A PHYLOGENETIC ANALYSIS OF THE 18S RIBOSOMAL-RNA SEQUENCE OF THE COELACANTH LATIMERIA-CHALUMNAE

Approximately 98% of the sequence of the 18S ribosomal RNA (rRNA) of the coelacanth Latimeria chalumnae was determined by a combination of direct RNA sequencing and sequencing of rRNA genes amplified by the polymerase chain reaction. This sequence was compared with 18S rRNA sequences of similar length from seven other vertebrate species, representing the taxa Petromyzontiformes, Holocephali, Elasmobranchii, Actinopterygii, Dipnoi, Amphibia, and Amniota, in order to determine the most likely sister group of the coelacanth. Maximum parsimony analysis of these sequences resulted in a single most parsimonious tree containing a number of anomalous relationships among these groups. A bootstrap analysis showed that none of the relationships in this tree was significantly supported at the 95% level, however. Addition of data from 15 other vertebrates (providing multiple representatives of most of the higher taxa) resulted in similar ambiguous groupings, as did a number of methods of editing the sites compared (designed to eliminate rapidly evolving positions). These results may be due to a relatively rapid radiation of the major lineages of osteichthyans, the resolution of which will require molecular information from a larger portion of the coelacanth genome.