HARMON, M.C. *; GAREY, J.R.; LIVINGSTON, B.T.; University of South Florida, Tampa; USF, Tampa; USF, Tampa: Position of the Ophiuroids within the Echinodermata Revisited

Both the ribosomal RNA gene sequences and the informative morphological characters within the Echinodermata have proven phylogenetically inconclusive with regards to the position of the ophiuroids. Littlewood et. al. report in their analysis of 18s ribosomal DNA sequences of echinoderm classes that ophiuroids are the sister taxa to the asteroids and the ophiuroid/asteroid clade is a sister to the holothuroid/echinoid clade (class topology of (C(A,O)(E,H))). Alternatively, their analysis of 28s rDNA sequences shows a class topology of (C(O(A(E,H)))), where the echinoid/holothuroid clade is a sister group to the asteroid taxa and the ophiuroids alone are a sister group to the e/h/a clade. We beleive that a resolution of the definitive position of the ophiuroids requires corroboration between the 18s and 28s rDNA analyses. We have re-examined the 18s molecular phylogeny using an alignment based on rRNA gene secondary structure. Sequences were aligned by eye using the MS-DOS program DCSE, trimmed to 1,548 nucleotides and exported into PAUP using the DOS CONVERSE program. A Gamma Shape parameter was estimated at 0.21 for the multiple alignment. The DOS program, Gambit, was used to analyze only the invariant and least variable sites in the alignment, and both Neighbor Joining and Minimum Evolution algorithms with k2p distances and alpha=0.21 were employed to generate phylogenetic trees. The 18s tree topology (C(A,O)(E,H))) proposed by Littlewood et. al. was duplicated, and the bootstrap values at the nodes in question were higher than those for previously generated trees. We are currently using similar methods to analyze 28s rRNA gene sequences from a number of species, including basal (based on cladistics and fossil records) individuals representing each class within the Echinodermata.