VARIATION IN THE RIBOSOMAL INTERNAL TRANSCRIBED SPACERS AND 5.8S RDNAAMONG 5 SPECIES OF ACROPORA (CNIDARIA, SCLERACTINIA) - PATTERNS OF VARIATION CONSISTENT WITH RETICULATE EVOLUTION

The ITS sequences of Acropora spp. are the shortest so far identified in any metazoan and are among the shortest seen in eukaryotes; ITS1 wa s 70-80 bases, and ITS2 was 100-112 bases. The ITS sequences were also highly variable, but base composition and secondary structure predict ion indicate that divergent sequence variants are unlikely to be pseud ogenes. The pattern of variation was unusual in several other respects : (1) two distinct ITS2 types were detected in both A. hyacinthus and A. cytherea, species known to hybridize in vitro with high success rat es, and a putative intermediate ITS2 form was also detected in A. cyth erea; (2) A. valida was found to contain highly (29%) diverged ITS1 va riants; and (3) A. longicyathus contained two distinct 5.8S rDNA types . These data are consistent with a reticulate evolutionary history for the genus Acropora.