MOLECULAR PHYLOGENETIC ANALYSIS OF rbcL IN THE PRYMNESIOPHYTA1

The nucleotide sequences of rbcL genes encoding the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) were determined from six species of Prymnesiophyta to clarify their phylogenetic relationships. Molecular phylogenetic trees were constructed using PAUP (Phylogenetic Analysis Using Parsimony). These analyses suggest that the Prymnesiophyta, except for the Pavlovales, area relatively stable monophyletic group. Pleurochrysis carterae, included in the Isochrysidales, is a sister species of a monophyletic group consisting of other members of the Isochrysidales, Gephyrocapsa oceanica and Emiliania huxleyi, members of the Coccosphaerales, Calyptrosphaera sphaeroidea and Umbilicosphaera sibogae var. foliosa, and a member of the Prymnesiales, Chrysochromulina hirta. The nucleotide sequence of rbcL from G. oceanica was identical to that from E. huxleyi within the region examined. Our trees show that G. oceanica and E. huxleyi are more closely related to C. hirta than to U. sibogae, C. sphaeroidea, and P. carterae. These results suggest that orders in the Prymnesiophyceae, including the above-mentioned genera, should be redefined.     

THE FLAGELLAR APPARATUS OF HYMENOMONAS CORONATA (PRYMNESIOPHYTA)1

The ultrastructure of Hymenomonas coronata Mills was reinvestigated to determine the microarchitecture of the flagellar apparatus. Cell morphology and flagellar apparatus structure are very similar to those of Pleurochrysis. Some important variations occur. First, a crystalline root (= compound root) is absent on microtubular root 1. Second, a two-stranded microtubular root emanates at a right angle from microtubular root 2. Third, a fibrous root emanates from the dorsal region between the basal bodies and extends to the cell's right, paralleling microtubular root 3. These similarities and variations in flagellar apparatus characters are discussed in reference to known variations in the Prymnesiophyta.     

MOLECULAR PHYLOGENY OF THE HAPTOPHYTA BASED ON THE rbcL GENE AND SEQUENCE VARIATION IN THE SPACER REGION OF THE RUBISCO OPERON

A phylogeny of 21 haptophyte algae was inferred by maximum parsimony, neighbor-joining, and maximum likelihood analyses of sequences of the plastid-encoded gene, rbcL. Sequence variation in the spacer region of the RUBISCO operon was also investigated. In all the rbcL trees constructed, the haptophytes form two distinct clades: one includes the Pavlovales and the other includes the Prymnesiales, Coccosphaerales, and Isochrysidales (all sensu Parke and Green 1976 . This relationship coincides with the recent taxonomic treatment splitting the division into two subclasses, the Prymnesidae and Pavlovidae ( Cavalier-Smith 1989 ) or the Prymnesiophycidae and the Pavlovophycidae using botanical suffixes ( Jordan and Green 1994 ), or into two classes, the Patelliferea and the Pavlovea ( Cavalier-Smith 1993 ). In the Prymnesiophycidae, all the coccolithophorids examined are placed in a single clade, which suggests a single origin of the coccolithophorids and the ability of coccolith formation in the haptophytes. The genus Chrysochromulina is polyphyletic. Species of Chrysochromulina with a very long haptonema and a compressed cell body (typical of species including the type C. parva Lackey) form a clade, including Imantonia, that is often classified in the Isochrysidales in the neighbor-joining tree, whereas some species possessing a nontypical cell body and cell covering form a clade with Prymnesium and Platychrysis in all trees. It is suggested that loss of the haptonema in Imantonia and the reduction in Prymnesium and Platychrysis occurred secondarily and independently in two different lineages. Within the coccolithophorids, four clades are recognized: Pleurochrysis, Calyptrosphaera-Cruciplacolithus-Calcidiscus-Umbilicosphaera, Helicosphaera, and Emiliania-Gephyrocapsa. A non-coccolith-bearing haptophyte, Isochrysis, is an ingroup of the Emiliania-Gephyrocapsa clade, suggesting its secondary loss of the ability to form a coccolith. Sequence comparison of the spacer region of RUBISCO operon supports most results obtained in the analysis of rbcL sequences. Monophyly of the Prymnesiales sensu Parke and Green is still unclear because of low (<50%) bootstrap support for this group.     

The architecture of the flagellar apparatus ofPrymnesium patellifera (Prymnesiophyta)

The flagellar apparatus of the small prymnesiophytePrymnesium patellifera has been analysed and a reconstruction is presented. Externally, the cell carries two sub-equal flagella and a short non-coiling haptonema. Within the cell, there are four microtubular roots and a number of fibrous bands, the latter interconnecting the two basal bodies and the haptonema base. One of the roots (r1) consists of a sheet of up to 25 microtubules originating close to the proximal extremity of the haptonema base, but the other three roots are composed of between 1 and 4 microtubules only. Distally, a large striated fibrous auxiliary connecting root extends across the anterior part of the cell linking root r1 and a mitochondrial profile on the opposite side of the cell. The arrangement of the components of the flagellar apparatus ofP. patellifera is commensurate with the general pattern found in many prymnesiophytes other than members of the Pavlovales, but there are a number of differences in detail from the other species described hitherto.