Nuclear DNA fractions with grossly different base ratios in the genome of the marine sponge Geodia cydonium.

The DNA of the marine sponge Geodia cydonium (G.c.), a member of the phylogenetically old phylum Porifera, was characterized by density gradient centrifugation and by determining its genetic complexity by reassociation kinetics. At least five subcomponents were identified by curve-fit analyses of analytical density gradient centrifugation profiles of total G.c.-DNA. Four of these subcomponents were isolated from total G.c.-DNA by preparative density gradient centrifugation. The GC-contents of the subcomponents were determined to be 36.4%, 44.0%, 58.7%, and 66.1%, respectively. To our knowledge, such an extreme heterogeneity of DNA composition has never before been observed for any organism. The genetic complexities within the subcomponents were determined by reassociation kinetics to 2.1×108, 2.8×108, 9.2×108, and 1.4×109 bp, respectively. The orders of magnitude of the genetic complexities clearly indicate that the DNA subcomponents mainly contain eukaryotic single copy DNA, since DNA of symbiotic prokaryotes should show significantly lower complexities. [ABSTRACT FROM AUTHOR]