Ultrastructure and phylogenetic relationships of the unicellular green algae Ignatius tetrasporus and Pseudocharacium americanum (Chlorophyta)

The phylogenetic relationships of two unicellular green algae, Ignatius tetrasporus Bold et MacEntee and Pseudocharacium americanum Lee et Bold were investigated by ultrastructural and molecular methods. The zoospores from both species were covered neither by scales nor cell walls. The flagellar apparatus of the zoospores commonly included these features: the upper basal bodies were displaced counterclockwise in half to two‐thirds of the basal body diameter and did not overlap with each other; the lower basal bodies were directly opposed or slightly displaced clockwise; the distal fiber had gently sigmoid central striations; terminal caps were absent from the ends of the basal bodies; a V‐shaped proximal sheath extended from the upper basal bodies; a posterior fiber lay between the opposite lower basal bodies; and the coarsely striated band linked the sinister rootlet to the lower basal body. The suite of these features was not identical to that of any other quadriflagellate swimming cells, but some features including the lower basal body orientation, the striated distal fiber, and the coarsely striated fiber resemble those of the several organisms of the Siphonocladales sensu Floyd and O’Kelly. Phylogenetic analysis using 18S rDNA sequence data revealed that I. tetrasporus and P. americanum formed a monophyletic clade within the clade of Ulvophyceae sensu López‐Bautista and Chapman, but was not nested within any of the orders of the class that were examined.