Chromobiote phylogeny: the enigmatic alga Reticulosphaera japonensis is an aberrant haptophyte, not a heterokont

Abstract

Chromobiote algae are unusual in having an additional smooth periplastid membrane outside their chloroplast envelope and in the location of the chloroplast/periplastid membrane complex within the lumen of their rough endoplasmic reticulum. They are often brown in colour because of the carotenoid fucoxanthin in addition to their usual accessory chlorophylls c _1–3. They are divided into two well-defined phyla: the heterokont Ochrophyta (or Ochrista, e.g. brown algae, diatoms, chrysophytes; sometimes informally known as ‘core chromophytes’) and the Haptophyta. Reticulosphaera is an enigmatic genus of net-like chromobiotes, with only two described species, which morphologically do not fall clearly into either category. We have therefore sequenced the 18S rRNA gene of R. japonensis, together with those of two known species of haptophyte (Pavlova salina and two independent isolates of Prymnesium patelliferum), in order to clarify its evolutionary position. Though it was previously classified with heterokont Ochrophyta, our maximum likelihood, maximum parsimony, and neighbor-joining phylogenetic analyses unambiguously show that Reticulosphaera japonensis was placed in the wrong phylum, and is in fact an aberrant haptophyte without a haptonema. We show that the deepest division within the Haptophyta is that between the two classes Prymnesiophyceae (synonym Patelliferea, including Prymnesium and the coccolithophorids) and Pavlovophyceae (Pavlova and other Pavlovales). R. japonensis branches within Prymnesiophyceae close to Emiliania and Phaeocystis, which have a reduced haptonema. Although Reticulosphaera species differ greatly morphologically from previously known Prymnesiophyceae, in view of their close relationship to Prymnesiophycidae it now seems unnecessary to continue to classify them in a separate class, Flavoretea. We therefore reduce Flavoretea in rank to a subclass, Flavoretophycidae, that is grouped with subclass Prymnesiophycidae in the revised haptophyte class Prymnesiophyceae. A new family Reticulosphaeraceae and order Reticulosphaerales are created for Reticulosphaera. A new family Dicrateriaceae and order Dicrateriales are proposed for Dicrateria, the only prymnesiophyte lacking both scales and a haptonema. Though ribosomal RNA sequences are most informative about the internal phylogeny of Haptophyta, we conclude after extensive analysis that they are unable to refute or confirm the monophyly of the Chromobiota (Haptophyta plus Heterokonta) or the kingdom Chromista (subkingdoms Chromobiota and Cryptista).

Key words :

• Dicrateria
• Flavoretophycidae
• Pavlova
• Prymnesium
• Reticulosphaerales
• rRNA phylogeny