Role of Endosymbiosis in Eukaryotic Evolution
In addition to providing a significant nutritional mode, the advent of endocytosis in an ancestor of living eukaryotes also enabled a completely new way to generate cellular change and complexity: endosymbiosis. Put simply, endosymbiosis is the process by which one cell is taken up by another and retained internally, such that the two cells live together and integrate at some level, sometimes permanently. Endosymbiotic interactions have been common in eukaryotic evolution, and many such partnerships persist today (Margulis, 1981). In two cases, however, endosymbiotic events had far-reaching effects on the evolution of life: these are the origins of mitochondria and plastids (chloroplasts).
Mitochondria are generally known as the energy-generating powerhouses of eukaryotic cells, where oxidative phosphorylation and electron transport metabolism takes place (Reichert and Neupert, 2004). They are also involved in several other jobs such as oxidation of fatty acids, amino acid metabolism, and assembly of iron-sulfur clusters (Lill et al., 1999; Lill and Kispal, 2000). They are bounded by two membranes, the innermost of which is generally highly infolded to form ‘cristae’ that take characteristic shapes, either flat, tubes, or paddle-shapes (Fig. 5) (Taylor, 1978). The presence of mitochondria is an ancestral trait in eukaryotes (Roger, 1999; van der Giezen and Tovar, 2005; van der Giezen et al., 2005; Williams and Keeling, 2003), although in certain anaerobes and microaerophiles they have radically reduced or transformed functions: in some cases they are not involved in energy production at all (e.g., the ‘mitosomes’ of microsporidia, diplomonads, and archaemoebae, or ‘hydrogenosomes’ of parabasalia, some ciliates, and some chytrid fungi) (Embley, 2006; Müller, 1993; Tovar et al., 1999; van der Giezen et al., 2005; Williams and Keeling, 2003). Mitochondria can be traced back to a single endosymbiosis of an alpha-proteobacterium (Andersson and Kurland, 1999; Gray et al., 1999; Gray and Doolittle, 1982; Gray et al., 2004; Lang et al., 1999).
Fig. 5. Mitochondria. There are three main structural types of mitochondria, defined by the shape of their cristae, or infolding of the inner membrane: from left to right are flat cristae (from the animal Cryptocercus punctulatus, © Kevin Carpenter), tubular cristae (from the diatom endosymbiont of Kryptoperidinium foliaceum, © Patrick Keeling and Kevin Carpenter), discoidal cristae from the euglenid Eutreptia pertyi (© 2009 Brian S. Leander), and a hydrogenosome (with no cristae) from the parabasalian Trichonympha acuta (© Patrick Keeling and Kevin Carpenter).
Plastids are the photosynthetic organelles of plants and algae. “Plastid” is a general term for all such organelles, including chloroplasts (in the green lineage), rhodoplasts (in the red lineage), leucoplasts (colourless plastids), etc (Fig. 6). Plastids have diverse functions in addition to photosynthesis, including the biosynthesis of amino acids, fatty acids and isoprenoids (Harwood, 1996; Herrmann and Weaver, 1999; Rohdich et al., 2001). As in the case of mitochondria, plastids in many lineages have been radically reduced or transformed, primarily through the loss of photosynthesis (e.g., the ‘apicoplast’ of Apicomplexa, and the relict plastids of many parasitic algae and plants (Gould et al., 2008; Ralph et al., 2004; Wilson, 2002)). Plastids can also be traced back to a single endosymbiosis event involving a cyanobacterium and the ancestor of the Archaeplastida (Reyes-Prieto et al., 2007; Rodriguez-Ezpeleta et al., 2005). However, unlike mitochondria, plastids then spread to other eukaryotic lineages by secondary and tertiary endosymbiotic events (Archibald, 2005; Gould et al., 2008; Keeling, 2004; McFadden, 1999). In these events, one eukaryotic cell took up another eukaryote that already contained a plastid (an alga), and this second, endosymbiotic eukaryote was then reduced and integrated. In most cases all that remains of this alga is the plastid surrounded by the remains of the endosymbiont’s plasma membrane. However, in cryptomonads and chlorarachniophytes a tiny relict of the algal nucleus called a “nucleomorph” is also retained, the study of which helped elucidate the complex evolutionary history of plastids (Archibald, 2005; Douglas et al., 2001; Gilson et al., 2006; McFadden et al., 1997). Other endosymbiotic relationships based on photosynthesis are also known (Johnson et al., 2007; Okamoto and Inouye, 2005; Rumpho et al., 2008), but typically these are not integrated to the extent that they are generally accepted to be ‘organelles’ rather than ‘endosymbionts’. One possible exception is the euglyphid amoeba Paulinella chromatophora, where a cyanobacterium similar to Synechococcus or Prochlorococcus has been integrated to an extent approaching that of canonical plastids (Nowack et al., 2008).
Fig. 6. Plastids. There are many different types of plastids, characterised by different pigments, structures, and envelopes. Here are a few examples, from left to right: the blue-green colored primary "chromophores" that were independently acquired by Paulinella chromatophora (© 2001 Michele Bahr and David Patterson), the red colored primary plastids of the red alga Porphyridium (© 2001 D. J. Patterson, L. Amaral-Zettler, M. Peglar and T. Nerad), the brown colored secondary plastids of Mallomonas insignis (© 2005 William Bourland), the green-colored secondary plastids of the euglenid Phacus (© Heather Esson and Brian S. Leander), and a TEM view of secondary green plastid of the euglenid Eutreptia pertyi (© Brian S. Leander).
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