Early evolution and phylogeny of Phaeophyceae deduced from the cell fine structure, photosynthetic pigments and molecular phylogeny.

点击翻译按钮获取中文摘要

Vegetative cells of Giraudyopsis stellifera (Chrysomeridales) have cell wall-like structures composed of mucilaginous substances, and form one or two swarmer(s) when fertile. The swarmers are pear-shaped, provided with a chloroplast having an eyespot and heterokont lateral flagella ; the posterior flagellum shows green fluorescence. After settlement to the substratum, the swarmer first develops into prostrate filaments which then produce erect filaments. The chloroplasts are of typical chromophyte-type having triple-layered thylakoids enclosed in a girdle lamella, and a pyrenoid embedded in the chloroplast. Presence of chlorophylls a,c, fucoxanthin, violaxanthin and diatoxanthin were shown as major photosynthetic pigments by HPLC analyses. Systematic position of Chrysomeridales including Giraudyopsis in higher rank is unclear, but the molecular phylogenetic analyses using rbcL gene sequences revealed that the species is phylogenetically close to Xanthophyceae, Phaeophyceae, Schizocladi … More ophyceae and Phaeothamniophyceae, but was not included in them, and formed clade with Tetrasporopsis (incertae cedis).A molecular phylogenetic analysis of the brown alga Discosporangium mesarthrocarpum using rbcL and 18S rDNA sequences revealed that the species forms a monophyletic clade with Choristocarpus tenellus that is sister to all other brown algae. Although D.mesarthrocarpum has unique disc-shaped plurilocular reproductive organs, D.mesarthrocarpum and C.tenellus share the following basic morphological features which are considered to be plesiomorphic characters in the brown algae : 1) Apical (and diffuse) growth ; 2) Uniseriate, subdichotomously branched filaments ; 3) Multiple chloroplasts per cell without pyrenoids ; 4) Lack of heterotrichy and phaeophycean hairs. The rbcL DNA sequence of an Australian D.mesarthrocarpum specimen showed considerable deviation from Mediterranean and Macaronesian specimens. Therefore, the presence of a second species in the genus is suggested, however, the taxonomic treatment of this putative species is suspended in the present report. Regarding the higher rank systematic position of D.mesarthrocarpum, reappraisal of Discosporangiaceae and Discosporangiales is proposed. Less