What factors drove the transformation of the cyanobacterial progenitor of plastids (e.g. chloroplasts) from endosymbiont to bona fide organelle? This question lies at the heart of organelle genesis because, whereas intracellular endosymbionts are widespread in both unicellular and multicellular eukaryotes (e.g. rhizobial bacteria, Chlorella cells in ciliates, Buchnera in aphids), only two canonical eukaryotic organelles of endosymbiotic origin are recognized, the plastids of algae and plants and the mitochondrion. Emerging data on (1) the discovery of non‐canonical plastid protein targeting, (2) (...) the recent origin of a cyanobacterial‐derived organelle in the filose amoeba Paulinella chromatophora, and (3) the extraordinarily reduced genomes of psyllid bacterial endosymbionts begin to blur the distinction between endosymbiont and organelle. Here we discuss the use of these terms in light of new data in order to highlight the unique aspects of plastids and mitochondria and underscore their central role in eukaryotic evolution. BioEssays 29:1239–1246, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

Proteins are targeted to plastids by N‐terminal transit peptides, which are recognized by protein import complexes in the organelle membranes. Historically, transit peptide properties have been defined from vascular plant sequences, but recent large‐scale genome sequencing from the many plastid‐containing lineages across the tree of life has provided a much broader representation of targeted proteins. This includes the three lineages containing primary plastids (plants and green algae, rhodophytes and glaucophytes) and also the seven major lineages that contain secondary plastids, “secondhand” (...) plastids derived through eukaryotic endosymbiosis. Despite this extensive spread of plastids throughout Eukaryota, an N‐terminal transit peptide has been maintained as an essential plastid‐targeting motif. This article provides the first global comparison of transit peptide composition and summarizes conservation of some features, the loss of an ancestral motif from the green lineages including plants, and modifications to transit peptides that have occurred in secondary and even tertiary plastids. BioEssays 29:1048–1058, 2007. © 2007 Wiley Periodicals, Inc. (shrink)

The dinoflagellates, a diverse sister group of the malaria parasites, are the major agents causing harmful algal blooms and are also the symbiotic algae of corals. Dinoflagellate nuclei differ significantly from other eukaryotic nuclei by having extranuclear spindles, no nucleosomes and enormous genomes in liquid crystal states. These cytological characteristics were related to the acquisition of prokaryotic genes during evolution (hence Mesokaryotes), which may also account for the biochemical diversity and the relatively slow growth rates of dinoflagellates. The fact that (...) the proliferation of many dinoflagellates is sensitive to turbulence may be due to the physiological requirements of the genome's liquid crystal states. Mechanical stress and anti‐microtubule drugs induce cell cycle arrest mainly in G1, implicating a role for the permanent cortical microtubular cytoskeleton in mechanotransduction. The cell cycles of photosynthetic dinoflagellates are also gated by the circadian rhythm, with cell division occurring mainly at the end of the dark phase. Cell growth and the biosynthesis of many toxins occur during the light phase, corresponding to G1 in the cell cycle. The dinoflagellates also embody several options for coupling cell cycle progression to cell growth, enabling them to make the best use of available resources and possibly preparing them for a symbiotic existence. BioEssays 27:730–740, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

The significance of horizontal gene transfer (HGT) in eukaryotic evolution remains controversial. Although many eukaryotic genes are of bacterial origin, they are often interpreted as being derived from mitochondria or plastids. Because of their fixed gene pool and gene loss, however, mitochondria and plastids alone cannot adequately explain the presence of all, or even the majority, of bacterial genes in eukaryotes. Available data indicate that no insurmountable barrier to HGT exists, even in complex multicellular eukaryotes. In addition, the discovery of (...) both recent and ancient HGT events in all major eukaryotic groups suggests that HGT has been a regular occurrence throughout the history of eukaryotic evolution. A model of HGT is proposed that suggests both unicellular and early developmental stages as likely entry points for foreign genes into multicellular eukaryotes. (shrink)

The cryptomonads and chlorarachniophytes are two unicellular algal lineages with complex cellular structures and fascinating evolutionary histories. Both groups acquired their photosynthetic abilities through the assimilation of eukaryotic endosymbionts. As a result, they possess two distinct cytosolic compartments and four genomes—two nuclear genomes, an endosymbiont‐derived plastid genome and a mitochondrial genome derived from the host cell. Like mitochondrial and plastid genomes, the genome of the endosymbiont nucleus, or ‘nucleomorph’, of cryptomonad and chlorarachniophyte cells has been greatly reduced through the combined (...) effects of gene loss and intracellular gene transfer. This article focuses on the structure, function, origin and evolution of cryptomonad and chlorarachniophyte nucleomorph genomes in light of recent comparisons of genome sequence data from both groups. It is now possible to speculate on the reasons that nucleomorphs persist in cryptomonads and chlorarachniophytes but have been lost in all other algae with plastids of secondary endosymbiotic origin. BioEssays 29:392–402, 2007. © 2007 Wiley Periodicals, Inc. (shrink)