Introductory Remarks

Abstract

Extracellular symbiotic algae like Prochloron ingested algal cells that may retain their ability to photosynthesize though they cannot multiply, intracellular symbiotic algae which can do both, cyanelles, and chloroplasts, represent a kind of functional continuum. There is a progressive reduction in nuclear or nucleoid genomes, as well as in the numbers and complexities of the walls and membranes that delimit the cells. Transport of their photosynthetic products may be correspondingly controlled or facilitated. Such a sequence of steps could reflect the evolution of plastids from prokaryotes by symbiogenesis. Prochlorophytes may be, or may resemble, stages in this evolutionary pathway. Molecular-biological studies provide means for testing this hypothesis, and objectively derived phylogenetic trees give us clues as to how plastids could have evolved.

When zoochlorellae were found in didemnid ascidians on the Great Barrier Reef of Australia, by Smith in 1935, they were regarded as nondescript algae of no great significance. Thirty years later, when these green cells were recognized as prokaryotes with the chlorophyll pigments of chlorophytes, and were baptized as Prochloron they elicited more attention. (For those of you who may still be unfamiliar with Prochloron and who have not yet referred to the illustrated volume of reviews and abstracts on this subject (Lewin and Cheng 1989), I might mention that its cells are green spheres, 12–30 µm in diameter, which occur tightly packed in special chambers of certain marine animals called didemnids, sessile compound ascidians common in tropical reef areas around islands in the Indopacific and south Pacific Ocean. Prochloron occurs less abundantly elsewhere, but always as an apparently obligate extracellular symbiont.) Some biologists regarded them as ancestral chloroplasts, or as descendants of putative chloroplast ancestors (PCA) that had somehow invaded and multiplied in alien eukaryote cells. The existence of Prochloron added support to an idea of Schimper, tentatively suggested in a footnote to an 1883 paper on plastid autonomy, and later put forward more forcefully, as “symbiogenesis,” in a thesis by Mereshkovsky submitted to Kazan University in 1905. The following quotations (my loose translations) are of interest. (Other relevant references are given in Lewin and Cheng 1989.)