Characterization of cytoplasmic and nuclear genomes in the colorless alga Polytoma. I. Ultrastructural analysis of organelles.

Abstract

EM studies have been made on the fine structure of the colorless biflagellate, P. obtusum, with main emphasis on the structural organization of the mitochondria and the leucoplast. Both organelles have been demonstrated to contain DNA aggregates as well as ribosomal particles within their matrix material. Reconstructions from serial sections showed that the mitochondria were highly convoluted and irregular in shape and size, and the leucoplast was a single cup-shaped entity, with large starch grains, localized at the posterior end and multiple sites of DNA aggregates. The starch-containing compartments appeared to be interconnected by narrow tubular or sheetlike bridges. Cytoplasmic invaginations into the plastid, often containing mitochondria, were of frequent occurrence and membranes of mitochondria and the leucoplast appeared to be closely apposed. Membranous elements, both sheetlike and vesicular, were also present in the matrix. The Polytoma leucoplast was, in certain respects, morphologically similar to the plastids of various photosynthetic mutants of Chlamydomonas, most of which show Mendelian segregation. It is suggested that Polytoma arose from Chlamydomonas-like ancestor, possibly through combined mutational processes of both chloroplast and nuclear genomes. Since Polytoma leucoplasts contain both DNA and ribosomal particles, it is probable that these organelles still possess semiautonomy and limited ability for protein synthesis.