PHYLOGENETIC POSITION OF THE PHACOTACEAE WITHIN THE CHLAMYDOPHYCEAE AS REVEALED BY ANALYSIS OF 18S RDNA AND RBCL SEQUENCES

Four genera of the Phacotaceae (Phacotus Pteromonas, Wislouchiella, Dy smorphococcus), a family of loricated green algal flagellates within t he Volvocales, were investigated by means of transmission electron mic roscopy and analysis of the nuclear encoded small-subunit ribosomal RN A (18S rRNA) genes and the plastid-encoded rbcL genes. Additionally, t he 18S rDNA of Haematococcus pluvialis and the rbcL sequences of Chlor ogonium elongatum, C. euchlorum, Dunaliella parva, Chloromonas serbino wii, Chlamydomonas radiata and C. tetragama were determined. Analysis of ultrastructural data justified the separation of the Phacotaceae in to two groups. Phacotus, Pteromonas, and Wislouchiella generally share d the following characters: egg-shaped protoplasts, a single pyrenoid with planar thylakoid double-lamellae, three-layered lorica, flagellar channels as parr of the central lorica layer, mitochondria located in the central cytoplasm, lorica development that occurs in mucilaginous zoosporangia that are to be lysed, and no acid-resistant cell walls. Dysmorphococcus was clearly different in each of the characters mentio ned. Direct comparison of sequences of Phacotus lenticularis, Pteromon as sp., Pteromonas protracta, and Wislouchiella planctonica revealed D NA sequence homologies of greater than or equal to 98.0% within the 18 S gene and 93.9% within the rbcL gene. D. globosus was quite different from these species, with a maximum of 92.9% homology in the 18S rRNA and less than or equal to 86.6% in the rbcL gene. it showed major simi larities to the 18S rDNA of Dunaliella salina, with 95.3%, and to the rbcL sequence of Chlamydomonas tetragama, with 90.3% sequence homology . Additionally, the Phacotaceae sensu stricto exclusively shared 10 (r bcL: 4) characters which were present neither in other Chlamydomonadal es nor in Dysmorphococcus globosus. Different phylogenetic analysis me thods confirmed the hypothesis that the Phacotaceae are polyphyletic. The Phacotaceae sensu stricto form a stable cluster with affinities to the Dunaliellaes and possibly Haematococcus pluvialis. Dysmorphococcu s globosus represented an independent lineage that is possibly related to Chlamydomonas moewusii and C. tetragama.