Ultrastructure of Cephaleuros virescens (Chroolepidaceae; Chlorophyta). III. Zoospores

Abstract

Transmission electron microscopic examination of Cephaleuros virescens Kunze growing on leaves of Camellia spp. and Magnolia grandiflora L. indicates that unreleased zoospores in mature zoosporangia are similar to those produced by the related genus Phycopeltis epiphyton Millardet and unlike the quadriflagellate motile cells produced by taxa in other families of Chlorophyta. The zoospores bear four smooth isokont bilaterally “keeled” flagella containing typical “9 + 2” axonemes and lacking scales. Flagellar insertion is apical and the parallel basal bodies overlap laterally at two levels. A cross section through the four basal bodies shows a trapezoidal arrangement wherein the two upper (anterior) basal bodies are closer together than are the lower (posterior) two. Serial sections indicate that diagonally opposing upper and lower basal bodies anchor flagella which emerge from the same side of the apical papilla. Each of the four basal bodies is associated with a microtubular spline which extends beneath the plasmalemma to the posterior end of the zoospore. A distinct multilayered structure is associated with each of the lower basal bodies. A nucleus, mitochondria (two of which are closely associated with the nucleus and spline microtubules), a chloroplast, and cytoplasmic haematochrome droplets are present in each zoospore. Pyrenoids and eyespots are absent. Flagellar insertion is characterized by “reversed bilateral symmetry”; and zoospores with both right-handed and left-handed arrangements are produced. The ultrastructure of the zoospores clearly indicates that: 1) the mode of flagellar insertion: 2) morphology, number, and arrangement of multilayered structures, and 3) bilaterally keeled flagella are characteristic of the Chroolepidaceae.