Prochlorophyte Evolution and the Origin of Chloroplasts: Morphological and Molecular Evidence

Abstract

Prochlorophytes have been distinguished from cyanobacteria by their use of chlorophylls a and b as light-harvesting pigments, their lack of phycobiliproteins, and their closely spaced thylakoids. The prochlorophytes share these characteristics with plant and euglenoid chloroplasts, however, which has led to a debate on whether these chloroplasts arose from an endosymbiotic prochlorophyte or a cyanobacterium. (Lewin and Cheng 1989; Stackebrandt 1989; Cavalier-Smith 1982). Recent analyses of 16S rRNA and RNA-polymerase gene sequence data suggest that none of the three known prochlorophyte lineages — Prochloron (Lewin and Cheng 1989), Prochlorothrix (Burger-Wiersma et al. 1986), and Prochlorococcus (Chisholm et al. 1988, 1991) — includes the endosymbiotic ancestor of the chloroplast. The problem we now face is to reconcile the morphological and molecular sequence data with a coherent picture of prochlorophyte and chloroplast evolution. To that end we summarize in this paper current evidence from both kinds of investigations, with an emphasis on our own research.