Intracellular Symbiosis of a Photosynthetic Prokaryote, Prochloron sp., in a Colonial Ascidian

Abstract

Cells of a symbiotic prokaryote, Prochloron sp., in colonies of a tropical ascidian, Lissoclinum punctatum, occur not only outside but also inside cells of the host. Cells of these photosynthetic symbionts of ascidians have previously been reported only extracellularly. The intracellular and extracellular symbionts do not differ morphologically. The host cells carrying the symbionts are freely distributed in the ascidian tunic. They probably endocytize the symbionts and then retain them within a vacuole. Since the intracellular prokaryotes showed no evidence of rejection or degeneration, this association between tunic phagocytes of L. punctatum and cells of Prochloron sp. seems to constitute a stable symrnbiosis, comparable to the postulated ancestral association between heterotrophic cells and the photosynthetic prokaryotes which gave rise to chloroplasts. Additional key words: algae, cyanobacteria, plastid evolution, prochlorophytes, tunicates Chloroplasts are photosynthetic organelles found in all photosynthetic eukaryotes. There is now convincing evidence from molecular biological studies on 16S-rRNA and RNA polymerase subunit (rpo Cl) and ribulose-bisphosphate carboxylase genes (Seewaldt & Stackebrandt 1982; Palenik & Haselkorn 1992; Urbach et al. 1992; Shimada et al. 1995) that these organelles originated from prokaryotic photosynthetic endosymbionts (cyanobacteria) engulfed and retained by heterotrophic host cells (Lewin 1981; Margulis 1981). Symbiotic photosynthetic prokaryotes are therefore of interest in relation to the evolution of chloroplasts. Prochloron is a genus of unicellular prokaryotes with the same chlorophyll pigments, chl a and b, as those in the chloroplasts of green algae and all other green plants (Lewin 1976). Prochlorophyta/Prochlorales was originally established (Lewin 1976, 1977; Florenzano et al. 1986) for prokaryotes that bear chl. a and b, lack bilin pigments, and generate oxygen in photosynthesis; molecular phylogenetic studies, however, indicate that this is a polyphyletic group whose members arose within the cyanobacterial radiation and should be treated as members of Cyanophyta/Cyanobacteria (Palenik & Haselkorn 1992; Urbach et al. 1992; Shimada et al. 1995). Cells of Prochloron occur in coral reef areas, almost exclusively as symbionts of colonial didemnid ascidians (Lewin & Cheng 1989). There the symbionts are normally associated with external or internal colony surfaces, but outside the host cells. Prochloron didemni LEWIN 1977 was originally described from the outer surfaces of didemnid colonies; but since no prochloron cells have been cultured in vitro, and specific distinctions remain to be established, we will refer to the symbionts in this paper simply as Prochloron sp. We present here the first report of Prochloron sp. as an intracellular symbiont in a didemnid ascidian, Lissoclinum punctatum KOTT 1977. Recent molecular biological data (Palenik & Haselkorn 1992; Urbach et al. 1992) indicate that the phylogenetic affinities of chloroplasts are closer to other cyanobacteria than to Prochloron, but we suggest that intracellular Prochloron can be regarded as a model of the ancestral green plastid.