. The chromatids released by dodecyl sulphate lysis were highly twisted and consequently the number of superhelical turns per circle could not be determined. Chromatids of Gyrodinium ranged from 25 μm to 35 μm and were 130–160 μm in Prorocentrum. Calculations based on quantitative DNA analysis and chromosome numbers suggest that the degree of polyteny in Gyrodinium is about 700 and in Prorocentrum about 1300. Their haploid DNA amounts are 3000 μm of double helix in Gyrodinium and 9860 μm in Prorocentrum or 2.1 and 7.0 times, respectively, that of the bacterium Escherichia coli.