Molecular Systematics of Planktonic
Prokaryotes
(^) Much insight has come from classifications of life forms based on differences in their
DNA sequences (Box 2.4), particularly those for the genes coding the RNA sequences
in ribosomes (rRNA) – the organelles that assemble proteins and are components of
every cellular organism. The rRNA is separable into a smaller (SSU rRNA) and a
larger subunit. Results from thousands of SSU rRNA sequence comparisons show a
deep division between the Archaea and Bacteria (Fig. 5.3).
Fig. 5.3 A molecular tree of life based on rRNA sequence comparisons. The diagram
compiles the results of many SSU rRNA sequence comparisons. Only a few of the
known lines of descent are shown. The gene sequences define three broad groups, two
of them prokaryotes, the third with a membrane demarcated cell nucleus (karyon,
hence the name Eukarya). Organisms represented by codes (e.g. BRC1) have not yet
been completely identified.
(^) (After Pace 2009.)