RELATIONSHIPS AMONG EUCHEUMA-DENTICULATUM, EUCHEUMA-ISIFORME AND KAPPAPHYCUS-ALVAREZII (GIGARTINALES, RHODOPHYTA) BASED ON NUCLEAR SSU-RIBOSOMAL-RNA GENE-SEQUENCES

The nuclear genes encoding small-subunit ribosomal RNAs (ssu-rDNAs) of the carrageenophytes Eucheuma denticulatum, E. isiforme and Kappaphyc us alvarezii were amplified by the polymerase chain reaction, cloned a nd sequenced. The sequences range from 1767 (K. alvarezii) to 1781 (E. isiforme) nucleotides in length, and have guanine+cytosine contents b etween 51.2% (E. isiforme) and 51.5% (E. denticulatum). Pairwise seque nce identities among these sequences ranged from 97.6% to 98.5%, level s comparable to some intergeneric identities within Gracilariales. In phylogenetic analyses, the two Eucheuma ssu-rDNAs group stably togethe r vis-a-vis the ssu-rDNA of K. alvavezii, and these three ssu-rDNAs fo rm a monophyletic group within a larger grouping of other carrageenoph ytes. The results demonstrate quantitatively that analysis of nuclear- encoded ssu-rDNA sequences is likely to be useful. in resolving taxono mic, phylogenetic and biogeographic questions among tribe Eucheumatoid eae Doty.