PHYLOGENY OF AUREOCOCCUS ANOPHAGEFFERENS AND A MORPHOLOGICALLY SIMILAR BLOOM‐FORMING ALGA FROM TEXAS AS DETERMINED BY 18S RIBOSOMAL RNA SEQUENCE ANALYSIS1

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ABSTRACT The 18S ribosomal RNA genes from isolates of the east coast “brown tide” alga Aureococcus anophagefferens (strain Pt‐1) and the Texas “brown tide” alga (strain TBA‐2) were sequenced and compared to the gene sequences of Pelagomonas calceolata, a member of the Pelagophyceae, and 10 other organisms. The genes of A. anophagefferens and strain TBA‐2 consisted of 1814 and 2236 bases, respectively. The difference in length was largely due to a 423‐base insert occurring in the gene of strain TBA‐2. Excluding the insert, 93% of the bases of the aligned sequences were identical. Phylogenetic analyses were performed based on two different alignment refinement methods (eye refinement and Gatesy refinement). Trees inferred from the Jukes‐Cantor distance matrices using the neighbor‐joining method were similar for both alignment methods. In both trees, A. anophagefferens, strain TBA‐2, and P. calceolata formed a monophyletic group with A. anophagefferens and P. calceolata being sister taxa and strain TBA‐2 occurring on a deeper‐rooted branch. Bootstrap data sets for both alignment methods gave strong support for the Pelagophyceae group and the branches within that group. Parsimony analyses using the two alignments gave one tree for the eye‐refined alignment and two trees for the Gatesy method. All three trees had nearly the same topology as the trees inferred from the distance method. In addition, the Pelagophyceae group and the branches within that group were supported by bootstrap analyses and decay indices. Based on the available data, A. anophagefferens and strain TBA‐2 should be placed in the Pelagophyceae but not in the same order and family as P. calceolata.